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Acta Materialia 57 (2009) 5174–5185

Structural and optical properties of CaTiO3 perovskite-basedmaterials obtained by microwave-assisted hydrothermal synthesis:

An experimental and theoretical insight

Mario L. Moreira a, Elaine C. Paris a, Gabriela S. do Nascimento a, Valeria M. Longo e,Julio R. Sambrano b, Valmor R. Mastelaro c, Maria I.B. Bernardi c, Juan Andres d,

Jose A. Varela e, Elson Longo e,*

a LIEC, Departamento de Quımica, UFSCar, Rod Washington Luiz, km 235, PO Box 676, CEP 13565-905 Sao Carlos, SP, Brazilb Grupo de Modelagem e Simulac�ao Molecular, DM, UNESP, PO Box 473, 17033-360 Bauru, SP, Brazil

c Instituto de Fısica de Sao Carlos, USP, PO Box 369, 13560-970 Sao Carlos, SP, Brazild Departamento de Quımica Fısica y Analıtica, Universitat Jaume I, 12071 Castello, Spain

e LIEC, Instituto de Quımica, UNESP, PO Box 355, 14801-907 Araraquara, SP, Brazil

Received 27 February 2009; received in revised form 8 July 2009; accepted 13 July 2009Available online 21 August 2009

Abstract

CaTiO3 powders were synthesized using both a polymeric precursor method (CTref) and a microwave-assisted hydrothermal(CTHTMW) method in order to compare the chemical and physical properties of the perovskite-based material as a function of the syn-thesis method. To this end, X-ray diffraction, Raman spectroscopy, inductively coupled plasma atomic emission spectroscopy and exper-imental Ti and Ca K-edge X-ray absorption near-edge structure spectroscopy, as well as measurements of photoluminescence (PL)emission, were used to characterize the typical bottom-up process of the CaTiO3 perovskite phase at different times. Detailed Rietveldrefinements show a random polycrystalline distortion in the powder structure, which can be associated with the tilting (a angle < O–Ti–O) between adjacent TiO6 octahedra (intermediate range) for CTHTMW samples and an intrinsic TiO6 distortion (short range) in relationto the polymeric precursor CTref sample. These properties were further investigated by first-principles calculations based on the densityfunctional theory at the B3LYP level. The relationship between this tilting on the PL profile is highlighted and discussed. Thus, a struc-tural model derived from both experimental results and theoretical simulations reveals a close relationship between this tilting and thepresence of intermediate energy states within the band gap which are mainly responsible for PL emissions.� 2009 Published by Elsevier Ltd. on behalf of Acta Materialia Inc.

Keywords: CaTiO3; Microwave-assisted hydrothermal; Rietveld; Photoluminescence; Ab initio calculation

1. Introduction

Order–disorder effects are the keys to many unsolvedstructural problems and unexplained structure-related prop-erties in solid materials. In particular, structural order–dis-order is always present in real materials and may play animportant role in technological applications by altering their

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* Corresponding author. Tel.: +55 16 3361 5215; fax: +55 16 3351 8350.E-mail address: [email protected] (E. Longo).

electronic and optical properties. Therefore, physical princi-ples that govern the structural state of a given perovskite andhow that state may change have long been the subject ofinvestigation and debate. Compounds with the perovskitestructure, ABO3, and its derivatives are perhaps the mostwidely investigated owing to their significance to both fun-damental research and the high potential for technologicalapplications because of their diverse physical properties[1–5]. Their electronic structures and photophysical proper-ties have been the goal of much research; in particular,

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M.L. Moreira et al. / Acta Materialia 57 (2009) 5174–5185 5175

several efforts have been devoted to studying the photolumi-nescence (PL) emission of titanates such as ATiO3 (A = Ca,Sr, Ba, Pb) [6–12].

The PL emission in perovskite materials is considered tobe closely related to the crystal structure and their corre-sponding distorted metal–oxygen polyhedra. In particular,TiO6 octahedra in this family exhibit marked structuralflexibility. Perhaps the most striking manifestation of suchflexibility is the fact that under ambient conditions manytitanates present structures with lower symmetries thatcan indeed be derived from the cubic aristotype structure(Pm3m space group) through the rotation or tilting of reg-ular, rigid octahedra or due to the presence of distortedTiO6 octahedra [13]. It is significant that although thestructure is distorted, the rotation/tilting does not disruptthe corner-sharing connectivity.

The tilting of the octahedral framework also plays a majorrole in determining the properties of oxide perovskites.Cooperative rotations can give rise to as many as Glazer tiltsystems [13–16]. In the seminal studies of Glazer [13] and,more recently, those of both Woodward [17,18], andHoward and Stokes [16], the changes in symmetry resultingfrom different types of octahedral tilting have been systema-tized and rationalized. Glazer’s notation specifies the magni-tude and phase of the rotation of the octahedra about thethree orthogonal axes of the aristotype cubic unit cell. Forexample, in the CaTiO3 (CT) the notation is a�b+a�, whichimplies unequal tilts about the x, y, and z axes. The positivesuperscript indicates neighboring octahedra tilt in the samedirection (in phase) while a negative superscript indicatesthe octahedra tilt in opposite directions (out of phase).Although the precise mechanisms of these transformationsremain elusive, both experimental and theoretical studieshave shed light on these interesting phenomena.

The present work focuses on links between the structureand the photoluminescent behavior of titanates with theparent mineral CaTiO3 (CT) selected as a case study. Inthe framework of a more extended project [19,20,21] aimedat the design and synthesis of perovskite-based materialswith PL properties. We investigate the role of microwaveprocessing on the structural and photoluminescent behav-ior of CT powders. The main objective is to investigatethe impact of structural order–disorder on the PL emissionassociated to octahedral tilting. This research involves fourcritical steps: (1) synthesis of powders; (2) structural disor-der characterization to elucidate their relationship with theparent ordered samples; (3) establishing the formationmechanisms of the disorder; and more importantly, (4)revealing the relationships between structural disorderand the PL property. The purpose is to join both the expe-rimental and theoretical results to explain the differentresponses of PL emission at room temperature by using astructural disorder motif. To this end, we have used differ-ent experimental techniques and first-principle calculationsbased on the density functional theory (DFT) to carry outa comprehensive investigation of the corresponding struc-tures and electronic properties.

2. Experimental and theoretical procedures

2.1. Sample preparation

Perovskite-based materials can be synthesized by aplethora of preparation methods. Mao et al. [22] describea number of advances that have been made in the synthesisof various ABO3-type oxides. The most popular techniquesinclude the traditional solid state reaction method [23–26],formation through a soft solution chemistry methodologysuch as a co-precipitation method [27], a hydrothermalmethod [28,29], a solverthermal method [30], alkoxidehydrolysis [31], metal–organic processing [32] and Pechini[33].

Hydrothermal media provide an effective reaction envi-ronment for the synthesis of numerous ceramic materialsbecause of the combined effects of solvent, temperature,and pressure on ionic reaction equilibrium. The conven-tional hydrothermal (CH) method has become an effectivesynthetic route for the materials science by dramaticallyincreasing control of the micro/nanometric morphologyand orientation [34]. In addition, this method is environ-mentally friendly and depends on the solubility of thechemical salts in water under temperature and pressureconditions. The key factors in this method are the vaporpressure and solubility of the chemical salts in water [35].In contrast to the CH method which requires a long time(typically half to several days) and high electric power (overa thousand watts) [36], microwave-assisted heating is agreener approach to synthesize materials in a shorter time(several minutes to hours) and with lower energy consump-tion (hundreds of watts). Consequently, the microwave-assisted route is a rapidly developing area of research[37–40] which has been demonstrated to be efficient in theprocessing of many oxides [41,42]. Recently, the impactof microwave frequency on the hydrothermal synthesis ofnanocrystalline tetragonal BaTiO3 has been analyzed bySuib et al. [39], indicating that the HTMW process canbe a potential and breakthrough way for the synthesis ofcrystalline CT powders at low temperatures and high heat-ing rates.

The condition of Ca/Ti = 1 corresponds to the composi-tion of the desired CaTiO3 product which was synthesizedusing TiCl4 (99.99%, Aldrich), CaCl2�2H2O (99.9%,Merck) and KOH (99%, Merck). Three solutions were pre-pared: in the first solution 0.05 mol of the TiCl4 was slowlyadded to 125 ml of de-ionized water at approximately 0 �Cunder vigorous stirring, forming TiO(OH)2 + H+ + Cl�.Similarly, 0.05 mol of CaCl2�2H2O was dissolved in thede-ionized water. Then, two precursor solutions containingTi4+ and Ca2+ ions were mixed, homogenized and sharedin five portions of 50 ml, to which 50 ml of the KOH solu-tion (6 M) was added to act as a mineralizer [35] and bringto pH = 14.

The suspension was loaded into a 110 ml Polytetraflu-oroethene autoclave reaching 90% of its volume, providingthe maximum pressure efficiency to the system [43]. The

5176 M.L. Moreira et al. / Acta Materialia 57 (2009) 5174–5185

autoclave was sealed and placed into a microwave-assistedhydrothermal system using 2.45 GHz microwave radiation(Support information S1) with a maximum output powerof 800 W [44]. The reaction mixture was heated to 140 �Cin less than 1 min (at 800 W) by a direct interaction ofwater with microwave radiation and was kept at this tem-perature for 10 (CT10), 20 (CT20), 40 (CT40), 80 (CT80)and 160 (CT160) min under pressure of 2.5 bar. Afterwardthe autoclave was naturally cooled to room temperature.Then the solid product was washed with de-ionized wateruntil a neutral pH was reached and then dried at 80 �Cfor 12 h. The CaTiO3 reference sample (CTref) was pre-pared by the polymeric precursor method [45,46] andheat-treated at 700 �C for 2 h in a conventional furnaceunder an air atmosphere and then naturally cooled to roomtemperature. This sample did not present photolumines-cence properties, and therefore the sample was used as areference sample (CTref).

2.2. Characterization techniques

Structural techniques were employed to provide infor-mation on different time and length scales. Each level hasits particular complexity, and the results therefore offerstructural insight into those length scales that determinemany important aspects of material phenomenology andtheir properties. Also, it is becoming increasingly clear thata detailed microscopic structural understanding of materi-als is necessary. Thus it is mandatory to use different tech-niques such as X-ray diffraction, which enables an averagedepiction of the structure (through pattern matching andRietveld analysis). Moreover, there is a wide range ofexperimental techniques that can be considered as comple-mentary tools to characterize specific properties of thematerials. Both Raman and UV spectroscopies are amongthe advanced nondestructive characterization tools used toacquire information on the estimate of the structural orderat short and medium ranges of a material. Therefore, theycan be used to estimate the crystal potential fluctuationsand local atomic arrangement. In addition, currently it iswell recognized that optical properties like PL depend onboth structural and electronic properties, including compo-sitional ordering and the presence of impurities and defects.PL depends on electronic excitations and thus is a neces-sary complement to spectroscopies concerning lattice exci-tations, yielding structural information of a completelydifferent character from the information obtained by dif-fraction-based techniques. The latter detected long-rangeorder while the former yielded information on the immedi-ate surroundings of an ion (particularly useful as a struc-tural probe in short-range amorphous structures).

X-ray absorption near-edge structure (XANES) spectros-copy provides information about the local order aroundcations in oxide materials [47–50]. Ti K-edge XANES exper-imental results of photoluminescent titanates revealed thecoexistence of two types of environment of titanium atoms,namely fivefold coordinated TiO5 (square-base pyramid)

and sixfold coordinated TiO6 (octahedron), before reachingthe complete structural order [47,48]. Based on the Ca K-edge L3,2 XANES spectra, Asokan et al. [49] reportedchanges in the local order around Ca atoms.

Powder X-ray diffraction (XRD) experiments were car-ried out on the as-prepared samples, and the correspondingmeasurements were obtained by Rigaku DMax 2500PCusing Cu Ka1 (k = 1.5406 A) and Cu Ka2 (k = 1. 54434 A)radiation. Data were collected from 20� to 120� in 2h rangewith an 0.5� divergence slit and an 0.3 mm receiving slitusing a fixed-time mode with a 0.02� step size and 1s/point.The Rietveld refinements [51] were carried out with GSASsoftware [52], which is specially designed to simultaneouslyrefine both structural and microstructural parameters usingthe least-square method. The peak profile function wasmodeled using the convolution of the Thompson–Cox–Hastings pseudo-Voigt (pV-TCH) with the asymmetryfunction described by Finger et al. [53]. The backgroundof each pattern was fitted by a polynomial function.Raman spectra were recorded on a RFS/100/S BrukerFourier transform Raman (FT-Raman) spectrometer witha Nd:YAG laser providing an excitation light at 1064 nmin a spectral resolution of 4 cm�1.

An inductively coupled plasma atomic emission spec-trometer, ICP-AES Simultaneous CCD–VISTA–MPX(Varian), with radial configuration, was used for chemicalanalysis. The dissolution procedure was carried out using10 ml of HCl (37% purity) and 3 ml of HNO3 (68% m/m)in closed vessels for 2 h at room temperature. Analyticalblanks were prepared following the same acid digestionprocedure. Background signal correction was carried outby the operating software of the instruments. The genera-tor frequency was 40 MHz, with an RF power of 100 kWand a plasma gas flow rate of 15 l min�1. Emission bandsat 396.8 and 366.1 nm were used to quantify calcium andtitanium content, respectively. Microstructural character-ization was performed by field emission scanning electronmicroscopy (FE-SEM, Zeiss SupraTM 35).

Titanium and calcium K-edge X-ray absorption spectrawere collected at the LNLS (National Synchrotron LightLaboratory) facility using the D04B-XAS1 beam line.The LNLS storage ring was operated at 1.36 GeV and160 mA. XANES spectra of grounded samples were col-lected at the Ti K-edge (4966 eV) and Ca K-edge(4205 eV) in the transmission mode at room temperatureusing a Si(1 1 1) channel-cut monochromator. XANESspectra were recorded between 4910 and 5100 eV for Tiand 4000 and 4200 eV for the Ca K-edge using energy stepsof 0.3 eV. For comparison purposes of the different sam-ples, all spectra were background removed and normalizedusing as unity the first EXAFS (extended X-ray absorptionfine structure) oscillation.

UV–visible absorption was recorded using the Cary 5Gspectrometer in total reflection mode by the integrationsphere. Photoluminescence (PL) spectra were collected witha Thermal Jarrel-Ash Monospec 27 monochromator and aHamamatsu R446 photomultiplier. The 350.7 nm exciting

Table 1Internal and lattice parameters used for theoretical calculations.

CTref CT10 CT160

Lattice parameters

a (A) 5.387 5.406 5.405b (A) 5.439 5.492 5.489c (A) 7.646 7.664 7.662

Internal parameters

Ti 0000, 0500, 0000 0000, 0500, 0000 0000, 0500, 0000Ca 0992, 0033, 0250 0989, 0044, 0250 0991, 0043, 0250O1 0072, 0489, 0250 0085, 0484, 0250 0086, 0479, 0250O2 0717, 0284, 0034 0706, 0292, 0037 0708, 0290, 0036

CTref = reference sample, prepared by the polymeric precursor method.


wavelength of a krypton ion laser (Coherent Innova) wasused, with the nominal output power of the laser kept at200 mW. All measurements were taken at room temperature.

2.3. Computational method and periodic model

The simulation was performed using a periodic approxi-mation as implemented in the CRYSTAL06 computer code[54]. Our computational method is the density functionaltheory in conjunction with Backe’s three parameter hybridnonlocal exchange functional [55], combined with the Lee–

Fig. 1. Different values of a angles for (<O–Ti–O) adjacent [TiO6]–[TiO6] clustsites for Ca and Ti atoms, respectively.

Yang–Parr gradient-corrected correlation functional,B3LYP [56], which has proven to be a very effective tool todeal with the present challenging problem. We have beenparticularly successful in employing this functional to studythe electronic and structural properties of bulk and surfacesof PbTiO3 [57,58], AZrO3 [7,59] and several other oxides[60,61]. The atomic centers are described by all electron basissets 86-511(d21)G for Ca [62], 86-411(d31)G for Ti [62] and6-31G* for O (Support information S2). The XcrysDen pro-gram [63] was used for the band structure drawing design.The analysis of the vibrational modes and their correspond-ing frequencies were calculated through numerical secondderivatives of the total energies as implemented in theCRYSTAL06 package [64]. Lattice constants and internalcoordinate data were obtained from XRD pattern refine-ment using the Rietveld method to better describe the struc-tural distortion derived from the experimental data.

CT crystallizes in an orthorhombic perovskite structurePbnm space group, in a single phase with four nonequiva-lent atoms per unit cell, with lattice parameters and inter-nal coordinations obtained from refined parameters.Internal coordinates and net parameters for CTref, CT10and CT160 samples are listed in Table 1. The alpha (a)angle values for adjacent TiO6 octahedra are schematicallyillustrated in Fig. 1 together with their octahedral and

ers and a schematic model for CTHTMW with octahedral and dodecahedral

Table 2Nominal and experimental compositions of CTHTMW samples.

Ceramic denomination Composition in molar fraction

Nominal Ca/Ti Analyzeda Ca/Ti

CT10 1 1.062 (±0.006)CT20 1 1.065 (±0.002)CT40 1 1.055 (±0.003)CT80 1 1.037 (±0.001)CT160 1 1.023 (±0.003)

a an = 5(a = analyzed, n = measures).

Fig. 2. X-ray patterns of CT samples annealed at 140 �C from 10 to160 min.


dodecahedral sites built on the lattice parameters obtainedfrom Rietveld refinements.

3. Results and discussion

3.1. ICP analysis

ICP-AES is capable of determining the Ca/Ti ratio aswell as the purity degree of sample. To estimate the accu-racy of the CT suspensions with lower salt concentrations,two series of 10 and 100� were diluted in de-ionized waterprepared from the original digested samples. Table 2 pre-sents nominal compositions of the CTHTMW compounds.Thus the current synthesis method can yield the high-purityCT powders with impurity concentrations of Fe, Sr andMg ions less than 1 ppm. The results depicted in Table 2are very important because tiny changes in the chemicalcomposition induce many changes in chemical and physicalproperties of the crystal structure. As secondary productswere removed by successive washes, all characteristicsand properties observed for these samples will be relativeonly to CTHTMW compounds.

)(6)()( 42 aqKOHaqTiClaqCaCl →++

Scheme 1. CaTiO3 formation reaction from aqu

3.2. X-ray diffraction and Rietveld refinements

Diffraction patterns of the samples are depicted inFig. 2, featuring a single phase of polycrystalline CT. Thefacility and low temperature synthesis of CT are attributedto the fast reaction owing to the direct microwave couplingwith water rotational barrier, allowing a uniform solutionheating startup [65,66]. Both calcium and titanium hydrox-ide were previously described from a Pourbaix potential–pH equilibrium diagram [67] in aqueous solution at roomtemperature. Lencka and Riman [35] suggested that signif-icant amounts of mineralizers are necessary (i.e., pH adjustagent) when non-alkaline precursors (absence of OH�

group) are used. The direct rotational water excitation bymicrowave radiation is able to uncouple OH� groups fromcalcium and titanium hydroxides, denuding the calciumclusters and enabling them to interact with the titaniumhydroxide clusters much more readily than calcium clustersthemselves in aqueous media [66]. Therefore, under micro-wave radiation heating and at a pH P 9, CT precipitation[35] is favored as indicated by Scheme 1, enhancing thekinetics crystallization of CT powders by one to two ordersof magnitude [38,41,42,68]. Thus, it may be presumed thatthe mobility of calcium and titanium clusters is higherunder hydrothermal conditions than at ambient pressureand temperature [69]. Effective collision rates occur whenparticles collide, producing irreversible oriented attach-ments [70] which offer favorable thermodynamic and kinet-ics conditions for CT shaping. Therefore, theaforementioned facts qualify the hydrothermal microwavesmethod as a typical bottom-up process. The formation ofCT from aqueous calcium and titanium precursors can beexpressed by Scheme 1.

The polycrystalline nature of CT powders is expressedby XRD patterns as shown in Fig. 2, which were identifiedas an orthorhombic phase with Pbnm space group. Theresults obtained from the Rietveld method (Support infor-mation S3) using the ICSD cif number 74212 are depictedin Table 3. The refinement was continued up to a conver-gence to be reached with the value of the quality factor(v2) approaching 1. In our case, great values of v2 for allpatterns are reached as depict in Table 3.

The Rietveld refined parameters depicted in Table 3 pro-vide small increases of a, b, and c lattice parameters for quasi-crystalline CTHTMW samples in relation to the crystallineCTref sample. Some modifications are observed in the valuesof alpha angles (a) between adjacent [TiO6]–[TiO6] octahe-dral clusters as shown in Table 3. and schematically reportedin Fig. 1 while only small modifications were found in the[CaO12] dodecahedral clusters. Therefore, the a value canbe used as a borderline between a quasi-amorphous(a� 158�) to a quasi-crystalline (a < 158�) structure. The

)(3)(6)( 23 lOHaqKClsCaTiO ++

eous calcium and titanium precursor salts.

Table 3Rietveld refined parameters of CTref and CTHTMW powders according to HTMW treatment time.

Ceramic composition a (A) b (A) c (A) v2 R-Bragg (%) Rwp (%) a (degree) Cell volume (A3)

CT10 5.406 5.492 7.664 1.14 39 11.64 154.28 227.59CT20 5.408 5.493 7.666 1.44 27 6.87 154.31 227.54CT40 5.407 5.492 7.665 1.25 47 11.60 154.04 227.64CT80 5.406 5.491 7.664 1.28 40 12.01 154.46 227.54CT160 5.405 5.489 7.662 1.22 44 11.47 155.22 227.40CTref 5.387 5.439 7.646 1.22 38 10.35 158.23 224.09

v2, goodness of fit; CTref, reference sample, prepared by the polymeric precursor method; Rwp, weighted error (%); a, angle between adjacent [TiO6]–[TiO6]octahedra.

Fig. 3. FE-SEM microscopy of the CT samples processed using amicrowave-assisted hydrothermal method for 10 min: (a) monodispersedmicro-cubes; (b and c) emphasis on multi-faced CT cubes.


value of the a angle for high ordered and non-photolumines-cent orthorhombic CTref powders (approximately 158�) ishigher than the values of CTHTMW samples as pointed outin Table 3 for titanium octahedron [TiO6] tilts in the z direc-tion (around y axis). This movement can actually be associ-ated with the existence of an orthorhombic–orthorhombicstructural transition, and this behavior can be related tothe tilt movement that dictates the orthorhombic symmetry[16].

3.3. FE-SEM microscopy

The micro-cube-shaped morphology of the CTHTMW

sample is quite similar to the morphology of analytical cal-cite crystals [71] as shown in the FE-SEM image of Fig. 3.

In Fig. 3a the monodispersed micro-cube-shaped mor-phology with an edge length around 2 lm is clearly seenwhich is different from the values obtained by the usualsynthesis methods [72], and in the current work it remainspractically unchanged. According to Wang [73], for face-centered cubic (fcc) materials the multiple twinning ofsmall cubes occurs by the sharing of common crystallo-graphic planes and then microstrains on CT cubes (seeFig. 3b and c) can appear. The structure of one cube isthe mirror reflection of the other cube of the twin plane,facilitating their adhesion and a possible crystal growthby a coalescence process which emerges at extremelyextended times (much longer than 160 min). At this point,it is important to remark that we can observe crystallinenanoparticles with ordered superstructures on the scale ofseveral hundred nanometers to micrometers (Fig. 3b andc) consisting of highly ordered polycrystalline buildingunits with scattering similar to a single crystal. Thisdescription coincides with the term mesocrystal introducedrecently by Colfen and Antonietti [74–76].

3.4. Raman spectroscopy

Raman spectroscopy is a well-known and useful methodfor investigating the behavior of symmetry changes in cera-mic compounds. For the orthorhombic D2h

16 symmetry ofCT powder in a Pbnm space group, 117 vibrational modeswith four formula units per primitive cell (ZB = 4) areexpected, but most of these modes cannot be detectedbecause of their low polarizabilities [77]. Hence, only nine

Raman active modes are commonly observed: 134 cm�1

for a vibration of Ca bonded to a TiO3 group (Ca–TiO3)lattice mode; 181, 224, 244, 287 and 339 cm�1 are associ-ated with O–Ti–O bending modes; 464 and 495 cm�1 corre-spond to Ti–O6 torsional (bending or internal vibration ofthe oxygen cage) modes and finally the 669 cm�1 associatedwith the Ti–O symmetric stretching mode [77–79]. Bothexperimental and theoretical values of Raman activemodes of CT powders are listed in Table 4 and depictedin Fig. 4, together with well-defined peaks for the CTref

sample [80]. Duran et al. [81] suggested that the symmetryof crystals observed by Raman spectroscopy indicates alocal and dynamic symmetry while the XRD measurementis an average and static symmetry. From this point of view,both methodologies confirm the orthorhombic structure ofCTHTMW samples.

Fig. 4. Raman shift for CTHTMW and CTref samples in orthorhombicsymmetry.


An analysis of the results depicted in Fig. 4 shows that inthe low-frequency range of 100–400 cm�1 the soft mode at135 cm�1 is not commonly active for first-order Ramanscattering in a orthorhombic perovskite-type structure.This experimental mode is seemingly due to a perturbationof the perfect crystal symmetry by grain boundaries or sec-ond order processes according to Zelezny et al. [82] while,in the theoretical simulation, the mode can be related to aperiodic distortion on the orthorhombic structure which iscertainly related to an increase in the lattice parameters a,b, and c. The bands centered at 181 and 224 cm�1 are asso-ciated with the O–Ti–O bending mode. It is known that thelocation of a morphotropic phase boundary depends on thedegree of orthorhombic lattice distortion, which is mainlyassociated with the Ag band at 181 cm�1 that disappearswhen phase transitions take place [78–83], indicating theexistence of a single orthorhombic phase for all situations.Moreover, in the low-frequency range, the best defined andstrongest mode at 244 cm�1 is assigned to hard mode B1g asa result of the rotation of the oxygen octahedron cage. Themodes at 287 and 339 cm�1 were dominated by Ti–O inte-rior vibrations from the tilt at the titanium octahedron [84].

An analysis of Fig. 4 report that in the high frequencyRaman region at 400–800 cm�1, no drastic spectral changeis observed for the mode at 464 cm�1 as compared to CTref

and CTHTMW samples. On the other hand, for a torsionalmode at 495 cm�1, there is a displacement in an upwarddirection rising to an intense and wide mode around537 cm�1 which is interpreted as a signal of the disorderedstructure by an increase in the tilts in the titanium octahe-dron cluster as demonstrated by Hirata et al. [77]. Fromthis viewpoint, the uncommon upward displacementtoward the high-frequency region is expected to be affectedby an increase in the average electronic density. The extentof distortion can be expressed by cell volumes which can tobe related to the average electronic densities at A-sites inABO3 compounds [85]. However, the ICP-AES resultsfrom this work proved that only calcium occupies the A-site. Even so, there is a significant volume increase in thecell (Table 3) caused by the processing method which takesthe angle changes between the titanium octahedra and con-

Table 4Theoretical and experimental Raman active modes (cm�1) for CaTiO3 sample

Vibrational modes Ref. [21] Ref. [83] Ref. [85] C

Ca–TiO3 128 – – 1Lattice mode 160 155 153 1

O–Ti–O 181 180 178 1Bending modes 220 226 222 2

245 247 244 2295 286 281 2339 337 333 3

Ti–O3 471 471 467 4Torsional modes 495 495 490 4

Ti–OStretching mode

644 639 – 6

CTref, reference sample; ref, reference paper; Exp, experimental results; Theo,

sequently distorts and polarizes the CTHTMW cell [77,83].From Table 3 it can be observed that the superpositionof three Raman active modes at 518 (B2g), 530 (B1g) and546 cm�1 (B3g) actually takes place, generating a wide bandcentered at 537 cm�1. This vibrational mode was not foundin the CTref sample from quantum mechanical vibrationalsimulations, just as it was not observed from experimentalspectrum (Fig. 4). Moreover, the very wide and low resolu-tion band centered at 669 cm�1 for CTHTMW powdersshifts to upward frequencies in relation to the CTref samplewhich is related to the Ag mode and described as a TiO6

distorted octahedron by Ti–O stretching. This fact is inagreement with the already published data [77,78,86]. Inthe theoretical simulations the absence of the vibrationalmode around 669 cm�1 can be attributed to low-resolutionmodes at this region in conformity with Raman spectra ofCTHTMW samples which is in agreement with theoreticaland experimental results regarding the existence of theTiO6 distorted octahedron for these samples.

3.5. XANES spectroscopy

It is well-known that perovskite-based materials (ABO3)consist of network corner-linked BO6 octahedra enclosinglarge cavities which form 12-coordination sites for the

s.

Tref Exp CTref Theo CTHTMW Exp CTHTMW Theo

27 137 134 –56 157 – 157

80 200 181 18124 227 224 22445 240 244 23788 285 287 25739 362 339 315

69 478 464 48895 499 537 518, 531, 546

45 – 669 –

theoretical simulations.

Fig. 5. The theoretical results for CT10 (HTMW): (a) CT160 (HTMW), (b) and CTref, (c) samples I–V report the band structure, total density of states,the total and projected density states of axial oxygen atoms, the total and projected density states of equatorial oxygen atoms, and the projected densitystates of titanium atoms, respectively.


A-site. In the present case, B- and A-sites correspond to Tiand Ca elements, respectively, as represented in Fig. 1. PLmeasurements induce energy transfer processes and specificstructural rearrangements occur; a remarkable evolution ofthe distortion across the octahedral and dodecahedral clus-ters can be sensed. Therefore, this fact may indicate a sym-metry-breaking process along the network of both [BO6]and [AO12] clusters, leading to a lower symmetry. Unavoid-ably, one must consider the whole [BO6] and [AO12] poly-hedra (i.e. both the octahedral and dodecahedral clusternetwork) to study the overall pattern.

The luminescence property of titanates has been studiedby our group in recent years [87–91]. Studies show that,from the analysis of XANES results, the quasi-amorphousstructures were associated with the presence of distorted[TiO6]–[TiO5] clusters which are responsible for the lumi-nescent emission. In addition, this behavior was relatedto the organization of the sample, i.e. when the system pre-sents a high level of crystallinity or is composed only ofordered [TiO6]–[TiO6] clusters [92], the PL emission disap-pears. In the present case, for CTref samples the Ti K-edgeXANES experimental results revealed the coexistence oftwo types of environment for titanium atoms, namely, a

fivefold coordinated TiO5 (square-base pyramid) and a six-fold coordinated TiO6, (octahedron) before reaching thecomplete structural order. Fig. 6a displays the pre-edgeregion of the XANES spectra of CTHTMW and CTref sam-ples representing both situations, i.e. [TiO6]–[TiO5] (six-fold–fivefold) and [TiO6]–[TiO6] (sixfold–sixfold). As canbe observed in Fig. 6a, the Ti K-edge spectra report a smallpeak situated around 4971 eV, which is ascribed as 1s-3delectronic transition [8,91]; this forbidden electronic transi-tion can normally be allowed if the mixture of oxygen 2pstates and empty titanium 3d states [87] occurs. Theincrease in the intensity of this peak indicates that the localenvironment of Ti is noncentrosymmetric, distorting theoctahedral configuration [8,87,93]. XANES spectra remainunchanged for all CTHTMW treatment times. However, theintensity of the Ti K-edge pre-edge peak (Ti) is higher withrespect to the CTref crystalline sample and lower withrespect to the CTref disordered sample. These results pointout the existence of a noticeable disorder on stillunchanged TiO6 clusters of CTHTMW samples [94]. Thisfact can be related to a quasi-crystalline structure by meansof tilted titanium octahedra in agreement with Rietveldresults Table 3. In other words, the Ti K-edge XANES

Fig. 7. (a) Evolution of the PL profile for CaTiO3 samples annealed by 10up to 160 min under a 350.7 nm excitation wavelength at room temper-ature, (b) their schematic representation of octahedral tilt with (c) wideband model to illustrate the three steps of PL emissions.

Fig. 6. Ti (a) and Ca K-edge (b) XANES spectra of the CaTiO3 samplesprocessed in HTMW at 140 �C from 10 to 160 min and by the polymericprecursor method (CTref).


spectra present an unchanged local structure for CTHTMW

samples if the processing time remains from 10 min up to160 min, indicating the same type of octahedral distortion.

The pre-edge region of the XANES spectra collected atthe Ca K-edge (Fig. 6b) shows no significant differencesbetween the CTHTMW samples compared to the CTref sam-ples (pre-edge Ca). Thus, the energy transfer processes pre-sented by the quasi-crystalline powders can be attributed tothe intrinsic disorder between [TiO6] and [CaO12] clusters,originating a polarized orthorhombic CaTiO3 structure(Fig. 1) without the presence of disordered [CaO12] and/or [TiO5] clusters, which is in agreement with both Ramanspectra and theoretical data.

3.6. Theoretical modeling

The orbital distributions as well as the density of states(DOS) are depicted in Fig. 5. To analyze the changes in theelectronic structure, it is beneficial to consider the bandstructure, which can be compared independent of the crys-tal space group. Figs. 5a-I, b-I and c-I present band struc-tures for the CT10 (a), CT160 (b) and CTref (c) models,respectively. The top of the valence band (VB) is locatedat the C point in the same way that the bottom conductionband is also located at the C point. Thus a direct band gap

was achieved for all models and corresponds to 3.87, 3.86and 3.98 eV for CT10, CT160 and CTref models, respec-tively. The calculated direct band gap of the CTref modelagrees with the experimental optically measured gap.Moreover, for both CTHTMW and for CTref samples(Fig. 5), the upper valence band is mainly composed ofoxygen (O) 2p states, which are predominantly influencedby changes on axial oxygen atoms (O9–O12), involved withthe alignment between two adjacent tilted TiO6 octahedra.In the Ti octahedral site the axial oxygen corresponds toatoms placed above and below the Ti–O plane in the octa-hedral cluster Fig. 1 as a result of orthorhombic structure.The O13–O20 represent de equatorial atoms Figs. 1 and 7in Ti–O plane to orthorhombic structure by the CRYS-TAL06 definition. The Ti (3d) contributions are clearlypredominant in the conduction band region. The structuraldistortions existing in the titanium octahedron mustdirectly affect both the conduction and valence regions atthe same time.

An analysis of Fig. 5a and b points out the downwardslope of the energies for both valence and conductionbands for CTHTMW in relation to the CTref systemFig. 5c; the 0.1 eV reduction of the band gap value issensed. This behavior can be associated with the changesof 2p and mainly 3d orbital redistributions in the valenceand conduction bands, respectively, as already indicatedby Ti K-edge spectra. UV–visible optical absorbance mea-surements of CTHTMW (Support information S4) render

Fig. 8. Area percentage with respective errors bars for the PL evolution ofCaTiO3 samples annealed at 140 �C using a HTMW process.


band gap values of 3.53 and 3.60 eV for CT10 and CT160quasi-crystalline structures, respectively. Thus this value is0.3 eV smaller than the corresponding CTref (3.85 eV) crys-talline sample, which is in agreement with both experimen-tal and theoretical results.

Density of states (DOS) for titanium 3d orbitals in theconduction band region are more significant and hencemore strongly affected than oxygen 2p orbitals in the sameregion by the structural modifications. Unlike within thevalence band, the oxygen 2p orbitals are more affected bystructural distortion in the quasi-crystalline arrangement,particularly the axial oxygen atoms (O9–O12) as depictedin Figs. 5a(III), b(III), and c(III). These statements are sup-ported by the observed changes among band structures inFigs. 5a–c(I). Thus, the theoretical calculation agrees withdistortions around titanium clusters as previouslydescribed by Rietveld refinements, which are associatedwith the value of a angle and octahedral distortions.

3.7. Photoluminescence properties

Fig. 7 illustrates the PL evolutions of CTHTMW samplessynthesized by a hydrothermal microwave route under dif-ferent times which can be considered as a trace for the for-mation of CT mesocrystals. In the photoluminescentresponse for the CTHTMW samples with a 350.7 nm excita-tion source, it can be seen that the luminescence behavior iscomposed of a broad luminescence band in the range of350–580 nm with a more defined peak at 614 nm coveringa wide range of visible spectra. In addition, the profile istypical of a multiphonon process, i.e. a system in whichrelaxation occurs by several paths involving the participa-tion of numerous states within the band gap of the material[95] originating from by intrinsic defects of the materialowing to the absence of secondary phases as well as anyline of organic matter (recently reported for BaTiO3 alsoprepared by the microwave-assisted hydrothermal method)[19].

The wide band model illustrated in Fig. 7c provides thethree necessary steps for a PL emission to occur: (i) in thefirst step, the excitation source, hm, corresponds to theenergy to promote a photon absorption from O 2p statesat the valence up to Ti 3d states inside the forbidden bandgap; (ii) after excitation, the recombination process occursamong the excited 3d states closer to the conduction band;and (iii) a wide band PL emission due to allowed 3d ? 2ptransitions, associated with a multiple hm‘ energy, can bemeasured.

To discuss the PL profile of Fig. 7, the luminescencespectra are broken up into five peaks, and each was fittedto a symmetric Gaussian function (Support informationS5) Fig. 8 depicts the typical peak-resolution results, show-ing that five Gaussian curves constitute the overall lumines-cence in the visible region from approximately 380–580 nm.These broad bands are composed of B1 and B2 (blue com-ponents), followed by a G3 (green component), a Y4(yellow component), and a O5 (orange component). In

addition, in Fig. 8 the contribution of each of the abovepeaks as a function of time is presented. From the analysisof these luminescence spectra, the following trends can bederived.

(i) These transitions can be associated with intrinsicdefects in the CTHTMW quasi-crystalline (mesocrys-tal) structure.

(ii) Both B2 and O5 present the large contributions.(iii) The relative intensity of these bands tends to decrease

as the processing time increases except for the O5peak in the orange region, which is now displacedto a higher wavelength and a rise in the relative inten-sity with increased processing time.

(iv) The contribution of the peaks measured by their cor-responding area can be separated into three differentbehaviors. First B1, G3 and Y4 present slight varia-tions, B2 decreases from 42.8% to 33.5% while forO5 an opposite behavior is sensed, rising from19.2% to 38.9%.

(v) The wavelength values for B1, B2, G3 and Y4 can beassociated with electron–hole recombination centerspromoted by high energetic defects while O5 can berelated to low energetic defects [88].

(vi) The relative contribution of these transitions arerelated to high energetic defects in the B1 peak, whichare not favored by an increase in the microwavehydrothermal processing time; an opposite trend isobserved for the low energetic defects correlated withthe O5 peak.

Therefore, HTMW processing promotes an intrinsic dis-order in a medium-range order in crystalline orthorhombicpowders by different angles concerning the TiO6 octahedra,which are responsible for the PL property in the CTHTMW

quasi-crystalline samples. Thus, PL spectroscopy can beused as a very sensitive probe to analyze the structuralorder–disorder degree in the lattice of ceramic materials.


In several cases [96,97], it has been proposed that theintensity of the PL emission increases with a decrease inthe grain size, which is closely correlated with interfacesof nanocrystallites (<100 nm) and d-surface states insidethe forbidden band gap by the distortion of TiO6 octahe-dra. However, in our case Fig. 3 the samples present amicro-sized cube shape, and thus in our case the PL emis-sions cannot be attributed only to the size of the crystals(nano-sized) as previously discussed. Then the origin ofthe structure polarization and the consequent PL emissionremain related to the structural distortion that is associatedwith TiO6–TiO6 tilting coupled with their internaldistortions.

4. Conclusions

In summary, the present work reports on micro-cube-shaped CaTiO3 powders prepared using HTMW methods.Their crystallization and optical properties were carefullyinvestigated and compared to powders synthesized bymeans of the polymeric precursor method. Using X-ray dif-fraction, Raman spectra, ICP-AES spectroscopy and FE-SEM microscopy, it was verified that the HTMW processis an efficient method to obtain CaTiO3 single-phase com-pounds in a short processing time at low temperature. Thismethod is highly cost-effective and environmentallyfriendly, qualifying this technique as a bottom-up process.Raman results combined with UV–visible absorbencies andthe PL analysis demonstrate that structural disorder at ashort and intermediate range is present in CTHTMW pow-ders. On the basis of the TiO6 tilted octahedra and theirconnectivity (measured by the O–Ti–O angle between adja-cent TiO6 clusters and the distorted octahedral site as sug-gested by Ti K-edge spectra), PL properties wererationalized. The disorder coupled to the tilt of TiO6–TiO6 adjacent octahedra was mainly responsible for thephotoluminescent emission of a CTHTMW orthorhombicstructure as a result of the remaining structural distortion.Theoretical results point out that these defects generatelocalized electronic levels essentially above the valenceband. This result was explained by the analysis of the bandstructure and subsequent redistribution of the density ofstates mainly around titanium 3d and axial oxygen 2pstates. It is important to note that all distortions presentin the CTHTMW samples are dependent on the syntheticroute employed, a finding that needs further, more detailedevaluation.

Acknowledgements

The authors acknowledge the support agencies CAPES,CNPq and FAPESP/CEPID 98/14324-8. J.A. acknowl-edges to Ministerio de Educacion y Cultura of the SpanishGovernment to provide research mobility. Thanks forXANES facilities provided by LNLS, Campinas, SP, Bra-zil. Thanks to Rorivaldo Camargo and Madalena Tursifor the technical support.

Appendix A. Supplementary materials

Supplementary data associated with this article can befound, in the online version, at doi:10.1016/j.actamat.2009.07.019.

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