Journal of Non-Crystalline Solids 355 (2009) 2466–2471

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Journal of Non-Crystalline Solids

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Optical properties of zinc molybdenum phosphate glasses

M.A. Ghauri a, S.A. Siddiqi a,*, W.A. Shah b, M.G.B. Ashiq a, M. Iqbal a

a Centre for Solid State Physics, University of the Punjab, Quaid-e-Azam Campus, Lahore 54590, Pakistanb PCSIR Labs, Ferozepur Road, Lahore 54600, Pakistan

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 July 2008Received in revised form 26 July 2009Available online 5 October 2009

PACS:72.40+w78.66.Jg82.80.Gk

Keywords:Optical spectroscopyAbsorptionFTIR measurementsPhotoconductivityPhosphates

0022-3093/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.jnoncrysol.2009.08.043

* Corresponding author. Tel.: +92 42 5839387; fax:E-mail addresses: saadat@khwarizmi.org, saadat.a

qi).

Zinc molybdenum phosphate (ZnO–MoO3–P2O5) glasses of different compositions were prepared andsynthesized and some of their properties were studied. The optical band gaps have been deduced fromspectral dependence curves. The infrared absorption spectra of these investigated glasses were foundanalogous to SrO–Fe2O3–P2O5. Annealing at different temperatures does not show a prominent changein absorption band positions. We correlate our findings with a possible structural explanation of theseglasses.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction Glass technologists are taking tremendous interest in the study

The measurement of optical absorption in the ultraviolet regionand visible region gives an idea of the energy gap in crystalline andamorphous materials. The principle involved is that photons withenergy greater than the band gap energy are absorbed and lessenergetic photons are transmitted. The absorption edge at low en-ergy follows the Urbach law [1]. The band gap is estimated fromabsorption coefficient, a, by extrapolating to a = 0. Tauc [2] sug-gested that absorption edge can arise in amorphous materials frominterband transition involving the tails of localized states and thesestates fall of exponentially in the band gap. Mott and Davis [3] re-viewed the data available on chalcogenide glasses and oxideglasses containing transition metal oxides and reported the valueof Eopt in phosphate glasses, containing V2O5 as major component,is in the range 2.4–3.2 eV [3–13].

Davis and Mott [14] proposed general expression for opticalabsorption,

aðxÞ ¼ Bð�hx� EoptÞ2

�hx

where B is constant. The value of optical band gap is obtained byextrapolation of the (a�hx)1/2 versus �hx curve to (a�hx)1/2 = 0.

ll rights reserved.

+92 42 9231139.nwar@gmail.com (S.A. Siddi-

of structural properties of glassy materials using spectroscopicanalysis techniques, including the structure of phosphate glasses,especially those containing alkali metal oxides or transition metaloxides [15–23] showed the fundamental frequencies at 500 cm�1

and the second fundamental frequency lies in the range at 1030–1000 cm�1 in orthophosphate glasses. The P@O, double bond liesin the range 1285–1205 cm�1 in the polyphosphate chain. The P–O group is characterized by a frequency at 1110 cm�1 and P–O–P,ring frequency occurs at 805–740 cm�1 for cyclic phosphates[15–18]. Sayer and Mansingh [19], qualitatively discussed the posi-tions of the vibrational frequencies, proposing that the phosphatetetrahedron is dominant in the structure of Mn, Ni, Co, Cu contain-ing glasses. The evidence is supported by measurements ofcopper–calcium–phosphate glasses where P–O–Cu+ bonds are cre-ated in the glasses having a higher copper content [20]. El-Desokyet al. [21] and Wang et al. [22] found a band at 515 cm�1 whichwas attributed to the overlapping vibrations of iron–oxygenpolyhedron and P2O7 groups. The spectra of samples of 40Fe2O3–60P2O5 showed a shoulder peak at about 660 cm�1 that is due tosome iron–oxygen band [23]. However El-Desoky [24] reportedthat IR spectra of 40Fe2O3–60P2O5–xS glasses where x = 0, 2, 4, 6,8 mass% using S (sulphur) crystalline powder, are very similar tothose of sulphur free iron phosphate glasses.

Xiaoyan-et al. [25] proposed a relationship between O and P.They determined the structure of iron phosphate glass having13.5Fe2O3–86.4P2O5, content and assumed, if ratio is O/P = 2.74,

Table 1Measured properties of ZnO–MoO3–P2O5 glasses.

Specimen Composition mol% Density d ± 0.002 (g/cm3) Molar volume (cm3) O/P Mo/P Zn/P Optical band gap Eopt (eV)

P2O5 MoO3 ZnO

a 50 5 45 2.87 39.99 3.10 0.05 0.45 3.25b 50 10 40 2.89 40.80 3.20 0.10 0.40 3.19c 50 15 35 2.92 41.45 3.30 0.15 0.35 3.10d 50 20 30 2.94 42.23 3.40 0.20 0.30 3.04e 50 25 25 2.95 43.16 3.50 0.25 0.25 3.0f 50 30 20 3.00 43.48 3.60 0.30 0.20 2.92g 50 35 15 3.03 44.07 3.70 0.35 0.15 2.85h 50 40 10 3.04 44.96 3.80 0.40 0.10 2.78i 50 45 5 3.06 45.69 3.90 0.45 0.05 2.72j 50 50 – 3.08 46.07 4.00 0.50 – 2.67k 50 – 50 2.80 39.90 2.70 – 0.50 3.50

Fig. 1. X-ray diffraction pattern of described glass samples.

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then PO4 tetrahedra joined at two corners of P–O–P bond to formlong chain of PO4 group. Similarly if ratio is O/P = 3, structure ofglass corresponds to meta-phosphate. They also reported that ifthe ratio is O/P = 3.5, there is formation of pyro-phosphate glassin which two PO4 tetrahedral joined together by one P–O–P bondto form P2O7 group. El-Desoky and Shahrani [26] found five bandsrespectively at 1030 cm�1, 935 cm�1, 750 cm�1, 630 cm�1 and540 cm�1 in CaO–PbO–Fe2O3–P2O5 glasses. A band at 1030 cm�1

is attributed to symmetric stretching vibration of the (PO3)2� andband at 935 cm�1 is assigned to symmetric stretching vibrationof P–O–P linkage. The band at 750 cm�1 is presence of asymmetricvibration of linkage [19] and absorption at 630 cm�1 may be ofbending mode of PO4 units. Binary phosphate glasses can beformed over a wide range of composition of two metal oxides ofmolybdenum and tungsten. If alkali oxides are added in MoO3–P2O5 or in WO2–P2O5 glass series, especially K2O keeping the con-tent of P2O5 fixed in the range 20–40 mol% [27–29], then, thisshowed the presence of [MoO4]2� ions [30], and Damodaran andRao [31] proposed that structure of these glasses built up from cor-ner sharing [MoO6/2] octahedral and [POO3/2] tetrahedral units.MuthuPari and Rao [32] analyzed the results obtained by infraredspectroscopy, in case of MO3–P2O5 and Na2O–MO3–P2O5 (M = Moor W) glass composition, on the basis of a model of corner sharing[MoO6/2] or [WO6/2] octahedral. The structural analysis of lead

phosphate glasses by chemical durability method also proposesthat structure of these glasses is dependent to corner-sharing ofPO4 group. Addition of Fe2O3 to a phosphate glass P–O–P bondsare replaced by more chemically durable P–O–Fe(II) or P–O–Fe(III)bonds [33].

In the present investigation, a series of glasses a–k, as mentionedin Table 1, are prepared with the variation of ZnO content in theMoO3–P2O5 matrix, xP2O5 + yMoO3 + [1�(x + y)] ZnO, keeping thevalue of xmol% P2O5 constant. ZnO were added in the compositionbecause it is a good photoconductor and is a wide band gap material[34,35]. The optical band gap of the whole series is evaluated fromthe famous equation of Davis and Mott [14]. For elucidation of thestructural evolution of the newly prepared glasses, as a function ofcomposition, other properties, such as density, molar volume andchemical durability, are investigated. The structural changes werestudied with the help of FT-IR spectroscopy.

2. Experimental work

2.1. Glass preparation

Reagent grade MoO3 (99.9%), ZnO (99.9%), P2O5 (99.9%) wereused as raw materials. The P2O5 percentage was kept constant at50 mol% in all samples a–k, as described in Table 1, while the per-

Fig. 2. Distilled water durability of described glasses as a function of ZnO contentafter immersion at room temperature for 24 h. The error of weight loss is estimatedat ±10%.

Fig. 3. Water durability of the described glasses containing 20% and 40% ZnO asfunction of immersion time19 days at room temperature.

Fig. 4. Optical absorption edges of the described glass samples.

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centage of ZnO and MoO3 was changed between 0 mol% and50 mol% after weighing these oxides compounds in an aluminacrucible in a batch of 35 g wt. The materials were mixed with thehelp of alumina rod. The crucible was placed initially at 450 �Cfor 1/2 h to minimize the evaporation of P2O5 and then was trans-ferred to an electric muffle furnace at 1050 �C for 1 h. Each meltwas poured on a clean thick stainless-steel mold, hot at 200 �Cimmediately quenched by pressing with another stainless-steel so-lid cylinder. The casts were in the form of discs of size 2 � 2 cm2

with thickness 3 mm. These bulk glass discs were annealed at dif-ferent temperatures by varying the time period from 1 h to 3 h. Therough surface of glass samples were removed by using 200 gradesilicon carbide powders on a rotating wheel. Finally for fine polish-ing, Al2O3 powder of 1.0 lm was used. Each glass, as is evidentfrom Fig. 1 was X-ray amorphous.

2.2. Density measurement

The density of each annealed specimen of xP2O5 + y-MoO3 + [1�(x + y)] ZnO was measured at room temperature by

Archimedes method using toluene as immersion liquid. The calcu-lated densities of different compositions as well as molar volumesare described in Table 1.

2.3. Chemical durability

The chemical durability of polished bulk glasses of circular shapewere evaluated from the weight loss/cm2 in distilled water (pH 6.9)at room temperature. The dimensions of circular disks of thickness3 mm were measured before and after suspension in Pyrex beakers,by a nylon thread, containing 100 ml distilled water. Each samplewas washed and dried in an oven at 100 �C for one hour beforesuspension. Loss in weight versus time or days is shown in Figs. 2and 3. The error in weight loss is estimated at ±10%.

2.4. Optical measurements

The absorption edges of all glasses are also listed in Table 1. Thethin film of glass samples, were prepared by blowing, using alu-mina tube rod, for absorption edges. The thickness of the blownsamples was in the range 150–350 lm. A Labomed UV spectrome-ter, UVD-3500, was used for measurements, in the range 200–900 nm: all measurements were carried out at room temperature.The optical band gap was estimated by plotting the quantity(a�hhx)1/2 as a function of �hx as proposed by Davis and Mott[14] for forbidden indirect transition. A selection of typical resultsis shown in Fig. 4.

Table 2Infrared absorption band position of ZnO–MoO3–P2O5 glasses.

Specimen Infrared absorption band position (cm�1)

a 500 – 760 – 927 – 1060 – 1252 – – – – 2365b 500 – 760 – 927 – 1060 – 1263 – – – 1654 2365c 500 – 760 – 928 – 1060 – 1263 – – – 1654 2365d 500 – 760 – 933 – 1060 – 1261 – – – 1654 2361e 500 – 765 – 933 – 1056 – 1264 – – – 1654 2362f 500 – 760 – 933 – 1059 – 1270 – – – 1653 2362g 500 – 755 – 934 – 1057 – 1283 – – – 1654 2360h 500 – – – 932 – 1050 – 1283 – – – 1653 2362i 500 – – – 935 – 1060 – 1284 – – – 1656 –j 500 – – – 935 – 1067 – 1283 – – – 1654 –k 500 724 – 894 – – – – – – 1375 1450 1654 –Z1 MoO3 – – 850 – 950 984 – 1068 – – – – – –Z2 ZnO 500 – 784 – – – – – 1260 1350 – 1484 – –

Fig. 5. Infrared absorption spectra for a series of (MoO3–ZnO–P2O5) glasses.

M.A. Ghauri et al. / Journal of Non-Crystalline Solids 355 (2009) 2466–2471 2469

2.5. Measurement of infrared spectra

The IR spectra and data of all glasses composition as given inTable 2, were measured from 500 cm�1 to 4000 cm�1, (using aconventional KBr laboratory reagent, supplied by BDH Chemical

Ltd., England) by pellet method on an RXI Perkin–Elmer, FT-IRspectrometer. Each pellet of thickness one millimeter, transparentto light, was prepared by mixing about 5 mg of glass with 150 mgof anhydrous KBr. The pellets were formed by pressing the admix-ture at 10 tons for few minutes under vacuum. The IR spectra forall compositions are shown in Figs. 5–7.

3. Results

The X-ray diffraction patterns of samples a–k, as given in Fig. 1,show one broad maxima starting around 2h�20�. The chemicaldurability of glass samples was evaluated from the weight lossesobserved for 1, 4, 7, 10, 13, 16 and 19 days. The weight loss/cm2

observed at room temperature for 24 h of all the compositions a–k is shown in Fig. 2. The samples containing 20% ZnO and 40%ZnO, sample f and b, respectively, were also evaluated for waterdurability for 19 days as given in Fig. 3. The energy band gap is esti-mated from the extrapolation of (a�hx)1/2 versus �hx as in Fig. 4 forall the compositions.

The infrared absorption spectra of all the glasses of composi-tions ZnO–MoO3–P2O5 along with spectra of crystalline ZnO,MoO3–P2O5 are described in Figs. 5 and 6. The infrared spectra ofglass composition 20%ZnO–30%MoO3–50%P2O5, sample f, annealedat different temperatures were also recorded but no significantchange in absorption band position was observed except the peaksas shown in Fig. 7 are broadened with the increase of temperature.

4. Discussion

The diffraction patterns as shown in Fig. 1 depict lack of latticeperiodicity which is the characteristic of the amorphous materials.The pattern obtained is analogous to the reported structure ofLi2O–MoO3–P2O5 glasses [36]. The density for sample j having com-position 50MoO3–50P2O5, is 3.08 g/cm3 which is slightly greaterthan the value as reported by Hckmat et al. [37] that may be dueto less heating time. Table 1 indicates that density decreases linearlywith the increase of mol% of ZnO which leads to thinking that O–P–Obond is breaking resulting in a decrease in the rigidity in the struc-ture [38]. The chemical durability is highly dependent on glass com-position. The glasses containing less than 20% ZnO are more durable.

The extrapolation of density verses mol% ZnO gives the value3.08 for 50MoO3–50P2O5 which is slightly The curves of (ax)1/2

versus �hx, as shown in Fig. 4, lead to acceptable value of Eopt byextrapolation. When (a�hx)1/2 = 0 the curve at lower values dis-plays deviation from linearity. The tails to the curves are morenearly exponential in energy. Redfield and Afromowitz [39] sug-gest that this region of absorption edge is related to imperfectionsin the material.

Fig. 6. Infrared absorption spectra of crystalline (Z1) MoO3, (Z2) ZnO and (k)50%ZnO–50%P2O5 glasses.

Fig. 7. Infrared absorption spectra of 20%ZnO–30%MoO3–50%P2O5 glass annealed at(f1) 100 �C, (f2) 200 �C, (f3) 300 �C and (f4) 400 �C.

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The positions of the absorption bands obtained in IR spectrameasurements are described in Table 2. The comparison of theposition of absorption bands of crystalline ZnO, and MoO3 are closeto the absorption spectra described by Nyquist and Kagel [40]. Theabsorption band at 500 cm�1, which appears in all glasses,irrespective of content of the composition in glasses, is known tobe the fundamental frequency of the [PO�3 ]2� group creating O–P–O bending. The fundamental frequency signifies the formationof meta-phosphate glasses [40,41]. Similar absorption band wasfound at 540 cm�1 which is assigned to harmonics of bendingvibration of O–P–O linkage in CaO–PbO–Fe2O3–P2O5 glasses [26].The band at 755–765 cm�1 is the P–O–P stretching frequency inring system due to the presence of MoO2�

4 ions, or in ZnMoO4, (zincmolybdate) [40]. The bonds are broken, non-bridging oxygen ap-pears, close to the neighboring Mo-cation which enhances the pathway suitable for migration of Mo-ions [40].

Within the range of ±10 cm�1, the intensity band at 927–935 cm�1 could be assigned to the (PO4)3� group, forming zinc-orthophosphate [40] or Mo–O stretching frequency associatedwith the structural unit [MoO4/2O2]2, A Similar peak around930 cm�1 is due to Mo–O stretching of [MoO6/2] unit. Also the cat-ion-oxygen attraction of P–O–Mo or P–O–Zn has been observed at

peak range 900–960 cm�1 as in P–O–Cu+ glasses [42]. The band at1050–1067 gives the presence of P–O, ionic stretching frequency of[POO3/2] units as in case of Na2O–MoO3–P2O5 glasses [15–23]. Thelast band existing at 1252–1283 cm�1 is due to the presence ofP@O vibration which is similar to MoO3–In2O3–P2O5 glasses [43].

The 724 cm�1 and 894 cm�1 frequency in 50 mol% ZnO–50 mol% P2O5 when compared, it is evident that these peaks belongto Zn4 (P2O7), pyrophosphate group [44]. Similarly the peaks at1654 cm�1 for the same composition is due to the presence ofzinc-orthophosphate, Zn (PO4)2 groups [44] as well as in all otherglasses containing MoO3. The bands around 2344–2365 cm�1 wereattributed to OH-stretching of strongly H-bonded as in cobaltphospho-molybdates [45], which is similar to Zn4 (P2O7) and seemto be related to small amount of water absorbed during recordingof IR-spectra or possibly overtones.

The estimation of Mo/P ratio is increasing whereas Zn/P ratio isdecreasing which is the indicative of the fact that Zn(PO3)2 isbreaking and MoO2 (PO3)2 units are forming [33,44–46]. The O/Pratio, in the present ZnO–MoO3–P2O5 glass system is changingfrom 3.10 to 4.0 which mean phosphate structure is changing frompyrophosphate to orthophosphate units similar to the system as inMoO4–Fe2O3–P2O5, and SrO–Fe2O3–P2O5 glasses [46–48] or pyro-phosphate (P2O7)4� units are breaking which causes an increasein non-bridging oxygen ions [33,47] into meta-phosphate group.It means the mixed structure arises.

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5. Conclusions

ZnO component was added in MoO3–P2O5 glass matrix. X-raydiffraction of the ternary glass structure shows amorphous in nat-ure. Glasses less than 20% ZnO are durable. (a�hx)1/2 verses �hx givethe most satisfactory values of the optical band gap as proposed byDavis and Mott [14]. The fundamental frequency of (PO4)3� groupis maintained in the glasses. The behavior may be quantitativelysummarized by suggesting the structure of mixed form of phos-phate of MoO2�

4 as well as Zn tetrahedral [49].

Acknowledgement

The authors are grateful to Dr. P.I. Qazi, Head Glass and Ceram-ics Division, PCSIR Labs, Lahore, for his help in the preparation ofglass samples by providing the laboratory facilities.

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