15

2. LITERATURE SURVEY

2.1 Alkaline-earth metal, lead and cadmium tungstates:

Scheelite is the mineral name for calcium tungstate, CaWO4. The crystal structure of

scheelite is tetragonal consisting of calcium surrounded by eight oxygens with the

isolated tetrahedra of WO4 being nearly regular with four equal W-O distances [1].

MWO4 type oxides with scheelite structure where M=Ca, Sr, Ba, Pb and Cd have been

reported to be useful for laser host materials [2], scintillators [3], oxide ion conductors

[4,5] and as humidity sensors [6]. Synthesis of MWO4 were reported by several

methods like solid-state method, co-precipitation, solvothermal, sol-gel, reverse

micellar reactions, microwave hydrothermal, combustion and hydrothermal. The

detailed discussion of which are given below.

Shan. Z et al reported the synthesis of MWO4 (Ca, Sr, Ba) powders by solid-state

reaction [7]. The photocatalytic activities of these tungstates for decomposing methyl

orange were investigated. They showed that the photocatalytic activity is in the

increasing order of CaWO4 < SrWO4 < BaWO4 under both neutral and acidic

conditions. Their further investigation proves that the higher structural openness

degree, corresponding to a lower packing factor, leads to the better photocatalytic

activity.

Cavalcante et al [8] also reported formation of BaWO4 powders by co-precipitation and

processed in a domestic microwave-hydrothermal at 413K for different times. XRD

patterns showed that the BaWO4 powders present a scheelite-type tetragonal structure

and free of impurity phases. The evidence of a tetragonal structure due to W-O

stretching vibration into the [WO4] tetrahedral groups was proved by FT-Raman

spectrum. FT-IR spectra revealed a strong shoulder on the v3 bands in the transmittance

16

spectra of the powders. FEG-SEM micrographs indicated that the powders present an

octahedral-like morphology with agglomerated nature and polydisperse particle size

distribution. PL emission at room temperature of BaWO4 powders exhibited a

maximum emission at around 542 nm (green emission). PL behaviour was explained

through distortions on the [WO4] tetrahedron groups and also due to the structural

defects in the BaWO4 lattice.

Sulawan. K et al [9] reported formation of CaWO4 by solvothermal synthesis at a

temperature of 1600C for 6h, using Ca(NO3)24H2O and Na2WO4.2H2O as precursors

by using various solvents such as glycerol-water ( or propylene glycol-water) as the

reaction medium. They provided the evidence of the scheelite structure with W-O

stretching vibration in [WO4]2- tetrahedral at 705-875 cm-1.

Zalga et al [10] reported synthesis of calcium and barium tungstates by aqueous sol-gel

method using tartaric acid as complexing agent and heating the xerogel obtained at

5000C for 1hr with subsequent annealing at 8000C for 1hr. The obtained gels were

studied by thermal (TG/DSC) analysis. They showed that these tungstates can be easily

doped with lanthanum ions without changing their crystal structure.

Kwan et al [11] reported the synthesis of monocrystalline BaWO4 nanorods using a

reversed micelle templating method. The nanorods were found to be uniform and have

large aspect-ratio. Those nanorods were observed in both as-made materials and

Langmuir–Blodgett monolayer assemblies.

Kisla P. F. S et al [12] reported microwave-hydrothermal preparation of alkaline-earth-

metal tungstates using metal nitrates and sodium tungstate hydrate as starting materials.

The processing conditions were 1100C, for times ranging from 5 to 20 minutes.

Characterization was done by using X-ray diffraction studies which showed crystalline,

17

single-phase materials were obtained. Electron microscopy showed nanostructured

particles with different morphologies. Raman spectroscopy was used to probe short-

range ordering and also to obtain a reliable set of spectra containing all the Raman-

active bands predicted by group-theory calculations.

Ryu. J. H et al [13] reported the microwave-assisted synthesis of CaWO4 using citrate

complex precursor. The precursors were heated at temperatures from 3000 to 6000 C for

3 hrs. They reported that crystallization was detected at 4000C and completed at

temperature of 5000C.

Yiguo. S et al [14] reported the synthesis of CaWO4-based red phosphors with

codoping of Eu3+ and Na+. Their results reported showed that it is possible to control

chemical composition of oxide nanostructures for structural decoration and

luminescence property tailoring via codoping aliovalent ions.

Thangadurai etal [15] reported the synthesis of ABO4 (A=Ca, Sr, Ba, Pb; B=Mo,W)

powders at room temperature by metathetic reactions using aqueous solutions of

alkaline or lead chloride, and sodium molybdates or tungstates. The reaction is

completed within a minute. The particle size as determined by SEM is of order of

several micrometers.

Parhi etal [16] reported synthesis of metal tungstates using solid-state metathetic

approach assisted by microwave energy. Where as single-phase ZnWO4 is synthesized

by SSM reactions at ambient conditions and MWO4 (M=Ni, Mn) are synthesized after

subjecting the amporphous product to moderate temperature of heating around 5000 C

for 6 hours.

18

Guoxin Zhang et al [6] reported the synthesis of CdWO4 by the reaction of CdCl2 and

NaWO4 in the presence of mercaptoacetic acid (TGA) as capping reagent.

Photoluminesence properties of quantum dots (QD) were studied by

photoluminescence spectroscopy (PL). The results showed that the single QD with

diameter of about 8±2 nm was single-crystal. The particle size distribution of QDs was

normal. Infrared absorption bands of carboxylic group on the surface of CdWO4 QDs

were observed around 1610-1550 cm-1 (non-symmetrical vibration of –COO-) and

1400 cm-1 (symmetricvibration of C-O). With reaction-time going, PL peak position

shifted from 498 to 549 nm and intensity of PL increased first and then decreased. PL

peak position of QDs was blue-shift compared with 570 nm WO6-6 liminescence center

of bulk CdWO4.

Chang Sung Lim et al [17] reported the synthesis of CdWO4 particles using cyclic

microwave irradiation assisted by a solid-state metathetic reaction. The reaction

crystallized at 400-6000C, showing a fine structure with a self-assembled rod-like

morphology and a crystallographic orientation with sizes of 1-3µm. The synthesized

CdWO4 particles were characterized by X-ray diffraction, Fourier transform infrared

spectroscopy, scanning electron microscopy and transmission electron microscopy. The

optical properties were investigated by photoluminescence emission and Raman

spectroscopy.

Won-Chun Oh et al [18] reported the microwave-assisted synthesis of Ag incorporated

SrWO4/zeolite composites by a SSM route. The composites were formed at 6000C for 3

hours showing a immobilized morphology with a particle size of 3-5 µm. They reported

that the stretching vibration in FTIR was detected as a strong W-O stretch in the

[WO4]2−tetrahedra at 823 cm−1.

19

Xiufeng Zhao et al [19] developed a procedure for SrWO4 via a precipitation reaction

between SrCl2 and Na2WO4 in presence of polymethacrylic acid. These authors

synthesized the WO3 spheres with the as-prepared SrWO4 hollow spheres as both

precursors and templates. After soaking in HNO3 and calcinations, the SrWO4 were

completely transformed into WO3 while the hollow structures were perfectly retainted.

The “polymer-cation template” model was proposed to describe the formation of the

SrWO4 hollow spheres.

Recently, Chang Sung Lim [20] reported formation of BaWO4 using a solid state

metathetic route assisted by cyclic microwave irradiation with subsequent heat

treatment at 600oC for 3hrs, showing a fine and homogeneous morphology with particle

sizes of 1-2 µm. They have concluded that PL intensities of BaWO4 particles prepared

at 6000C were much stronger than that of the samples prepared at 400 and 5000C. The

Raman modes for the BaWO4 particles were detected at 925, 831, 795, 352, 344 and

332 cm-1, the free rotation mode was detected at 189 cm-1 and the external modes were

localized at 148 cm-1.

Recently Sofronov et al [21] reported microwave synthesis of CdWO4 using Cd(NO3)2

and ammonium tungstates as precursors. According to these investigators CdWO4

Scheelite phase exists upto 2000C and thereafter transforms to monoclinic structure.

2.2 Transition metal tungstates:

MWO4 (M2+ = Mn, Fe, Co, Ni and Zn) type divalent transition metal compounds have

been reported to be useful for humidity sensors [22], photocatalysts [23], photochromic

[24] and as photoanodes [25]. 3d transition metal tungstate powders have been

synthesized by different techniques such as solid-state reaction, chemical synthesis,

hydrothermal, microwave hydrothermal, self propagation, template synthesis,

20

combustion, molten salt and aqueous salt metathetic reaction. The detailed discussion

of which are given below.

Bhaskar Kumar. G et al [26] synthesized ZnWO4 ceramic powder by a solid-state

reaction method. Phase formation takes place at 10000C. A broad and blue emission

(472 nm), which orginates from its wolframite structure. The particles are agglomerated

and the average diameter of the grain size of 530 nm.

Montini. T et al [27] reported the chemical synthesis of tungstates of divalent transition

metals (MIIWO4, M=CoII, NiII, CuII, ZnII) by reaction of transition metal nitrates with

sodium tungstate. The precipitates were calcined at 5000C. According to their results

higher photocatalytic activity for decolourization of Methylene Blue and Methyl

Orange is due to ZnWO4 compared to that of the other tungstates which correlates with

its strong tendency of excitons self-trapping.

Angana Sen et al [28] reported chemical synthesis of MWO4 where M = Ca(II), Co(II),

Cu(II), Ni(II) and Zn(II) powders from complete evaporation of a polymer based metal-

complex precursor solution. The metal ions forms complexation with triethanolamine

which are dispersed in a polymeric reagent composed of an aqueous solution mixture of

sucrose and polyvinyl alcohol (PVA). The mesoporous carbon rich precursor powders

which is obtained on complete dehydration of the precursor solution generates the

respective metal tungstate phase after complete removal of carbonaceous residue at a

heat treatment temperature less than 5000C. The average particle sizes are around ~15-

40 nm.

Kalinko . A et al [29] synthesized ZnWO4 powders by co-precipitation method and

annealed in air at different temperatures in the range of 80-8000C. These authors

reported that well crystalline ZnWO4 is obtained after annealing at 8000C. They further

21

stuided the temperature dependence of the Raman scattering which indicates a dramatic

variation of the vibrational properties in ZnWO4 powders. Photoluminescence band

shift to longer wavelength, comparing to single crystal is also observed for powders

annealed at temperatures below 4500C, where as the band position for ZnWO4 powders

annealed at higher temperatures is identical to that in single crystal. The annealing

temperarture has also strong influence on the bandwidth of the photoluminescence band

and decay kinetics.

Kalinko. A et al et al [30] synthesized ZnWO4 by the reaction of ZnSO4.7H2O and

Na2WO4.2H2O as precursors by co-precipitation technique. The PL spectra and

luminescence excitation spectra of pure microcrystalline and nano-sized ZnWO4 as

well as the ZnxNi1-xWO4 solidsolutions were studied using vaccum ultraviolet (VUV)

synchrotron radiation. They reported that the shape of photoluminescence band at 2.5

eV, being due to radiative electron transitions within [WO6]6- anions, becomes

modulated by the optical absorption of Ni2+ ions in the ZnxNi1-xWO4 solid solutions.

And also showed that no significant change in the excitation spectra of Zn0.9Ni0.1WO4

is observed compared to pure ZnWO4. The shift of the excitionic band in the excitation

spectra of nanosized ZnWO4 is caused by the local structural relaxation. The number of

bands observed in the excitation spectra due to one-electron transitions from the top of

the valence band to quasi-localised states.

Hongbo Fu et al [31] prepared PbWO4 and ZnWO4 photocatalysts by hydrothermal

crystallization process. For ZnWO4 the aqueous solutions of Na2WO4. 2H2O and

Zn(NO3)2 were mixed and pH was adjusted to 11 by using NaOH or HCl. Where as

for preparation of PbWO4, the aqueous solutions of Pb(NO3)2 and H2WO4 were added

and then pH was adjusted to 11 by using KOH or HNO3.The reaction mixtures were

22

sealed in a Teflon-lined stainlesssteel autoclave and heated to 1800C under autogenous

pressure for 24 hrs. After cooling the product was filtered, washed and dried at ambient

temperature. They reported the single phases of the tetragonal, and monoclinic

structures for PbWO4 and ZnWO4 respectively. The rhodamine-B photodegradation in

aqueous medium was employed as a probe reaction to test the photoactivities of the

prepared samples. ZnWO4 only displayed high photoactivity under UV irradiation.

However, PbWO4 showed poor photoactivity under any light irradiation according to

their reports.

Qiang Wang et al [32] reported the synthesis of ZnWO4 nanostructures via

hydrothermal method in presence of surfactant polyethylene glycol 20000 (PEG-

20000). Synthesis of ZnWO4 were made by simple inorganic salts as precursors. The

shape and size of ZnWO4 was controlled by pH value of the reaction system, dosage of

surfactant and other hydrothermal parameters. The composition and PL properties of

the sample have been investigated by X-ray diffraction pattern and fluorescence

spectrophotometer respectively. They showed that ZnWO4 with different morphologies

exhibited the blue peak at 506 nm with an excitation wavelength at 253 nm.

Theo Kloprogge et al [33] reported the microwave-assisted synthesis of FeWO4,

MnWO4, ZnWO4, CaWO4 and PbWO4 nanocrystalline minerals. According to these

authors the crystals formed are of submicrometer size and show equidimensional and

needle like crystals. Increasing the synthesis time and temperature results in the

disappearance of the needles and the growth of the equidimensional crystals. The

Raman spectra are consistent with those reported for the natural equivalents of these

tungstates.

23

Tiaotiao Dong et al [34] reported the preparation of ZnWO4 and Eu3+ doped ZnWO4

via a facile self-propagating combustion method. The photoluminescent property of

Eu3+ doped ZnWO4 indicated significant energy transfer from WO42- groups to Eu3+

and suggested an effective doping of Eu3+ into the lattice of ZnWO4. Eu3+ doped

ZnWO4 exhibited higher photocatalytic activity in the photocatalytic degradation of

RhB than pure ZnWO4 and the highest activity was observed for Eu3+ doped ZnWO4

with 4% Eu3+.

Hogjun Zhou et al [35] reported ambient template synthesis of MnWO4 by taking

aqueous solutions of MnCl2 and Na2WO4 as precursors. The formed MnWO4

nanowires lengths ranges in microns.

Larisa Gigorjeva et al [36] reported simple combustion method for the synthesis of

ZnWO4 powders with grain size in range 20 nm–10 µm. Solution of dissolved tungsten

in hydrothermal peroxide was mixed with zinc acetate solution. Then ethylene glycol

and nitric acid were added. The mixed reactants were heated for 3-4 hrs upto 2500C

until burning of gel gets started. They reported the phase pure ZnWO4 after

calcinations at 600-10000C for 2hrs. The characterization of the sample was identified

by maximum absorptions in IR spectrum. The photocatalytic activities of powders were

tested by degradation of methylene blue solution under UV light. PCA was controlled

by degradation of methylene blue solution under Hg lamp full spectra irradiation. It

was shown that PCA depends on powder grain size and is the highest for powders with

the highest effective area (SBET). The PL spectra of nanopowders were identical with

spectra of the ZnWO4 single crystal, however the decay kinetics are nanocrystal size

dependent.

24

Yakubovskaya et al [37] reported the synthesis of nanocrystalline zinc tungsate by

molten salt method in the form of elongated grains of width 30-40 nm and lengths upto

80 nm and nanorods of cadmium tungstate of width 25-30 nm and lengths upto 120 nm.

The molten salt method in LiNO3 at a low temperature was used. They showed that the

scintillation materials based on nano-sized crystalline powders of zinc tungstate have

light output comparable with single crystals and have improved kinetic characteristics

of luminescence.

Sagrario M. Montemayor et al [38] reported the synthesis and electrochemical

characteristics of lithium insertion into several tungstates of general formula MWO4

(M= Mn, Co, Ni, Cu) by aqueous salt metathesis reaction. Thermal analysis showed the

crystallization temperature to be between 400 and 5000C. The electrochemical study

has shown that these compounds are not good hosts for insertion reactions because of

the irreversibility of the process although they could be used as electrodes in primary

cells.

Jian Zhang et al [39] reported hierarchical star-like FeWO4 nanostructures by

hydrothermal process in resece of L-cysteine. The L-cysteine concentration appears to

clearly affect the morphological evolution of FeWO4 from nanowires to polyhedrons

and eventually to stars with symmetry. The FeWO4 nanostars exhibit a small

ferromagnetic ordering at low temperature, and the Neel temperature is calculated to be

63.5 K.

Miroslaw Maczka et al [40] reported two different synthesis routes for MnWO4 (i) a

hydrothermal method using ethanolamine and CTAB and (ii) by annealing a precursor

obtained by co-precipitation. In their second method, XRD patterns were indicative of

25

nanocrystalline MnWO4 formation with crystallite size smaller than 20 and 26 nm

when the synthesis was performed at 250 and 400ºC, respectively.

Kisla P. F. Siqueira et al [41] reported the microwave synthesis of nanosized transition

metal tungstates by taking metal nitrates and sodium tungstates as starting materials.

Raman spectroscopy was used to probe short-range ordering and also obtain a reliable

set of spectra containing all the Raman-active bands predicted by group-theory

calculations.

Rajagopal S et al [42] prepared FeWO4 and CoWO4 nanostructures by the

hydrothermal method using sodium tungstate, ferrous ammonium sulphate and cobalt

chloride solutions as precursors. The characterization techniques render that the

products obtained belong to the monoclinic crystal system and P2/a space group, with

average sizes of nanoparticles of about 150 nm and 70 nm in the case of FeWO4 and

CoWO4 respectively.

Jeong Ho Ryu et al [43] reported the synthesis of ZnWO4 nanocrystalline powders by

polymerized complex method using zinc nitrate and tungstic acid as starting materials.

The polymeric precursors were heat-treated at temperatures from 300 to 6000C for 3

hours. Crystallization of ZnWO4 was detected at temperatures of 6000C. The particles

heat treated at 400 and 5000C showed primarily co-mixed morphology with spherical

and silkworm-like forms, while the particles heat treated at 6000C showed more

homogeneous morphology with a narrow size distribution.

Fu-Shan Wen et al [44] reported the preparation of zinctungstate crystals with various

morphologies and particle sizes using sodium tungstate and zinc acetate as the raw

materials. Eu3+-doped ZnWO4 was also obtained from hydrothermal systems and the

26

photoluminescent properties of products were investigated. Significant energy transfer

from WO42- groups to Eu3+ ions has been observed.

Di Chen et al [45] reported the low-temperature synthesis of metal tungstate, MWO4

(M= Ca, Sr, Ba, Cd, Zn, Pb) nanocrystallites by reaction between metal chloride and

sodium tungstate in ethyl glycol at 1800C for 10 hours. PL measurements reveals that

the obtained CaWO4, CdWO4 and PbWO4 show excitonic peaks at about 430, 500 and

500 nm, respectively. The solvent and reaction conditions are important in the

formation of the products.

Sofronov. D. S et al [46] reported the synthesis of zinc and cadmium tungstates by the

reaction of metal nitrates with ammonium paratungstate by microwave irradiation.

These reactions begin at the melting point of crystallohydrates (1000C) and finish at the

complete decomposition of the mixture (400-4500C) with tungstate formation. They

reported that during the microwave activation of the interaction between cadmium

nitrate and ammonium paratungstate, cadmium tungstate with the scheelite structure is

formed. This phase does not appear during thermal heating of the same reaction

mixture.

2.3 BaSnO3 and BaZrO3:

Perovskite titanates, stannates and zirconates are industrially important ceramics

because of their highly useful technological applications. BaSnO3 is a useful

component in fabricating thermally stable capacitors and it also shows considerable

promise as gas-phase sensor for the detection of carbon monoxide and carbon dioxide.

Traditionally, BaSnO3 is synthesized by prolonged heating of stoichiometric mixture of

BaCO3 and SnO2 at 12000C. One of the important drawbacks associated with this

27

process is the co-existence of Ba2SnO4 phase which leads to non-stoichiometry

through the formation of alternate layers of perovskite BaSnO3 and rock salt BaO along

the C-direction [47]. BaSnO3 has also been prepared by other methods like

hydrothermal, coprecipitation, self heat-sustained reaction, self propagating high

temperature synthesis, wet chemical route, polymerized complex method, reverse

micelle method, molten salt synthesis, and solid-state reaction. The detailed discussion

of which are given below.

Vivekanadan et al [48] reported the hydrothermal synthesis of BaSnO3 by taking

SnO2.H2O gel and Ba(OH)2 solutions as precursors. First this leads to formation of

hydrated phase BaSn(OH)6.3H2O. On heating in air or pressure in situ ~ 2600C

BaSn(OH)6.3H2O converts into BaSnO3 fine powder which involves the formation of

intermediate oxyhydroxide BaSnO(OH)4. They reported the particle size in the range of

µm.

Wensheng Lu et al [49] reported the synthesis of BaSnO3 powder with a crystallite

size of 27.6 nm through a hydrothermal reaction of a peptised SnO2·xH2O and

Ba(OH)2 at 2500C and the following crystallization of this hydrothermal product at

3300C. The peptisation of the SnO2·xH2O gel is dependent on the pH value. The

BaSn(OH)6 phase in the as-prepared powder transforms into an amorphous phase at

2600C, from which the BaSnO3 particles nucleate and grow with an increase in

temperature.

Udawatte. C. P et al [50] reported the synthesis of cubic perovskie-type BaSnO3 under

hydrothermal conditions. Hydrothermal treatment of highly reactive co-precipitated

stannic hydroxide gel and barium hydroxide resulted to form a well-crystallized single

phase of barium stannate powder via an intermediate phase of BaSn(OH)6. The

28

reported the optimum hydrothermal conditions of 573 K for 120 s for formation of

phase pure of BaSnO3. The synthesized product had a fine microstructure, with a grain

size of 7 ~ µm. The formation of BaSnO3 strongly depends on the reaction temperature.

Young Jung Song et al [51] reported the preparation of BaSnO3 by the oxalate

coprecipitation method. The white oxalate precipitates were prepared by coprecipitation

of Ba2+ and Sn4+ using the oxalic acid as the precipitant. The oxalate precipitates were

then converted to the fine BaSnO3 powders by the calcinations at 10500C. The fine

BaSnO3 powders were cubic phase with the lattice parameter of 0.4119 nm. The

morphology reported in µm.

Shanwen Tao et al [52] reported the preparation of BaSnO3 by chemical precipitation

method by using SnCl4.5H2O and Ba(NO3)2 as precursors. The cubic BaSnO3 phase

forms at 4000C when a precipitation method is applied. Thermal analysis indicates that

the decomposition temperature of BaSn(OH)6 is below 6000C. The trace amounts of

BaCO3 in the precipitate are reported due to the reaction of barium ions with the

atmospheric CO2 in a strong basic solution. Conductance measurement between 200

and 5500C indirectly demonstrates the gas-sensing mechanism of BaSnO3 might be a

surface-controlled process. The gas-sensing properties of BaSnO3 to ethanol was

reported in this paper. The sensor made of the BaSnO3 prepared by a precipitation

method exhibits low responses to LPG, petrol, H2 and CO but high response and good

selectivity to ethanol.

Abdul-Majeed Azad etal [53] reported the synthesis of BaSnO3 via self heat-sustained

reaction technique in which they heated Ba(NO3)2 with metallic tin powder at 11000C

for 12 hours which showed the presence of Ba2SnO4 along with BaSnO3 and formation

29

of phase pure BaSnO3 was achieved only by further heating of calcinated mixture at

12000C for 24 hours.

Aguas etal [54] reported self propagating high temperature synthesis (SHS) of BaSnO3

from barium peroxide and different tin sources like Sn, SnCl2 and SnO2. According to

these investigators, the product at 950-10500C was predominatly BaSnO3 with some

SnO2 and Ba2SnO4 impurities. Annealing of SHS powders at 10000C for 2 to 72 hours

formed phase pure BaSnO3.

Cerda. J et al [55] reported synthesis of perovskite-type BaSnO3 particles by a new

simple wet chemical route based on a sol–gel process. Phase formation takes place at

temperature of 11000C for 8 hours. Their results based on XRD, Raman spectroscopy

and TEM can concluded that the precursor gel contains a small amount of BaCO3 that

decomposes in further calcinations of the precursor. BaCO3 traces are no longer seen in

X-ray diffractograms above T = 11000C and in Raman spectrum above T = 14000C,

then the formation of pure cubic perovskite-like BaSnO3 is reached.

Cerda. J et al [56] reported the perovskite-type BaSnO3 powders from a simple wet

chemical route. The resulting powders calcined at different temperatures have been

studied by XRD, Raman and TEM. The BaCO3 impurities removal has also been

discussed in function of the calcinations temperature of the material. The electrical

characterisation of BaSnO3 thick films has been performed both as a function of

temperature and as a function of the gas concentration. The resistance variations of the

BaSnO3 sensor have been measured in the presence of O2, CO and NO2. O2 shows a

maximum sensitivity at T=7000C while CO and NO2 at T=6000C. The response of

BaSnO3 sensor to NO2 reported to be very slight.

30

Chandana Preakumara Udawatte et al [57] reported the preparation of perovskite-type

BaSnO3 powders by the polymerized complex method at reduced temperature.

SnCl4.xH2O was first dissolved in ethylene glycol (EG) and anhydrous citric acid (CA)

was then added. After complete dissolution, BaCO3 powder was added and the mixture

was stirred at 800C for several hours until the solution became transparent,

demonstrating the presence of Ba2+- and Sn4+-citric acid complexes. This solution, with

molar ratio Ba/Sn/CA/EG =1:1:10:40, was heated at 1300C to produce a resin without

any precipitation via polymerization with citric acid and ethylene glycol including the

complexes. Pyrolysis of the resin at 3500C yielded a precursor of BaSnO3 and further

heat treated in air at ≥ 6000C for 2 hours. They reported the lattice parameter a0

=0.4118± 0.0003 nm and powder formed at 8000C consisted of faceted microcrystals of

nm in size.

Jahangeer Ahmed et al [58] reported the preparation of BaSnO3 nanoparticles by

reverse micelle method by taking Ba(NO3)2, SnCl4.5H2O, NH4OH solution as

precursors. After the final annealings at 6500C for 72 hours showed the particle size

were in the range of 20-40nm. X-ray diffraction and HRTEM studies confirmed the

phase purity of the prepared materials.

Ramdas. B et al [59] reported the preparation of perovskite structured stannated (Ba1-

xSrxSnO3, x=0.0-1.0) powders by molten salt synthesis (MSS) method using KOH as

the flux at lower temperature (4000C). The phase formation was confirmed by FT-IR

spectroscopy, powder X-ray diffraction and the microstructure was analysed by

scanning electron microscopy. XRD patterns reveal the formation of single phase

products for parent and substituted products with good crystallinity throughout the

range (x=0.0-1.0). The morphology of the particles of BaSnO3 is spherical.

31

Abdul-Majeed Azad et al [60] reported the synthesis of BaSnO3 by two different routes

i.e., by solid-state reaction route and self-heat-sustained (SHS) route. Ba(NO3)2

crystals and SnO2 powder were taken as precursors in solid-state reaction. The

stoichiometric amounts of the two precursors was then ball-milled for 4 hours in

isopropyl alcohol medium in airtight polystyrene bottles using clean zirconia balls as

the milling medium. The mixture was calcined first at 8000C for 8 hrs, crushed,

repelletized and fried again at 10000C for 24 hours in air. Where as in SHS route

metallic tin powder was intimately mixed with anhydrous Ba(NO3)2 crystals in a 1:1

molar ratio. The mixture was placed in platinum boat and first heated slowly to and

maintained at 2500C for 4 hrs so as to facilitate complete melting of metallic tin and its

uniform dispersion under gravitational flow in the liquid state. The temperature was

then raised gradually to 8000C and maintained for another 4 hrs. The mixture was next

calcined at 11000C for 12 hrs. XRD patterns revealed that phase pure BaSnO3 was

formed by solid-state reaction at 10000C for 24 hours. Where as in SHS method

formation of phase initiated at about 8000C and at 11000C showing peaks due to

mixture of Ba2SnO4 and BaSnO3 and complete phase formation takes place at 12000C.

Shail Upadhyay etal [61] reported synthesis of BaSnO3 by solid-state ceramic route

using BaCO3 and SnO2 by calcination at 1475K for 6 hours followed by sintering at

1525K for 6 hours. Koferstein etal [62] reported the precursor route synthesis of nano-

sized BaSnO3 from barium tin 1,2-ethane di-olato complex precursor by a rate

controlled calcination process at 8200C. The sintering behaviour is compared between

fine and coarse-grained BaSnO3 powders. They found that the fine grained BaSnO3

achieve a relative density of 90% after sintering at 1600°C for 1h and at 1500°C and a

32

soaking time of 30h, whereas coarse-grained powder compacts reach only 80% of the

relative density at 1650°C (10h).

BaZrO3 is a high melting refractory ceramic with melting point of 26000C and is

useful as an inert crucible material for sintering of super conductors [63], thermal

barrier coating for supersonic jets [64], host for UV white light emitting diode [65],

sensor applications in H2 atmosphere[66], photocatalyst [67] and as dopant in BaTiO3

matrix. Synthesis of BaZrO3 has been reported by solid-state reaction, hydrothermal,

co-precipitation, combustion, spraypyrolysis, solgel and sonication. The detailed

discussion of which is given below.

Abdul-Majeed Azad et al [68] reported the synthesis of BaZrO3 by a solid-state

reaction technique using nitrate precursors. Reactive powders consisting of submicron

particles and narrow particle size distribution were obtained by heating a 1:1 molar

mixture of barium nitrate and zirconyl nitrate at 8000C upto 8 hours. Simultaneous

thermal analysis (TG-DTA) assisted in elucidating the probable reaction pathways

leading to the formation of the target compound in the BaO-ZrO2 system. Systematic

structural and microstructural characterization on the green powders and the compacts

sintered upto 17000C were carried out. A two-stage sintering schedule consisting of a 6

hours soak at 16000C followed by slow heating upto 17000C with no dwell, led to

highly dense microstructural features.

Xiaohua et al [69] reported the preparation of BaZrO3 doped with Eu3+ by solid-state

reaction. They has shown that formation of pure cubic perovskite phase upto 4 mol%

doping concentration of Eu3+. The PL emission intensity regularly increases with

europium content upto about 2 mol%, and then decreases indicating the concentration

quenching.

33

Aimable et al [70] reported the hydrothermal synthesis of BaZrO3 in supercritical

conditions. In this work these authors has taken three barium precursors: barium

hydroxide Ba(OH2), barium acetate Ba(CH3COO)2, barium nitrate Ba(NO3)2. Two of

them [Ba(CH3COO)2, Ba(NO3)2] led to the pure perovskite phase but the synthesis

conducted from Ba(OH2) led to amorphous phase, which revealed a mixture of BaZrO3

and ZrO2 after a heat treatment of 2 hrs at 9000C under air atmosphere.

Brzezinska Miecznik. J et al [71] reported the coprecipitation technique to prepare an

intimate mixture of barium carbonate and hydrous zirconia gel. A part of barium ions

became incorporated in the zirconia gel structure. Heat treatment of the system results

in the crystallization of the BaO in ZrO2 solid solution of tetragonal symmetry. The

solid solution, when heated without contact with BaCO3, decomposes at elevated

temperatures. It results in the formation of BaZrO3 and monoclinic zirconia solid

solution. No solid solution of monoclinic symmetry appears when barium carbonate is

present in the system. In this case, the reaction of the tetragonal solid solution with

BaCO3 leads to the synthesis of BaZrO3.

Padma Kumar et al [72] reported the synthesis of BaZrO3 by single-step combustion

process of an aqueous solution containing Ba and Zr ions by using citric acid as

complexing agent and liquor ammonia as fuel. The X-ray diffraction studies have

shown that the as-prepared powder was single phase, crystalline and has a cubic

perovskite structure (ABO3) with a lattice constant a=4.19A0. The average particle size

calculated is 30 nm. They sintered nano BaZrO3 to a density of 99% of the theoretical

density at 16500C in 2 h without the use of any sintering aids. The dielectric constant

and loss factor values obtained at 10MHz for a well-sintered barium zirconate pellet

has been found to be 32.2 and 1x10-4 respectively, at room temperature.

34

Miroslaw M. Bucko et al [73] reported the preparation of BaZrO3 nanopowders by

spray pyrolysis method. This work investigated application of ultrasonic spray

pyrolysis method based on thermal decomposition of barium and zirconium nitrate

solution for preparation of fine barium zirconate powders. The aerosols of nitrate

solutions of 0.1, 0.01 and 0.001 M were thermally treated at 800, 1000 and 12000C.

The prepared powders were composed of the spherical particles with sizes, from 90 to

500nm, were reported mainly depended on the concentration of nitrate solution and less

depended on the pyrolysis temperature. Where as particles consisting of 25-60 nm in

size depended on the pyrolysis temperature and to lesser extent on the solution

concentration.

Abdul Majeed Azad et al [74] reported the synthesis of barium metazirconate (BaZrO3)

by an equimolar ball-milled mixture of barium and zirconyl nitrates. The phase

formation was reported at 8000C. The cubic perovskite structure was found to be stable

up to 17000C, the maximum sintering temperature reported by these authors.

Badica et al [75] reported the synthesis of BaZrO3 by freeze drying (FD) and

conventional mixing. Precursor powders of Ba/Zr=1:1 chloride have been prepared by

(FD) and conventional mixing. A laboratory-made apparatus for sublimation of the

frozen water from cryo-particles is presented in this paper. Both powders have the same

phase formation behaviour. Upto to 500-6000C, Zr chloride transforms into ZrO2. At

high temperatures, BaCl2 starts melting and decomposing and two subsequent

processes takes place: a) chlorination of the Zr atoms from the oxide and b) formation

of BaZrO3 phase. These processes depend on treatment atmosphere and reactivity of

the starting powders. They reported the freeze-dried powders with higher reactivity

and/or O2 atmosphere are suitable to obtain BaZrO3 phase. In freeze-dried powder,

35

decomposed in O2 atmosphere at 11900C for 20 hours, 90% of BaZrO3 phase content

was attainted. No new phases were observed during decomposition of the freeze-dried

powder. In the as-synthesized freeze-dried powder some XRD lines remained

unidentified.

Taglieri [76] reported the synthesis of BaZrO3 by citrate route method. This route

consists of complexation of metallic ions by ciric acid and ammonia in an aqueous

solution. After an accurate drying of the intial solution, the crystalline phase of barium

zirconate is reported at 7000C. The powders are characterised during the various steps

by thermal analysis, XRD, FITR, granulometric, SEM and dilatometric measurements.

Yupeng Yuan et al [77] reported the synthesis of BaZrO3 via a Pechini-type process.

Hydrogen was produced from pure water over BaZrO3 photocatalyst without the

assistance of any cocatalysts under ultraviolet light radiation and maximum apparent

quantum efficiency is upto 3.7%. High hydrogen production rate of BaZrO3 was

attributed to the highly negative potential of photoinduced electrons, 1800 Zr-O-Zr

bond angle, and large dispersion of conduction band bottom composed of Zr 4d

orbitals.

Borja- Urby et al [67] reported the BaZrO3 by a facile hydrothermal method at 1000C.

The estimated band gap in the 2.4-4.9 eV range, depending on Bi concentration ,

suggests nanocrystalline BaZrO3:Bi as a useful visible-light activated photocatalyst

under excitation wavelengths

36

2.4 BiFeO3 and LaFeO3:

Bismuth ferrate, BiFeO3 is a multiferroic that exhibits both ferroelectric and

antiferromagnetic ordering with a curie temperature Tc=1083K and Neel temperature

TN=657K respectively. The magneto-electric coupling in a multiferroic generates a

unique magneto electric effect which allows the polarisation to be tuned under external

magnetic field or magnetization to be tuned under external electric field. Consequently

BiFeO3 finds use for several applications such as non volatile information storage,

spintronic sensors, wireless sensors, digital memories, spin filters etc. BiFeO3

crystalyses in a rhombohedrally distorted perovskite structure of R3C symmetry in

which all metal ions are displaced along the (111) direction relative to the ideal

centrosymmetric position and the oxygen octahedra surrounding Fe are rotated

alternatively around the axis. The antiferromagnetic ordering remains upto Neel

temperature. Synthesis of BiFeO3 has been reported by several methods that include

solid-state, wet chemical, hydrothermal, mechano chemical, solution combustion,

microwave hydrothermal and sol-gel. The detailed discussion of which is given below.

Chandrasekhar et al [78] reported the solid-state synthesis of BiFeO3 by taking Bi2O3

and Fe2O3 as a starting precursors. They have reported that BiFeO3 ceramic crystallizes

in a rhombhohedral perovskite phase. The ferroelectric hysteresis loop measured at

room temperature demonstrates a loosy loop with unsaturated behaviour and symbolise

a partial reversal of polarisation. A dielectric constant with temperature measurement

for BiFeO3 ceramic represents an anomaly around 3500C for all frequencies and

37

intimately associated with antiferromagnetic to paramagnetic phase transition of

BiFeO3.

Sverre M. Selbach et al [79] reported the synthesis of BiFeO3 by wet chemical route as

as well as the solid-state method at 8250C. Polymeric BiFeO3 precursors were obtained

from solutions of nitrate salts and carboxylic acids with and without ethylene glycol

added as polymerization agent. The polymeric precursors were shown to decompose

above 2000C with successive nucleation and growth of BiFeO3 above 4000C. The phase

purity of the product was shown to depend on the type of carboxylic acid used, and

tartaric, malic and maleic acids resulted in nanocrystalline phase-pure BiFeO3.

Chao Chen et al [80] reported hydrothermal synthesis of perovskite bismuth ferrite.

Effects of initial KOH concentration, reaction temperature and duration time on the

phase evolution, the particle size and morphologies of BFO crystallites were studied.

The KOH concentration of 4M was benefit to refraining the formation of any impurity

phases and growing BFO crystallites into single-phase perovskites. The hydrothermal

reaction to grow BFO crystallites was described by the dissolution-crystallization

process.

Szafraniak et al [81] reported the mechanochemical synthesis of BiFeO3 by using

Bi2O3 and Fe2O3 as precursors. Their studies shows that the i) BiFeO3 powder consists

of loosely packed grains with a broad distribution of sizes between a few nm and 45 nm

ii) the grains have core/shell structure iii) grains of sizes larger than about 30 nm

exhibit well-developed crystalline structure.

38

Fruth et al [82] reported the preparation and characterization of BiFeO3 nanopowders

by solution combustion route. After annealing 3 hours at 6000C the precursor

transformed in nanopowder containing the BiFeO3 compound with parameters of the

hexagonal elementary cell a=5.577A0 and c=13.866A0 at room temperature. After

sintering dense bodies were obtained. The microstructure shows a compact structure

with grains nanosize dimension well interconnected. Their results show that sintering

of BiFeO3 via combustion route has a profound effect on densification and

microstructure. The densification kinetics during sintering is strongly enhanced by the

nature of precursor powder.

Biasotto Glenda et al [83] reported the microwave assisted hydrothermal method to

synthesize crystalline BiFeO3 at temperature of 1800C with times ranging from 5 min

to 1 hr. For comparison BFO powders were also crystallized by a soft chemistry route

in a conventional furnace at a temperature of 8500C for 4 hours. They verified that

XRD results shows the formation of perovskite BFO crystallites while infrared data

showed no traces of carbonate.

Qing-hui Jiang et al [84] reported the synthesis and properties of multiferroic BiFeO3

ceramics by sol-gel process i.e., so called Pechini method. The conventional sintering

and spark plasma sintering process has been used to fabricate BiFeO3 ceramics.

Ferroelectric and magnetic loops have been observed in the BiFeO3 ceramics at room

temperature. Dielectric constant is stable between 100 Hz and 10 MHz, and the loss

could decrease to 0.1%. Polarisation and magnetization should be improved for

practical applications.

39

Sushmita Ghosh et al [85] reported the synthesis of BiFeO3 by ferrioxalate precursor

method. Oxalic acid is used as chelating agent. The oxidation of ferrioxalate precursor

by HNO3 was accompanied by the evolution of various gases (such as CO2 , NO2 and

water vapour) and the gas evoution helped the product to obtain a fine grain structure.

Oxalic acid and nitric acid present in the solution play the key role for the synthesis of

BiFeO3 at a low temperature.

Carmen Paraschiv et al [86] reported the synthesis of nanosized BiFeO3 by a

combustion method starting from Fe(NO3)3.9H2O-Bi(NO3)3.9H2O-glycine or urea

systems. They showed that the presence into the precursor of both reducing (glycine

and urea) and oxidising (NO3-) components, modifies their thermal behaviour

comparative with the raw materials, both from the decomposition stoichiometrics and

temperature occurrence intervals points of view. Also, the thermal behaviour is

dependent on the fuel nature but practically independent with the fuel content. The fuel

nature influences also some characteristics of the resulted oxides (phase composition,

morphologies). In case of oxides prepared using urea as fuel, a faster evolution toward

a single phase composition with temperature rise is evidenced, the formation of the

BiFeO3 perovskite phase being completed in the temperature range of 500-5500C.

Andreja Gajovic et al [87] reported the preparation of BiFeO3 by a hydrothermal

process without the introduction of any metal cations, other than Fe3+ and Bi3+. They

have choosed strong organic hydroxide was used as precipitation agent. They showed

that pure phase of BiFeO3 was achieved in the case of a hydrothermal treatment after

the coprecipitation of a solution of bismuth and iron salts containing an equimolar ratio

of Fe3+ and Bi3+ ions.

40

Haibo Zhang et al [88] reported the hydrothermal synthesis and size-dependent

properties of multiferroic bismuth ferrite crystallites. Effect of initial potassium

hydroxide concentration, starting materials, reaction temperature and duration time on

the crystallinity and morphologies of BFO were reported in this paper. Results show

that the particle size of BFO increases with the increasing concentration of KOH, and a

slight decrease of the molar ratio of Bi to Fe efficiently prohibits the presence of the

secondary phase. The results of Raman measurement show that the intensity of the first

normal A1 mode peak at 137.5cm-1 decreases with the decreasing particle size. The

Neel temperature TN decreases from 378.20 to 365.80C as the particle size of BFO

powders decreases from 569 to 56 nm. The decrease in TN of BFO powders could be

related to the decrease in spontaneous polarisation and the number of antiferromagnetic

interactions with decreasing particle size. The decrease in curie temperature TC, is

attributed to the decreasing rhombohedral distortion of the unit cell with the decreasing

crystallitesize in BFO powders.

Yongming Hu et al [89] reported synthesis of mulifferoic bismuth ferrite with narrow

size distribution via wet chemical route using bismuth nitrate and iron nitrate as starting

materials and excess tartaric acid and citric acid as chelating agent respectively,

followed by thermal treatment. They showed that crystallization starts at 3500C and

completed at 5500C. They studied the ferromagnetic properties of BFO and

magnetization increased with reducing the particle size.

Yonggang Wang et al [90] reported the synthesis of various bismuth ferrite compounds

by a hydrothermal method assisted by alkali metal ions (K+, Na+ and Li+). It is

suggested that alkali metal ions (K+, Na+ and Li+) played an important role in the

41

formation of rhombohedral BiFeO3, orthorhombic Bi2Fe4O9 and cubic

Bi12(Bi12Fe0.5)O19.5 by means of changing the solubility of Bi3+ and Fe3+ hydroxides.

Miao Hongyan et al [91] reported coprecipitation/hydrothermal route to fabricate pure

phase BFO powders using FeCl3.6H2O and Bi(NO3)3.5H2O as starting materials,

ammonia as precipitant and NaOH as mineralizer. They showed the results that,

through preparing precursor by coprecipitation method, hydrothermal reaction

temperatures and concentrations of NaOH were brought down to 1800C and 0.15mol/L

respectively. The micro-morphology of synthesized BFO powders changed with

different reaction temperature and concentration of NaOH, when reaction temperature

raised from 1600C to 1800C, the micro-morphology of synthesized BFO powders

varied from lamellar structure to nano-lines. When concentration of NaOH decreased

from 0.2 mol/L to 0.15 mol/L, the synthesized BFO powders grew from lamellar

structure into double-layered plates. The Neel temperature and decomposition

temperature of the synthesized BFO powders were detected to be 3010C, 8280C and

9640C respectively.

Wanju Luo et al [92] reported the nanosized multiferroic BiFeO3 powders by a

microwave combustion method using bismuth nitrate and iron nitrate as starting

materials. They confirmed that sample crystalline into a BiFeO3 phase with high purity

when G/N ranges in 0.5-2.0. Where as no obvious combustion reaction could be

observed and the sample would not form a crystalline structure when G/N < 0.5. The

combustion became more and more violent when G/N ratio was increased, and an

impurity phase of Bi25FeO39 would increase significantly when G/N was larger than

2.0. A magnetic transition from superparamagnetic to ferromagnetic is observed

42

accompanying a structural transition from an inhomogenous phase to homogeneous

phase.

Xiaobo He et al [93] reported the synthesis of phase pure BiFeO3 in molten alkali metal

nitrates (KNO3-NaNO3) at 5000C. They reported the plate-like morphologies of

BiFeO3.

Lisnevskaya et al [94] reported the low-temperature process for bismuth orthoferrite

synthesis. The key role in reducing the synthesis temperature is played by ammonium

nitrate additions, which melt and enter into highly exothermic redox reaction with

bismuth iron oxalate. To obtain only BiFeO3 with out any contamination due to

Bi2Fe4O9, an excess of bismuth oxide is needed.

Saeid Farhadi et al [95] reported the microwave-induced solid-state decomposition of

the Bi[Fe(CN)6].5H2O precursor in presence of CuO powder as strong microwave

absorber. BiFeO3 nanopowder prepared by this method showed a weak ferromagnetic

order at room temperature and could be a promising visible-light photocatalytic

material due to a strong absorption band in the visible region. The prepared BiFeO3

confirm multiferroic nature with Neel temperature at 3710C and curie temperature at

8300C.

E.C.Aguiar et al [96] reported the low-temperature synthesis of nanosized bismuth

ferrite by the soft chemical method. They reported the pure phase formation at 8500C

for 4 hours. Typical FT-IR spectra for BFO powders revealed the formation of a

perovskite structure at high temperature due to a metal-oxygen bond while Raman

modes indicated oxygen octahedral tilts induced by structural distortion.

43

Samar Layek et al [97] reported the preparation and studies on (1-x)BiFeO3-x

Li0.5Fe2.5O4(x=0.25 and 0.5) multiferroic nanocomposities by mixing the two phases

independently by two different methods followed by annealing at 6000C. X-ray

diffraction data shows that nanoparticles are single phase in nature and crystallize in the

same structure as the bulk compound. They reported that magnetization and coercive

field increases with increasing ferromagnetic phase. The increase in magnetization is

intrinsic and not resulting from any impurity phases. The dielectric constant is

increased to high values 103-104by making composite with Li0.5Fe2.5O4.

Muneeswaran et al [98] reported the synthesis of BiFeO3 powders by coprecipitation

method. Pure phase (BFO) powder was obtained by controlling the chemical

coprecipitation process, pH level and calcinations temperature. They reported that BFO

powders had R3c crystal structure under optimised conditions. Fourier transform

infrared spectra showed that strong band corresponds to the Fe-O stretching and O-Fe-

O bending vibrations. Higher dielectric and low leakage behaviours observed for the

powders prepared at pH 10.8 confirms better phase purity.

Bellaki et al [99] reported the synthesis and magnetic properties of BiFeO3 and

Bi0.98Y0.02FeO3 samples by a solution combustion method using oxayldihydrazide

(ODH) as a fuel. The crystal structure examined by X-ray powder diffraction (XRD)

indicates that the samples were of single phase and crystallize in a rhombohedral (space

group R-3c) structure. Magnetic measurements were carried out on the resultant

powders from 300 to 0K. They showed that significant increase in magnetization was

observed for Y-doped BiFeO3.

Qiu ZC et al [100] reported the hydrothermal synthesis of perovskite bismuth ferrite

crystallites with the help of NH4Cl. Pure BiFeO3 was synthesized in a wide

44

hydrothermal condition with the help of NH4Cl at low temperature of 140 degrees.

They reported that NH4Cl played a key role in the BiFeO3 formation and BiFeO3

morphologies. Part BiFeO3 samples exhibited weak magnetic properties.

Lanthanum ferrate, LaFeO3 is a p-type semiconductor with a distorted perovskite type

crystal structure Pbnm symmetry and exhibits an unusual variety of magnetic properties

and structural changes. Being chemically stable in reducing as well as oxidising

atmosphere, LaFeO3 is useful for several applications relating to sensing properties of

toxic and noxious gases NOx and CO [101], catalytic activity for oxidation of organic

pollutants in ambient conditions [102], diesel soot oxidation [103], solid oxide fuel

cells [104], magneto hydrodynamic power generation [105] and biosensing in dendritic

form for highly selective and sensitive determination of neurotransmitted compounds

of dopamine [106] etc. Several synthesis methods for LaFeO3 in terms of solid-state,

hydrothermal, sol-gel, wet chemical, sonochemical, combustion, thermal

decomposition of precursor, hotsoap method have been reported in literature. The

detailed of which is given below.

Benedict Ita et al [107] reported three different synthesis i.e solid-state, sol-gel and

nebulized spray pyrolysis of LaFeO3. Solid state reaction involved mixing of high

purity oxides of La2O3 and Fe2O3 thoroughly to ensure homogenization and firing at

1473 K for 20 h. In sol-gel synthesis, two types of experimental procedures are

followed (i) equi-molar amounts of lanthanum nitrate and iron nitrate were dissolved in

water and ethylene glycol is added. The resulting solution was dried in a oven at 350 K

and fired at 873 K for 12 h. (ii) equimolar amounts of lanthanum nitrate and iron

acetylacetonate were mixed (1:1 molar ratio) and ethylene glycol is added. The

resulting viscous solution was dried at 350 K and fired at 873 K for 12 h. In spray

pyrolsis, equimolar amounts of lanthanum acetylacetonate and ferric acetylacetonates

45

were dissolved in methanol solvent. The mixtue is nebulised and the corresponding

mist was kept at 673 K, using air as a carrier gas. The metal acetylacetonates

decomposed and yielded fine particles of LaFeO3. The powders prepared by all the

three low-temperature routes contain nearly spherical particles with an average

diameter of 40 nm.

Thirumalairajan et al [108] reported the synthesis of LaFeO3 dendritic nanostructures

by a well controlled, surfactant-assisted facile hydrothermal process. The systematic

investigation of growth mechanism imply that preferential growth along the [121]

direction by oriented attachment of LaFeO3 nanoparticles in the diffusion limit, leading

to the formation of LaFeO3 dendrites. The possible growth mechanism of the dendritic

morphology is discussed from the aspect of diffusion and oriented attachment based on

experimental results. The electrochemical measurements performed on LaFeO3

dendritic nanostructures deposited on the surface of a glassy carbon electrode exhibit a

strong promoting effect. The oxidation current is proportional to concentration in the

linear range of 8.2 x 10-8 to 1.6 x 10-7 M with a detection limit of 62 nM at S/N=3.

Meanwhile, the sensor effectively avoids the interference of ascorbic acid and uric acid,

and it is successfuly applied to determine the dopamine formulations with high

selectivity and senssitivity.

Hui Shen et al [109] reported the sol-gel auto-combustion synthesis of nanocrystlline

LaFeO3 powder from mixed aqueous solution of lanthanum and iron nitride, using

citric acid as the fuel. They reported that LaFeO3 so prepared shows the existence of a

weak ferromagnetism and mainly anti-ferromagnetic ordering of the spins in the

calcined sample. Compared with the powder calcined at 6000C and 7000C, the as-burnt

46

LaFeO3 exhibits much stronger ferromagnetic behaviour, with larger magnetization and

smaller coercive field.

Siva Kumar et al [110] reported sonochemical synthesis of nanocrystaline LaFeO3

involving the use of lanthanum carbonate and iron pentacarbonyl as reactants. They

reported the particle size of 30 nm. Study of the magnetic properties of nanocrystalline

LaFeO3 particles shows a coercivity of ~250 Oe, while the saturation magnetization is

~ 40 memu g-1.

Faye et al [111] reported the synthesis of LaFeO3 by self-combustion method. They

showed that LaFeO3 behave as very efficient and stable catalysts for the phenol

oxidation in aqueous medium using hydrogen peroxide as oxidant in ambient

conditions. Various LaFeO3 perovskites were generated by self-combustion method

using metal nitrate precursors and glycine as ignition promoter. The glycine/NO3- (τ)

value was shown to affect the structural (crystal domain size), textural (surface area)

and reducibility characteristics of the iron based mixed oxides. The use of a τ value = 1

allowed to synthesize well-crystalline LaFeO3 nanoperovskite exhibiting high specific

surface area with high amount of reducible surface iron active species.

Zhou Kaiwen et al [112] reported the preparation of LaFeO3 by calcining precursor

La2(CO3)2(OH)2-Fe2O3. 1.5 H2O in air. XRD analysis showed that precursor dried at

800C was a mixture containing orthorhombic La2(CO3)2(OH)2 and amorphous Fe2O3.

1.5 H2O. Orthorhombic LaFeO3 with highly crystallization was obtained when

La2(CO3)2(OH)2-Fe2O3. 1.5 H2O was calcined at 9000C in air for 2 hours. Magnetic

characterization indicated that the calcined product at 9000C behaved weak magnetic

behaviour at room temperature. Their reports revealed that thermal process of

47

La2(CO3)2(OH)2-Fe2O3. 1.5 H2O experienced five steps, and the final step involves

the formation of orthorhombic LaFeO3.

Tatsuo Fujii et al [113] reported the synthesis and anomalous magnetic properties of

LaFeO3 nanoparticles by hot soap method. This synthesis was based on the thermal

decomposition of organometallic compounds precipitated in a hot coordinating solvent.

Moderate heat treatment at low temperature far below the combustion point of organic

compounds produced spherical LaFeO3 nanoparticles with average diameter of about

15nm. Inspite of the antiferromagnetic nature of bulk LaFeO3, the obtained

nanoparticles exhibited anaomalous large magnetization. Superparamagnetic behaviour

with a blocking temperature of about 30K was observed in magnetization and

Mossbauer spectroscopy analyses.

Jianbo Wang et al [114] reported the synthesis and characterization of LaFeO3 nano

particles via the PVA sol-gel method by taking La(NO3)3.6H2O and Fe(NO3)3.9H2O

as precusors.LaFeO3 nano particles with perovskite structure with crystalline

temperature 4600C have been reported in this paper. Carbonate products are found in

the reaction process at lower temperature and the formation of the nano particle is due

to the carbonate at a higher temperature.

Rajendran et al [115] reported synthesis and magnetic properties of nanocrystaline

orthoferrite powders by employing an aqueous sol-gel process. Crystalline orthoferrites

were formed at a relatively low temperature of 6500C without any phase segregation to

individual rare earth oxide and iron oxide. The perovskite phase was formed directly

from the gel precursor without any crystalline phase segregation. The resultant

nanocrystalline orthoferrites exhibited weak ferromagnetic behaviour and reduced

moments.

48

Xiangting Dong, et al [116] reported the electrospinning preparation of LaFeO3

nanofibers. PVA/[La(NO3)3+Fe(NO3)3] composite nanofibres were fabricated by

electrospinning. Polycrystalline LaFeO3 nanofibres were synthesized by calcining the

relevant composite fibres at 6000C. XRD analysis revealed that the composite fibres

were amorphous in structure. The crystal structure of LaFeO3 nanofibres was

orthorhombic system with space group Pn*a. The surface of the LaFeO3 nanofibres was

smooth, and the diameter of the composite was about 180nm.

Khetre S. M et al [117] reported the combustion synthesis of polycrystalline LaFeO3

using glycine as fuel. Synthesized material has crystallite size of 28-63 nm. They

showed that the temperature dependent resistivity reflects the semiconducting

behaviour of the material. The dielectric dispersion with frequency has been explained

on the basis of an electron-hole hopping mechanism which is responsible for

conduction and polarization. Their measurements of dielectric constant and ac

conductivity with frequency suggest that the conduction in material is similar to the

conduction in ferrites and occurs due to polaron hopping.

Pei Song Tang et al [118] reported the synthesis of nanocrystalline LaFeO3 by

precipitation method using Fe(NO3)3 and La(NO3)3 as starting materials. It was shown

that as-prepared LaFeO3 shows strong visible-light absorption with absorption onset of

532 nm, indicating a narrow optical band gap of 2.33 eV. Furthermore, the as-prepared

LaFeO3 shows high visible-light photocatalytic activity for decomposition of

methylene blue in comparsion with the Degussa P25.

Xiwei Qi et al [119] reported auto-combustion synthesis of nanocrystalline LaFeO3.

The overall process involves three steps; formation of homogeneous sol: formation of

dried gel: and combustion of the dried gel. They showed that LaFeO3 dried gel derived

49

from citrate and nitrate sol exhibits self-propagating combustion at room temperature

once it is ignited in air. After auto-combustion, the desired nanocrystalline LaFeO3 was

acquired and no further calcinations was needed. The auto-combustion was considered

as a heat-induced exothermic oxidation-reduction reaction between nitrate ions and

carboxyl group. The coercivity of the nanocrystalline LaFeO3 is 98.82 G, while

saturation magnetization is only 2.75 emu g-1.

Zhi-Xian Wei et al [120] reported the synthesis, magnetization and photocatalytic

activity of LaFeO3 and LaFe0.9Mn0.1O3-δ. They were evaluated the photocatalytic

activity toward the degradation of methyl orange under the sunlight irradiation. Their

results show that the catalytic activities of the Mn-doped in LaFeO3 i.e.,

LaFe0.9Mn0.1O3-δ were much higher than those of LaFeO3 due to higher oxygen

vacancies, variable valency Mn ions and the strong absorption in visible light, and it

has higher magnetic property than that of LaFeO3 due to the existence of Mn3+/ Fe3+

and Mn3+//Mn4+ double exchange interaction in LaFe0.9Mn0.1O3-δ. The LaFe0.9Mn0.1O3-

δ is the photocatalyst with intrinsic magnetic property and visible light activity and it is

applicable to the magnetic separation process for its higher saturated magnetization,

lower coercivity, and remanent magnetization as well as the superparamagnetic

contribution in the sample.

Ting Liu et al [121] reported the synthesis of nanocrystalline LaFeO3 powders via sol-

gel route using glucose as a complexing agent The effect of the mol ratios of glucose to

metal ions (glucose/M) on the formation of LaFeO3 was investigated. They reported

that well crystalline LaFeO3 was obtained from precursor with glucose/M = 3:5 and

3:10 at 5000C. However, for precursor with glucose/M=3:20, pure LaFeO3 was not

obtained at temperature below 6000C.

50

Wei. Z. X et al [122] reported the preparation of perovskite –type LaFeO3 and alpha-

Fe2O3 with high specific areas with stearic acid-nitrates ratios by a novel stearic acid

solution combustion method.The catalytic activities of perovskite-type LaFeO3 and

alpha Fe2O3 for the thermal decomposition of octahydro-1,3,5,7-tetranitro-1,3,5,7-

tetrazocine (HMX) were investigated by TG and TG-EGA techniques. The

experimental results show that the catalytic activity of perovskite-type LaFeO3 was

much higher than that of alpha- Fe2O3 because of higher concentration of surface-

adsorbed oxygen and hydroxyl of LaFeO3.

Keita Taniguchi et al [123] reported the self-propagating high temperature synthesis of

LaFeO3. According to their results, perovskite type oxides are regarded to be one of the

alternatives to precious metal catalysts such as Pt for the oxidation of diesel particulate

matter. Significantly, the LFO samples exhibited good catalytic activity for the

oxidation of carbon black, though their surface area was smaller than that of Pt/Al2O3.

Bai ShouLi et al [124] reported the synthesis, catalytic activity and sensing properties

of LaFeO3. They reported the gas sensing property, especially toxic and noxious gases

of NO2 and CO. They have prepared the nanocomposites of LaFeO3 and LaFe1-

xMgxO3(x=0.02, 0.04, 0.06) by various methods. The sensors based on these nano

composites have been fabricated to examine the sensing responses to gases, and the

results show that these sensors exhibited high response to both oxidising gas (NO2) and

reducing (CO), and the response was greatly enchanced by the surface modification of

MgO.

Saeid Farhadi et al [125] reported the synthesis of LaFeO3 nanoparticles by

microwave-assisted decomposition of bimetallic La[Fe(CN)6].5H2O compound in

presence of SiC as the secondary heater. The obtained LaFeO3 with nanometer size

51

give a relatively high value in specific surface area of about 36.5m2g-1. The use of this

method can also be applicable to the synthesis of other mixed oxides such as SmFeO3,

NdFeO3 and GdFeO3.

2.5 BiVO4 and LaVO4:

Bismuth vanadate, BiVO4 exists in three crystalline structures- tetragonal zircon,

monoclinic scheelite and tetragonal scheelite form of which only BiVO4 of monoclinic

scheelite structure with a band gap of 2.4 ev exhibits higher visible light photocatalytic

activity. Several synthesis methods have been reported for BiVO4 in terms of solid-

state, mechanochemical, precipitation, sol-gel, sonochemical, hydrothermal, spray

pyrolysis etc. Generally, m-BiVO4 is obtained by high temperature processes while t-

BiVO4 is prepared in aqueous media by low-temperature processes. The detailed

discussion of which is given below.

Tojo et al [126] reported the synthesis of BiVO4 by mechanochemical solid state

reaction by using (Bi2O3) and vanadium oxide (V2O5) at ambient temperature. They

reported the following solid-state reaction has taken place mechanochemically during

the milling:

Bi2O3 + V2O5 → 2BiVO4

They reported that the mechanochemical reaction ratio between Bi2O3 and V2O5 is

saturated around 90%. Further heating above 3000C is necessary for the synthesis of

homogeneous BiVO4 with an increase in reaction ratio up to about 99.4%.

52

Fuentes-Martinez et al [127] reported the mechanochemical assisted solid-state

methathesis of BiVO4 by using Bi(NO3)3.5H2O-NH4VO3-NaOH as precursors.

Because of high lattice energy of byproduct (NaNO3) drives the reactions to produce

the desired products. By proper controlling the synthesis conditions and effective use of

precursors, they prepared the monoclinic BiVO4 which is of great interest to many

applications.

Abdul Halim Abdullah et al [128] reported the synthesis of BiVO4 by precipitation

methods. Two different bismuth precursors were used. The bismuth solutions were

mixed with ammonium metavanadate solution before titrated against ammonium

hydrogen carbonate solution. The precipitate formed using bismuth nitrate was calcined

at 4500C and 3000C where as the precipitate obtained by using bismuth acetate was

calcined at 4500C under air flow for 4hrs. A more distorted monoclinic scheelite type

BiVO4 was obtained when using bismuth acetate as starting material. The surface area

of BiVO4 produced from bismuth nitrate pentahydrate was higher than that produced

from bismuth acetate. The as-prepared BiVO4 causes the photodegradation of

Methylene Blue (MB) under visible light irradiation.

Dingning Ke et al [129] reported the hydrothermal synthesis of BiVO4 by using

cetyltrimethylammonium bromide as a template-directing reagent. They studied that

hydrothermal temperature influences the morphology, crystal phase, light absorption

and photocatalytic activity of the obtained BiVO4. The microsphere BiVO4 was formed

at low temperature (≤1600C) possesses a mixed crystal consisting of tetragonal and

monoclinic phases, where as lamellar BiVO4 has monoclinic phase which is obtained at

higher temperature (2000C) showed the best photocatalytic activity for O2 evolution.

53

Scott S. Dunkle et al [130] reported the synthesis of BiVO4 by ultrasonic spray

pyrolysis method. Their photocatalytic studies reveals that BiVO4 synthesized by this

procedure is significantly more active for O2 evolution under visible-light irradiation in

AgNO3 solution.

Chunguang Li et al [131] reported the synthesis of BiVO4 by molten salt method. The

authors reported that tetragonal BiVO4 completely transforms to monoclinic phase after

heating in molten LiNO3 at 2700C for 18 hrs. The photocatalytic activity is studied by

measuring decolorization of N,N,N1,N1-tetraethylated rhodamine dye solution under

visible-light irradiation. They reported that monoclinic BiVO4 structure with moderate

surface area exhibits the highest photocatalytic activity. This indicates that distortion of

the monoclinic BiVO4 structure and modification of electron structure in low

temperature molten salt system piay important role in its highest visible-light

photocatalytic activity.

Meng Shang et al [132] reported the solvothermal synthesis of BiVO4. The as-prepared

BiVO4 exhibits visible-light photocatalytic efficiency to 20 times more than that of

products prepared by traditional solid-state reaction and nitrogen doped TiO2 (N-TiO2).

Zhang. A et al [133] reported the hydrothermal synthesis of pure phase monoclinic

BiVO4. Their results revealed that the hydrothermal conditions (16 h at 140-2400C) are

favourable for the formation of monoclinic BiVO4 powders. UV-visible diffuse

reflectance spectra (DRS) of BiVO4 indicates that samples prepared at higher

hydrothermal temperatures may consist of larger particles with less aggregation, and

thus leads to smaller band gap.

Jianqiang yu et al [134] reported the synthesis of BiVO4 by co-precipitation process.

The aqueous solutions of Bi(NO3)3 and NH4VO3 are mixed using ammonia as

54

precipitating agent. The amorphous BiVO4 formed first heat treated to various

temperatures for crystallization of BiVO4. The crystallization leads to formation of

monoclinic scheelite phase. These authors reported that crystallinity rather than surface

area is a key factor for the photodecomposition of Methylene Blue.

Xi Zhang et al [135] reported the tetragonal and monoclinic BiVO4 selectively by

aqueous process. They studied the photocatalytic activity of the two phases of BiVO4

and reported that monoclinic phase with a band gap of 2.34 eV shows higher

photocatalytic activity than tetragonal phase with a band gap of 3.11 eV.

Yang Zhou et al [136] reported the synthesis of monoclinic scheelite BiVO4 by

template-free method by using Bi(NO3)3.5H2O and NH4VO3 as precursors in presence

of glucose under the high temperature pyrolysis. The as-prepared BiVO4 showed high

photocatalytic activity, which was done by degradation of acetic acid solution under

visible-light irradiation.

Zhiqiang Wang et al [137] reported the synthesis of BiVO4 by wet chemical method.

The photocatalyst prepared by the reaction of Bi(NO3)3.5H2O and NH4VO3 at a low

temperature of 600C. They further reported that photocatalytic water splitting activity

of BiVO4 prepared under these conditions under visible light irradiation is much higher

than that of sample synthesized by solid state reaction. The activity can be improved

significantly by annealing the BiVO4 at 4000C.

Lanathanide orthovanadates, LaVO4 crystallise in two polymorphic forms namely

tetragonal (t) phase with zircon structure and monoclinic phase with monazite structure.

With increase in ionic radius there is a strong tendency for lanathanide ions to prefer

monoclinic structure due to its higher oxygen coordination number 9 compared to 8 of

tetragonal form [138]. Since La3+ has the highest ionic radius among the tripositive

55

lanthanide ions, m-LaVO4 exists as thermodynamically stable state while t-LaVO4

exists as metastable state. Consequently, of the two forms m-LaVO4 can be easily

prepared by conventional solid-state reactions [139] where as t-LaVO4 can be prepared

by soft chemical processes under specified reaction conditions [140-143]. Of the two

forms m-LaVO4 has been reported to be neither a suitable host for luminescent

activators nor a good catalyst compared to t-LaVO4. It is therefore a challenge to

synthesize the metastable zircon type t-LaVO4 using solid precursors. The details of

various synthesis procedures are discussed below.

Jilin Zhang et al [138] reported the synthesis of tetragonal LaVO4 by hydrothermal

synthesis with assistance of ethylenediaminetetraacetic acid (EDTA) as chelating agent.

In this process EDTA facilitate the formation of t-LaVO4. They further synthesized

Eu3+-doped t-LaVO4 which showed red intense emission under near UV-light

excitation.

Jie Ma et al [144] reported the selective synthesis of monoclinic and tetragonal phase of

LaVO4 via oxides-hydrothermal route using precursors V2O5 and La2O3. With the

usage of EDTA, the phase conversion from m- LaVO4 to t-LaVO4 happened, and pure

t-LaVO4 nanorods are obtainted under proper conditions. They further reported that

Eu3+ doped t-LaVO4 showed different photoluminescent when compared to Eu3+ doped

m-LaVO4 suggesting that the former is more suitable to be a host for luminescent

activatiors.

Guocong Liu et al [145] reported the hydrothermal synthesis of LaVO4: Dy3+ with

tetragonal (t-) structure at 1600C for 24 hrs. The effects due to reaction time, pH value

and annealing temperature on the morphology and size of LaVO4: Dy3+ nanoparticle

have been studied. Their results has proved that polyhedron-like LaVO4: Dy3+

56

nanocrystals exhibits improved luminescence compared to LaVO4: Dy3+ nanocrystals

with other shapes.

Chun-Jiang Jia et al [146] reported the selective synthesis of monazite and zircon type

of LaVO4 nanocrystals by hydrothermal method. They reported that chealting agents

like EDTA induces the transition from stable m-LaVO4 to metastable t-LaVO4.Their

results revealed that t-LaVO4 could be obtained by the mediation of small amount of

Na2H2L. The steric hindrance effect of EDTA which is coordinated with La3+ makes it

stabilized with fewer coordination sites, and is the reason for the formation of t-LaVO4,

which is the driving force of polymorph transformation from m- to t- LaVO4.

Weiliu Fan et al [147] reported the selective synthesis of monazite and zircon type

LaVO4: Ln (Ln = Eu, Sm and Dy) nanocrystals by a facile hydrothermal method.

Tuning the pH of the growth solution is responsible and driving force for structural

transformation and shape evolution of the lanthanide-doped LaVO4 nanocrystals.

Sung Wook Park et al [148] reported the synthesis of LaVO4 by solid state reaction.

The XRD patterns of the Li-doped LaVO4: Eu3+ powder phosphors contains a mixture

of tetragonal and monoclinic phases. The tetragonal phase of LaVO4: Eu3+ phosphor

shows highest photoluminescent intensity than the monoclinic one.

2.6 Research objectives:

A critical review of literature on ABO3 and ABO4 type mixed metal oxides indicated

that several methods have been developed for the synthesis of these compounds in

terms of solid-state and solvent based reactions. However for commercial applications,

that require large amounts of phase pure samples, several of these solution based

methods find less application either because of the use of special apparatus like

57

autoclaves or because of the prohibitive cost of precursors that precludes the technique

to be cost effective. In the traditional solid-state method, the reactants in the form of

pure oxides and/or carbonates are ground together to form a homogeneous mixture

which then will be subject to heat treatment at elevated temperatures. Since these

reactions are diffusion dependent, they require relatively high temperatures for

completion of reaction besides intermitant grinding in many cases. So far very little

work has been due to exploit solid-state metathetic reaction to obtain the mixed metal

oxides at much lower temperatures compared to traditional solid-state methods.

Another advantage of SSM reaction is that, the precursors need not necessarily be

oxides or carbonates only but in their place one can use much cheaper reactants that are

easily available and cheaper. Hence, in this study it is proposed to synthesize some

technologically useful tungstates, stannates/zirconates, ferrates and vanadates by cost

effective solid-state metathetic reactions at considerably lower temperatures. The

synthesis and characterisation of metal tungstates MWO4 ( M= Ca, Ba, Sr, Pb, Cd, Mn,

Fe, Co, Ni and Zn), BaSnO3, BaZrO3, LaFeO3, BiFeO3, BiVO4, and LaVO4 is

therefore forms the main research objective of this work.

58

References:

[1] Muller. O, Roy. R, (1974), The Major Ternary Structural Families, Springer-

Verlag, New York.

[2] Chen. W, Inagawa. Y, Omatsu. T, Tateda. M, Takeuchi. N, Usuki. Y, Opt.

Commun. 194, (2001), 401.

[3] Takai. S, Sugiura. K, Esaka. T, Mater. Res. Bull. 34, (1999), 193.

[4] Nagirnyi. V, Feldbach. E, Onsson. L. J, Kirm. M, Lushchik. A, Lushchik. Ch,

Nagornaya. L. L, Ryzhikov. V. D, Savikhiu. F, Svensson. G, Tupitsina, I. A,

Radiat. Meas, 29, (1998), 247.

[5] Esaka. T, Solid. State. Ion, 136-137, (2000), 1.

[6] Guoxin. Z, Sefei. Y, Zheng. L, Lan. Z, Wei. Z, Haiying. Z, Hua. S, Yongxian.

W, Appl. Surf. Sci., 257, (2010), 302.

[7] Shan. Z, Wang. Y, Ding. H, Huang. F, J. Mol. Catal. A: Chem. 302, (2009), 54.

[8] Cavalcante. L. S, Sczancoski. J. C, Lima. L. F, Espinosa. J, J.W.M, Pizani. P.S,

Varela. J. A, Longo. E, Cryst. Growth. Des. 9, (2009), 1002.

[9] Sulawan. K, Titipun. T, Somchai. T, J. Ceram. Process. Res., 11, (2010), 432.

[10] Zalga. A, Moravec. Z, Pinkas. J, Kareiva. A, J. Therm. Anal. Calorim., 105,

(2011), 3.

[11] Kwan. S, Kim. F, Akana. J, Yang. P, Chem. Commun, (5), (2001), 447.

[12] Kisla. P.F.S, Roberto. L. M, Marcelo. V, Anderson. D, J. Mater. Sci., 45,

(2010), 6083.

[13] Ryu. J. H, Yoon. J. W, Lim. C. S, Shim. K. B, Key. Eng. Mater., 317, (2006),

223.

[14] Yiguo. S, Liping. L, Guangshe. L, Chem. Mater, 20, (2008), 6060.

http://www.sciencedirect.com/science/article/pii/S1381116908005712http://www.sciencedirect.com/science/article/pii/S1381116908005712http://www.sciencedirect.com/science/article/pii/S1381116908005712http://www.sciencedirect.com/science/article/pii/S1381116908005712

59

[15] Thangadurai. V, Knittlmayer. C, Weppner. W, Mater. Sci. and Eng B, 106,

(2004), 228.

[16] Parhi, P, Karthik. T. N., Manivannan, V, J. Alloys and Compd., 465,

(2008), 380.

[17] Chang Sung Lim, Kyoung Hun K, Kwang Bo, Shim, J. Ceram. Process. Res.,

12, (2011), 727.

[18] Won-Chun Oh, Jong Guen Choi, Chong Yeon Park, J. Ceram. Process. Res.,

12, (2011), 435.

[19] Xiufeng. Z, Teresa. L. Y. C, Xitian. Z, Dickon. H. L. Ng, Jiaguo. Yu, J. Am.

Ceram. Soc, 89, (2006), 2960.

[20] Chang Sung Lim, J. Ceram. Process. Res., 12, (2011), 544.

[21] Sofronov. D. S, Sofronova. E. M, Starikov. V. V, Baymer. V. N, Kudin. K. A,

Matejchenko. P. V, Mamalis. A. G, Lavrynenko. S. N, Mater. Manuf.

Processes., 27, (2012), 490.

[22] Bhattacharya. A. K, Biswas. R. G, Hartridge. A, J. Mater. Sci. 32, (1997),

353.

[23] Huang. G, Zhu. Y. Mater Sci. Eng.B, 139, (2007), 201.

[24] Kuzmin. A, Purans. J, Kalendarev. R, Pailharey. D, Mathey. Y, Electrochim. Acta

46, (2001), 2233.

[25] Pandey. P. K, Bhave. N. S, Kharat. R. B, Electrochim. Acta, 51, (2006), 4659.

[26] Bhaskar Kumar. G, Sivaiah. K, Buddhudu. S, Ceram. Inter.36, (2010), 199.

[27] Montini. T, Gombac. V, Hameed. A, Felisari. L, Adami. G, Fornasiero. P,

Chem. Phys. Lett. 498, (2010),113.

[28] Angana Sen, Panchanan Pramanik, J. Eur. Ceram. Soc, 21, (2001),745.

[29] Kalinko. A, Kuzmin. A, J. Lumin. 129, (2009), 1144.

60

[30] Kalinko. A, Kotlov. A, Kuzmin. A, Pankratov. V, Anatoli. I. P, Shirmana. L,

Cent. Eur. J. Phys. 9, (2011), 432.

[31] Hongbo Fu, Chengsi Pan, Liwu Zhang, Yongfa Zhu. Mater. Res. Bull, 42,

(2007), 696.

[32] Qiang Wang, Mingming Sun, Chunhong Li. Adv. Mater. Res, 328-330 (2011),

1580.

[33] Theo Kloprogge J, Matt L. Weier, Loc V. Duong, Roy L. Frost. Mater. Chem.

and Phys, 88, (2004), 438.

[34] Tiaotiao Dong, Zhaohui Li, Zhengxin Ding, Ling Wu, Xuxu Wang, Xianzhi

Fu. Mater. Res. Bull, 43, (2008), 1694.

[35] Hongjun Zhou, Yuen Yiu, Aronson. M. C, Stanislaus S. Wong, J. Solid State

Chem. 181, (2008), 1539.

[36] Larisa Gigorjeva, Donats Millers, Janis Grabis, Dzidra Jankovica. Cent. Eur.

J. Phys. 9, (2011), 510.

[37] Yakubovskaya. A. G, Katrunov. K. A, Tupitsyna. I. A, Starzhinskiy. N. G,

Nagornaya. L. L, Zhukov. A. V, Zenya. I. M, Baumer. V. N, Vovk. O. M.

Funct. Mater.18, (2011), 446.

[38] Sagrario M. Montemayor, Antonio F. Fuentes, Ceram. Int., 30, (2004), 393.

[39] Jian Zhang, Yan Zhang, Jing-Yi Yan, Shi-Kuo Li, Hai-Sheng Wang, Fang-Zhi

Huang, Yu-Hua Shen, An-Jian Xie, J. Nanopart Res. 14, (2012), 796.

[40] Miroslaw Maczka, Maciej Ptak, Michalina Kurnatowska, Leszek Kepinski,

PawelTomaszewski, Jerzy Hanuza, J. Solid State Chem.184, (2011), 2446.

[41] Kisla P. F. Siqueira, Anderson Dias, J. Nanopart. Res.13, (2011), 5927.

[42] Rajagopal. S, Nataraj. D, Khyzhun. O. Yu, Yahia Djaoued, Robichaud. J,

Mangalaraj. D, J. Alloys and Compd. 493, (2010), 340.

61

[43] Jeong Ho Ryu, Chang Sung Lim, Keun Ho Auh, Mater. Lett. 57, (2003),

1550.

[44] Fu-Shan Wen, Xu Zhao, Hua Huo, Jie-Sheng Chen, Shu-Lin. E, Jia- Hua

Zhang. Mater. Lett. 55, (2002), 152.

[45] Di Chen, Guozhen Shen, Kaibin Tang, Huagui Zheng, Yitai Qian, Mater. Res.

Bull. 38, (2003), 1783.

[46] Sofronov. D. S, Sofronova . E. M, Starikov . V.V, Voloshko. A. Yu, Baymer

V.N, Kudin . K.A, Matejchenko P.V, Mamalis A.G, Lavrynenko. S. N, J.

Mater. Eng and Performance: DOI: 10.1007/s11665-012-0200-9, (2012).

[47] Pfaff. G, J. Eur. Ceram. Soc 12, (1993), 159.

[48] Vivekanandan, T. R. N Knutty, Mater. Res. Bull. 22, (1987) 1457.

[49] Wensheng Lu, Helmut Schmidt, J. Euro. Ceram. Soc, 25, (2005), 919.

[50] Udawatte. C. P, Yoshimura. M, Mater. Lett. 47, (2001), 7.

[51] Young Jung Song, Seungwon Kim,