Available online at www.sciencedirect.com

Solid State Sciences 10 (2008) 1056e1061www.elsevier.com/locate/ssscie

Influence of microwave energy on structural andphotoluminescent behavior of CaTiO3 powders

V.S. Marques a, L.S. Cavalcante b,c,*, J.C. Sczancoski b,c, D.P. Volanti b,c, J.W.M. Espinosa b,c,M.R. Joya d, M.R.M.C. Santos a, P.S. Pizani d, J.A. Varela b,c, E. Longo b,c

a CCN, Departamento de Quımica, Universidade Federal do Piauı, 64049-550 Teresina, PI, Brazilb LIEC, Instituto de Quımica, UNESP, P.O. Box 355, 14801-907 Araraquara, S~ao Carlos, SP, Brazil

c LIEC, Departamento de Quımica, Universidade Federal de S~ao Carlos, P.O. Box 676, 13565-905 Araraquara, S~ao Carlos, SP, Brazild Departamento de Fısica, Universidade Federal de S~ao Carlos, P.O. Box 676, 13565-905 S~ao Carlos, SP, Brazil

Received 20 September 2007; received in revised form 21 October 2007; accepted 13 November 2007

Available online 22 November 2007

Abstract

Calcium titanate, CaTiO3 powders were prepared by the polymeric precursor method and annealed at different temperatures for 2 h in a con-ventional furnace and for 30 min on an adapted microwave oven. The effect of microwave energy on structural and photoluminescent behavior ofCaTiO3 powders was investigated by means of X-ray diffraction, micro-Raman scattering and photoluminescence measurements. The results ofthe CaTiO3 powders processed in the microwave oven showed a high structural organization compared to conventional treatment.� 2007 Elsevier Masson SAS. All rights reserved.

Keywords: Microwave; Calcium titanate; Structural orderedisorder; Photoluminescence

1. Introduction

Initially, mechanochemical synthesis known as solid-statereaction has been reported for the preparation of CaTiO3

(CTO) powders. For this method, CTO powders are obtainedby mixture and reaction of TiO2 and CaCO3 or CaO at temper-atures of approximately 1350 �C [1]. However, CTO powdersobtained by this method presented several problems, such asinhomogeneity, desired stoichiometry, impurity contamina-tion, high temperatures of processing and coarser powders ofdifferent size and nonuniform distribution [2]. These problemscan be reduced by wet chemical methods, such as solegel [3],coprecipitation [4], combustion [5], organiceinorganic solu-tion [6] and polymeric precursor [7].

* Corresponding author. LIEC, Departamento de Quımica, Universidade

Federal de S~ao Carlos, P.O. Box 676, 13565-905 Araraquara, S~ao Carlos,

SP, Brazil. Tel.: þ55 16 3361 5215; fax: þ55 16 3351 8214; mob: þ55 16

9176 49 43.

E-mail address: laeciosc@bol.com.br (L.S. Cavalcante).

1293-2558/$ - see front matter � 2007 Elsevier Masson SAS. All rights reserved.

doi:10.1016/j.solidstatesciences.2007.11.004

In recent years, researches have increased for the develop-ment of new methods of materials’ processing that offermore advantages in relation to the conventional process cur-rently employed. Generally, solid-state reaction requires hightemperatures for a long time, leading to an increase in electricenergy costs [8]. A simple alternative for reduction of thesefactors can be the microwave energy. This energy is able to pro-mote a rapid heat treatment to some inorganic oxides that pres-ent susceptibility to this energy with frequency of 2.45 GHz[9]. Therefore, this technology has received great attentiondue to some important advantages, which include reduced costsof processing, better quality of production, formation of newmaterials and products, among others. The microwave energypromotes an increase in the diffusion rate, a reduction in the ac-tivation energy by polarization, an increase of the temperaturein internal regions of solids and a dielectric relaxation of thematerials [10]. To obtain CTO powders well crystallized bythe polymeric precursor method high temperatures are neces-sary for a long time (approximately 2 h) [11]. CTO powderspresent interesting photoluminescence (PL) properties at

1057V.S. Marques et al. / Solid State Sciences 10 (2008) 1056e1061

room temperature. These PL properties are probably related tothe level of structural organization, preparation method andthermal treatment conditions [12]. Thus, materials’ processingmethods more simple and rapid of materials are interesting toinvestigate the several levels of structural organization, whichare linked to PL emission at room temperature.

In this communication, we investigate the effect of micro-wave processing on structural and photoluminescent behaviorof CTO powders prepared by the polymeric precursor method.In addition, the obtained results by the microwave processingare compared with those obtained by the conventional treat-ment, using the same temperature conditions.

a

b

Fig. 1. XRD patterns of CTO powders annealed at different temperatures for

(a) 2 h in CF and for (b) 30 min in MO.

2. Experimental details

CTO powders were synthesized through the polymeric pre-cursor method [11]. Calcium carbonate (99.9% purityAldrich), titanium(IV) isopropoxide (99.9% purity Aldrich),ethylene glycol (99% purity J.T. Baker) and citric acid anhy-drous (99.5% purity Synth) were used as starting materials. Ti-tanium(IV) isopropoxide was quickly added in aqueoussolution of citric acid to avoid the hydrolysis reaction of thealkoxide with air environment. Clear and homogenous tita-nium citrate is formed under constant stirring at 80 �C for sev-eral hours. In the following step was realized the gravimetricprocedure for correction and determination of the stoichiomet-ric value correspondent to the mass of TiO2 in grams of tita-nium citrate. Calcium carbonate was dissolved in citratesolution and then added in a stoichiometric quantity of Ticitrate. After of the homogenization of the solution containingCa2þ cations, ethylene glycol was added to promote the citratepolymerization by the polyesterification reaction. The citricacid/ethylene glycol mass ratio was fixed at 60:40. Thesolution was annealed at 100 �C under constant stirring toeliminate water, occurring to the formation of a polymericresin. The obtained polymeric resin was then placed in a con-ventional furnace and heated at 350 �C for 4 h, promotinga pulverization on powders. The disordered powders obtainedwere heat treated at different temperatures for 2 h with a heat-ing rate of 1 �C/min in a conventional furnace (CF) and for30 min with a heating rate of 25 �C/min in a domestic micro-wave oven (MO), more details in the Ref. [8]. The crystallinepowders were analyzed by X-ray diffraction (XRD) patternsrecorded on a Rigaku-DMax 2500PC (Japan) with Cu Karadiation in the 2q range from 5� to 75� with 0.02�/min. Themicro-Raman scattering (MRS) measurements were performedusing a 514.5 nm line of an argon ion laser as excitation source.The power was kept at 9 mW and a 100 mm lens was used. Thespectra were recorded using a T-64000 (Jobin-Yvon) triplemonochromator coupled to a CCD detector. PL spectra of pow-ders were taken with a U-1000 Jobin-Yvon double monochro-mator coupled to a cooled GaAs photomultiplier and to theconventional system of photon counting. The 488.0 nm excita-tion wavelength of an argon ion laser was used, with thelaser’s maximum output power kept at 30 mW. A cylindricallens was used to prevent the sample from overheating.

The slit width used was 100 mm. All measurements were takenat room temperature.

3. Results and discussion

The XRD patterns of CTO powders annealed at differenttemperatures in CF and MO are displayed in Fig. 1(a) and (b).

The presence of diffraction peaks can be used to evaluatethe structural order at long-range. CTO phase was confirmedby comparing XRD patterns with the respective Joint Commit-tee on Powder Diffraction Standards card no. 42-0423. All thediffraction peaks are related with orthorhombic structure. CTOpowders annealed at 400, 450 and 500 �C for 2 h in CF pres-ents a structural disorder. This behavior is typical of disor-dered or amorphous materials (see Fig. 1(a)). However, CTOpowders annealed in the same conditions of temperatures for30 min in a microwave oven present diffraction peaks, which

1058 V.S. Marques et al. / Solid State Sciences 10 (2008) 1056e1061

are characteristics of ordered or crystalline materials (seeFig. 1(b)).

Fig. 2 illustrates the MRS spectra and direction and interac-tion of flow heat in the heating of the CTO powders annealedat different temperatures in CF and MO.

The Raman-active phonon modes for CTO powdersannealed at 400, 450 and 500 �C for 2 h in a conventional fur-nace were not detected, evidencing a structural disorder. TheRaman-active phonon modes were only observed for theCTO powders annealed at 550 and 600 �C for 2 h, confirmingthe structural order at short range (see Fig. 2(a)). All powdersannealed at different temperature for 30 min in the presence ofmicrowave energy showed a high degree of structural organi-zation, as can be seen in the Raman spectra (see Fig. 2(b)).However, some of these Raman modes are not well definedfor CTO powders annealed at 400, 450 and 500 �C. All theRaman modes are ascribed to the orthorhombic perovskitephase in agreement with the Refs. [13,14]. The vertical arrowindicates the presence of a peak in approximately 494.5 cm�1,assigned to TieO torsional (bending or internal vibration ofthe oxygen cage) modes, this value is similar to that reportedby Balachandran and Eror [15] (see Fig. 2(b)). The two hori-zontal arrows presented in the inset of Fig. 2(b) indicate thepresence of two small peaks, suggesting the simultaneouspresence of [TiO6] and [TiO5] clusters, which are reveled bymeans of X-ray absorption near-edge structure (XANES)

Fig. 2. MRS spectra of CTO powders annealed at different temperatures for (a) 2 h

Direction and interaction of heat flow in the heating of CTO powders in (c) CF a

[16]. An incomplete organization of CTO lattice designed bydefects in a bonding is caused by oxygen vacancies ðV�OÞ be-tween the clusters: ½ðTiO6eTiO5$V�OÞ.�. The formation of(TieO) bonding is observed after heat treatment at 550 �Cfor 30 min, when the CTO lattice is more ordered. The peaklocalized in approach of 641 cm�1 is assigned to the TieOsymmetric stretching vibration, suggesting the presence ofa system more organized when annealed at 600 �C for 2 h inFC and 30 min in MO.

These differences verified in Raman-active phonon modefor different treatments employed can be attributed the differ-ent energies used for the organization of the CTO powders. Inthe CF, the direction and interaction of the heating flow in theprocessing of CTO powders are presented in Fig. 2(c). Heatingprocesses in CF start from the surface to the interior of CTOpowders (see Fig. 2(c)). On the other hand, the heating pro-cesses in MO start from the interior to the surface of CTOpowders (see Fig. 2(d)). In particular, the chemical methodemployed for the synthesis of CTO powders presents residualorganic compounds arising from citric acid and ethylene gly-col. These organic compounds when pulverized generateamorphous carbon powders, that in particular absorb micro-waves (2.45 GHz frequency) most rapidly, rising the tempera-ture at 1283 �C in just 1 min in a MO operating at 1 kW [9]. Inthis experiment, MO presents the maximum power of 900 W.The phase formation occurs rapidly in a microwave processing

in CF and for (b) 30 min in MO. The zoom shows the 600e680 cm�1 region.

nd (d) MO.

1059V.S. Marques et al. / Solid State Sciences 10 (2008) 1056e1061

by the reaction of TiO2 and CaO. According to literature [17],the fast phase formation in this system is because of TiO2

absorbs partially the microwave irradiations. Moreover, CaOalso shows a poor coupling with microwave energy due thismaterial to be semitransparent. In general, few materials arecompletely transparent or absorbants [18]. Fig. 2(d) showsthat the heat flow in the heating occurs in great majority onthe interior of the CTO powders and in minor parts on thesurface. We believe that this phenomenon is responsible bythe fast heat treatment of CTO powders. It was verified inCaBi4Ti4O15 thin films crystallized in a domestic microwaveoven at 700 �C for 10 min [19].

CTO powders presented an orthorhombic structure witha space group Pbnm. The experimental values of the latticeparameters of CTO crystalline phase were a¼ 5.4188(2) A,b¼ 7.6410(7) A and c¼ 5.3701(2) A. These parameterswere calculated using the least squares refinement of the UNI-TCELL-97 program [20]. From obtention of this optimizedstructure, we built a 1� 1� 2 supercell as a periodic modelto represent the ordered CTO-o model, (see Fig. 3(a)). Thisordered model can be designed as [TiO6]e[TiO6], since eachTi atom is surrounded by six O atoms. We assumed that beforethe powder became completely crystallized, i.e., before theheat treatment reached 600 �C, the structure was composedof an aleatory mixture of TiO6 octahedra linked by Ca ions.The disordered CTO-d model is formed by the displacementof one Ti atom in the [001] direction. This displacement isthe simplest way to represent the two environments of Ti,

Fig. 3. Supercell representation 1� 1� 2 of the orthorhombic C

½TiO5$VzO� complex clusters, where Vz

O ¼ VxO; V�O and V��O

square-base pyramid, and [TiO6] octahedron. Therefore, theCTO-d structure can be designed as ½TiO5$Vz

O�e½TiO6� (seeFig. 3(b)).

This assumption was based on our XANES results ofthe crystallization process of SrTiO3 [21], PbTiO3 [22] andCaTiO3 [23], which indicated the coexistence of the two typesof environments for the titanium, namely fivefold titaniumcoordination ([TiO5] square-base pyramid) and sixfold titaniumcoordination ([TiO6] octahedron), from citrate solution to thedisordered powder, when synthesized by the polymeric precur-sor method. This slight degree of order in structurally disor-dered materials was to be expected, since two or more atomsarranged close to each other in a stable configuration mustnecessarily have some degree of order because of theexistence of a minimum potential energy. Our purpose withthis CTO-d model is to provide a simple scheme to help shedlight on the effects of structural deformation of the electronicstructure without completely suppressing the geometry of thecell. Using the same kind of distorted model, we have success-fully explained the PL of various perovskite titanates [24e26].

Fig. 4 shows the PL spectra of CTO powders annealed atdifferent temperatures in CF and MO.

The presence of structural disorder at long and short rangeis observed in CTO powders annealed at 400, 450 and 500 �Cfor 2 h in CF, which are shown in Figs. 1(a) and 2(a). In con-sequence, these powders present intense PL emission at roomtemperature (see Fig. 4(a)e(c)). So, PL is a powerful tool to

TO structure: (a) ordered CTO-o and (b) disordered CTO-d.

1060 V.S. Marques et al. / Solid State Sciences 10 (2008) 1056e1061

analyze the level of structural organization at medium range.CTO powders annealed at 400 and 450 �C for 30 min, presentan intense PL emission due to presence of a certain structuralorderedisorder degree to medium range linked to the defectscreated in the surface after heat treatment in MO (seeFig. 4(a) and (b)).

The increase of temperature creates [TiO6]e[TiO6] clustersin the lattice leading to the reduction of defects linked to½TiO5$V�O� complex clusters (see Fig. 4(c) and (d)). We believethat the same mechanism takes place on structurally disor-dered CTO powders treated in FC and MO. The defects inthe structure and surface of CTO powders can be due to theoxygen vacancies ðV�OÞ in complex clusters or twists in bond-ing between the [TiO6]e[TiO6] clusters.

The model suggests that the increase of heat treatment re-duces the disorder in CTO lattice creating electron-capturedoxygen vacancies, according to equations using KrogereVink notation [27]:

½TiO6�xþ�TiO5$Vx

O

�/½TiO6�0þ

�TiO5$V�O

�ð1Þ

½TiO6�xþ�TiO5$V�O

�/½TiO6�0þ

�TiO5$V��O

�ð2Þ

�TiO5$V��O

�þ 1

2O2/½TiO6� ð3Þ

where [TiO6]0 is donor, ½TiO5$V�O� is donoreacceptor, and½TiO5$V��O � is acceptor.

a

d e

b

Fig. 4. PL spectra of CTO powders annealed at (a) 400, (b) 450, (c) 500 and (d) 550

General PL spectra of CTO powders annealed at several temperatures for 2 h in (

The presence of [TiO6]0 with ½TiO5$V�O� complex clustersstabilize the defects via charge compensation. Therefore,½TiO5$V�O� species were associated with oxygens vacanciesðV�OÞ and ½CaO11$Vx

O�, ½CaO11$V�O�, ½CaO11$V��O � complexclusters, possibly found in this material. The increase ofthermal treatment reduces these vacancies in which an elec-tron of conduction band looses its energy and re-occupiesthe energy levels of e0/h� in the valence band [12]. InFig. 4(e) and (f) it can be observed that the different heattreatment lead to changes in photoluminescent behavior ofCTO powder. Thus, we showed that the PL emission is sen-sible to the level of organization of the system. In both cases,the heat treatment of CTO powders at 600 �C for 2 h in CFor 30 min in MO causes the extinction of PL emission atroom temperature and indicates a complete structuralorganization.

4. Conclusions

In summary, these results show the versatility and advan-tages of the microwave processing on the CTO powders.Thereby, it is possible to obtain these powders in short timeand with low energy costs in relation to the CF. In addition,the different processes employed (MO or CF) were interestingto accompany the structural evolution on the material. Photo-luminescence proved to be a sensitive probe to detect thecrystalline phase formation by the variation of structural or-deredisorder degree at medium range.

f

c

�C in FC and MO. Inset shows PL spectra of CTO powders annealed at 600 �C.

e) FC and for 30 min (f) MO.

1061V.S. Marques et al. / Solid State Sciences 10 (2008) 1056e1061

Acknowledgements

The authors thank the financial support of the Brazilianagencies CAPES, CNPq and FAPESP.

References

[1] H.F. Kay, P.C. Bailey, Acta Crystallogr. 10 (1957) 219.

[2] R. Evans, J.A.K. Howard, T. Sreckovic, M.M. Ristic, Mater. Res. Bull. 38

(2003) 1203.

[3] G. Pfaff, Chem. Mater. 6 (1994) 6.

[4] H.S. Gopalakrishnamurthy, M.S. Rao, T.R.N. Kutty, Thermochim. Acta

13 (1975) 183.

[5] M. Muthuraman, K.C. Patil, S. Senbagaraman, A.M. Umarji, Mater. Res.

Bull. 31 (1996) 1375.

[6] S.J. Lee, Y.C. Kim, J.H. Hwang, J. Ceram. Process. Res. 5 (2004) 223.

[7] Y. Pan, Q. Su, H. Xu, T. Chen, W. Ge, C. Yang, M. Wu, J. Solid State

Chem. 174 (2003) 69.

[8] D. Keyson, D.P. Volanti, L.S. Cavalcante, A.Z. Sim~oes, I.A. Souza,

J.S. Vasconcelos, J.A. Varela, E. Longo, J. Mater. Process. Technol.

189 (2007) 316.

[9] K.J. Rao, B. Vaidhyanathan, M. Ganguli, P.A. Ramakrishnan, Chem.

Mater. 11 (1999) 882.

[10] E.T. Thostenson, T.W. Chou, Compos. Part A-Appl. Sci. Manuf. 30

(1999) 1055.

[11] F.M. Pontes, C.D. Pinheiro, E. Longo, E.R. Leite, S.R. de Lazaro,

J.A. Varela, P.S. Pizani, T.M. Boschi, F. Lanciotti, Mater. Chem. Phys.

78 (2003) 227.

[12] L.S. Cavalcante, M.F.C. Gurgel, E.C. Paris, A.Z. Sim~oes, M.R. Joya,

J.A. Varela, P.S. Pizani, E. Longo, Acta Mater. 55 (2007) 6416.

[13] Y. Li, S. Qin, F. Seifert, J. Solid State Chem. 180 (2007) 824.

[14] T. Hirata, K. Ishioka, M. Kitajima, J. Solid State Chem. 124 (1996)

353.

[15] U. Balachandran, N.G. Eror, Solid State Commun. 44 (1982) 815.

[16] S. de Lazaro, J. Milanez, A.T. de Figueiredo, V.M. Longo, V.R. Mastelaro,

F.S. De Vicente, A.C. Hernandes, J.A. Varela, E. Longo, Appl. Phys. Lett.

90 (2007) 111904.

[17] J. Ma, M. Fang, P. Li, B. Zhu, X. Lu, N.T. Lau, Appl. Catal., A 159

(1997) 211.

[18] W.H. Sutton, W.E. Johnson, United States Patent 4,219,361. Available

online in: <http://www.freepatentsonline.com/4219361.html>.

[19] A.Z. Sim~oes, C.S. Riccardi, M.A. Ramırez, L.S. Cavalcante, E. Longo,

J.A. Varela, Solid State Sci. 9 (2007) 756.

[20] T.J.B. Holland, S.A.T. Redfern, Miner. Mag. 61 (1997) 65.

[21] F.M. Pontes, E. Longo, E.R. Leite, E.J.H. Lee, J.A. Varela, P.S. Pizani,

C.E.M. Campos, F. Lanciotti, C.D. Pinheiro, Mater. Chem. Phys. 77

(2003) 598.

[22] E.R. Leite, E.C. Paris, F.M. Pontes, C.A. Paskocimas, E. Longo,

F. Sensato, C.D. Pinheiro, J.A. Varela, P.S. Pizani, C.E.M. Campos,

F. Lanciotti Jr., J. Mater. Sci. 38 (2003) 1175.

[23] A.T. de Figueiredo, V.M. Longo, S. de Lazaro, V.R. Mastelaro, F.S. De

Vicente, A.C. Hernandes, M.S. Li, J.A. Varela, E. Longo, J. Lumin.

126 (2007) 403.

[24] R.C. Lima, J.W.M. Espinosa, M.F.C. Gurgel, E.C. Paris, E.R. Leite,

M.R. Joya, P.S. Pizani, J.A. Varela, E. Longo, J. Phys., Lett. 100

(2006) 034917.

[25] A. Souza, M.F.C. Gurgel, L.P.S. Santos, M.S. Goes, S. Cava,

M. Cilense, I.L.V. Rosa, C.O. Paiva-Santos, E. Longo, Chem. Phys.

322 (2006) 343.

[26] M.F.C. Gurgel, J.W.M. Espinosa, A.B. Campos, I.L.V. Rosa, M.R. Joya,

A.G. Souza, M.A. Zaghete, P.S. Pizani, E.R. Leite, J.A. Varela, E. Longo,

J. Lumin. 126 (2007) 771.

[27] F.A. Kroger, H.J. Vink, in: F. Seitz, D. Turnbull (Eds.), Solid State

Physics, third ed. Academic Press, New York, 1956, p. 307.