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Preparation and characterization of powders and thin films of Bi2AlNbO7 and Bi2InNbO7 pyrochlore oxides
Materials Chemistry and Physics 124 (2010) 552–557
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Preparation and characterization of powders and thin films of Bi2 AlNbO7 and Bi2 InNbO7 pyrochlore oxides Zaine Teixeira ∗ , Larissa Otubo, Rubia Figueredo Gouveia, Oswaldo Luiz Alves Solid State Chemistry Laboratory, Institute of Chemistry, University of Campinas (UNICAMP), P.O. Box 6154, 13083-970. Campinas, SP, Brazil
a r t i c l e
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Article history: Received 2 November 2009 Received in revised form 8 January 2010 Accepted 8 July 2010 Keywords: Thin films Atomic force microscopy (AFM) Oxides Optical properties
a b s t r a c t Powders and thin films of Bi2 InNbO7 and Bi2 AlNbO7 with pyrochlore-type structures were prepared by metalorganic decomposition processes (MOD). The structural evolution of the powders was characterized by X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). The powders were obtained at lower temperatures and faster than with conventional solidstate reactions. The films were further characterized by ultraviolet–visible–near-infrared (UV–vis–NIR) absorbance spectroscopy and atomic force microscopy (AFM). A pyrochlore-type phase in the films was obtained at even lower temperatures than the powders. These presented high transmittances in the vis–NIR range, uniform thicknesses and low roughnesses. A blue shift in the optical gap was observed as the thickness decreased. This was mainly attributed to the Brus effect, since the films showed the presence of nano-sized clusters. The film properties make them very interesting for technological applications such as in photocatalytic processes. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Pyrochlore-type structures have been the subject of numerous studies due to their wide spectrum of characteristics, such as their electrical, magnetic, dielectric, optical, and catalytic properties [1]. The general formula of pyrochlore oxides is described as A2 B2 O6 O, with four crystallographically non-equivalent kinds of atoms, a Fd3m space group and eight molecules per unit cell. The pyrochlore oxides present mainly ionic character, which allows a number of crystallographic substitutions as long as ionic radii and neutrality are respected. Many phases with A and B elements in higher oxidation states exhibit dielectric, piezo, and ferroelectric properties [1]. The pyrochlores show several technological applications such as: gas sensors (Cd2 Sb2 O7 ), transistors (Cd2 Nb2 O7 ), thermistors (Bi2 Ru2 O7 ), switching elements and thin film resistors (Cd2 Os2 O7 and Ca2 Os2 O7 ) [2,3]. Furthermore, the Bi2 O3 –ZnO–Nb2 O5 (BZN) pyrochlore systems are promising materials for electronic devices in microwave communications [4,5]. Recently, thin films of PbO–ZnO–Nb2 O5 pyrochlore have shown high-permittivity and low-loss dielectric tunable that make this material promising for wireless communication [6]. Pyrochloretype compounds are obtained mainly by solid-state reactions [1]. An alternative technique is the metalorganic decomposition pro-
∗ Corresponding author. E-mail addresses: [email protected] (Z. Teixeira), [email protected] (O.L. Alves). URL: http://lqes.iqm.unicamp.br (Z. Teixeira). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.07.009
cess (MOD), which produces films and powders faster and at lower temperatures compared to conventional methods. In this technique, an organic solution containing a stoichiometric mixture of metalorganic components is pyrolysed, generating the oxide [7,8]. Some examples of thin films of pyrochlore oxides obtained by this method are Cd2 Nb2 O7 that presented high electrical resistivity [9,10], and Bi–Zn–Ti–Nb–O (BZTN) with pronounced dielectric properties [11]. The Bi2 MNbO7 (M = Al, In, Fe, Ga) pyrochlores obtained by solid-state reactions presented photocatalytic properties in the production of H2 and O2 from aqueous methanol and cerium sulfate solutions [12–14]. In these papers, the authors suggested that M3+ and Nb5+ , doping B2 site in A2 3+ B2 4+ O7 might cause an increase in hole (carrier) concentrations and might provide a change in magnetic, electrical transport and photophysical properties [14]. A sol–gel route for the preparation of Bi2 MNbO7 (M = Al, In, Fe, Sn) showed that the solids prepared at 400 ◦ C presented higher photocatalytic activity for methylene blue degradation than similar compounds prepared by solid-state reaction or even TiO2 P25 [15]. Recent papers with regards to these Bi2 MNbO7 solids have mainly focused on structural investigations and dielectric properties [16–18]. In this work, powder and thin films preparations by MOD processes of Bi2 InNbO7 and Bi2 AlNbO7 pyrochlore-type oxides are described. The optical properties of the films were also investigated. 2. Experimental details Bismuth(2-ethylhexanoate), Bi(hex)3 and niobium ethoxide were purchased from Strem Chemicals. The 2-ethyl-hexanoic acid, H(hex), was purchased from Across Organics. Indium chloride, aluminum chloride and ammonium hydroxide
Z. Teixeira et al. / Materials Chemistry and Physics 124 (2010) 552–557 were purchased from Merck. Commercially available reagents were used without further purification. Niobium tri-(ethoxy)-di-(2-ethylhexanoate) precursor, Nb(OEt)3 (hex)2 , was prepared by a metathesis reaction described elsewhere [9,19]. Aluminum(2-ethylhexanoate), AlOH(hex)2 , and indium(2-ethylhexanoate), In(hex)3 , precursors were obtained by double-decomposition from the ammonium salts as described by Vest and Singaram [19]. The precursor solutions for the MOD process were obtained by dissolution of the individual precursors for Bi2 InNbO7 in xylene and for Bi2 AlNbO7 in dried tetrahydrofuran. The final concentrations were chosen to obtain 0.09 and 0.18 mol L−1 Bi2 InNbO7 solutions, which were labeled In1 and In2, respectively. The Bi2 AlNbO7 final concentrations were 0.045 and 0.09 mol L−1 , which were labeled Al1 and Al2, respectively. Two concentrations were used to evaluate how this factor affects film optical properties, thickness, crystallite size and average transmission. In MOD powder preparation, an aliquot (3 mL) of the precursor solutions (In1 and Al1) was placed into a platinum crucible and pyrolyzed in a muffle using a heating rate of 10 ◦ C min−1 , from room temperature to a plateau, ranging from 400 to 1100 ◦ C for each sample, each being held at the final temperature for 4 h. For comparison, solids were also obtained by solid-state reactions as described by Zou and co-workers [12]. The metal oxides were weighted in the stoichiometric ratio, mixed in an agate mortar, pressed into pellets, placed into a platinum crucible and heated in air at 1100 ◦ C for 2 h. The solids were re-mixed and re-pressed for 3 times following this procedure. In the final step, the pellets were maintained at 1100 ◦ C for 48 h. The films were obtained by the dip-coating method from the two concentrations for each composition. The solution precursors were deposited on borosilicate glass slides at a 2 cm min−1 rate. After coating, the film was removed from one of the slide faces with a piece of cotton. The samples were placed in a quartz tube furnace preheated to 400 ◦ C for 15 min for the pyrolysis step. The coating process was repeated five times in order to get the desired thickness. The samples were finally annealed at 600 ◦ C under an ambient atmosphere for 8 h. The individual precursors were investigated by thermogravimetric analysis (TGA), infrared spectroscopy (IR), X-ray diffraction (XRD) and gravimetric characterizations (data not shown). The precursor solutions were investigated by TGA (Shimadzu 50-WS) using a heating rate of 10 ◦ C min−1 under synthetic air (50 cm3 min−1 ). The powders and film samples were characterized by X-ray diffraction (Shimadzu XRD-6000, Cu K␣, = 0.154 nm), and scanning electron microscopy (SEM) (Jeol JSM-6063LV) with secondary electrons or energy dispersive spectroscopic (EDS) detection. Optical characterization of the films was carried out by UV–vis–NIR absorbance (Cary 5G spectrophotometer, Varian). The film thicknesses and roughnesses were measured by a perfilometer (Alpha Step 200). Atomic force microscopy (AFM) experiments were performed on a Shimadzu WET-SPM 9500J3 instrument. The noncontact AFM mode was used to obtain topography information about the thin film surface. Image processing was analyzed in a PC microcomputer using Shimadzu software.
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Fig. 1. (—) TG and (- - -) DTG curves of stoichiometric precursor solutions used in the preparation of (a) Bi2 InNbO7 (0.18 mol L−1 ) and (b) Bi2 AlNbO7 (0.09 mol L−1 ).
tograms, mainly the (1 1 1), (3 1 1), and (2 2 2) ones. The atoms in structural positions B (16d) and A (16c) for the space group Fd3m have diffraction on opposite faces for the Miller index [20]. Heavy atoms in A sites results in a reduction of intensity of the peaks which have an odd sum of their Miller index. Thus, in the present compounds, we propose that the octahedral BO6 are first organized, given that the (3 1 1) intensity is higher than the (2 2 2) intensity in the solids treated at lower temperatures. At 700 ◦ C, an inversion in the intensity of (3 1 1) and (2 2 2) is observed, which may be related to A site organization. Thus, the pyrochlore-type structure is
3. Results and discussion The individual precursors showed the required characteristics for the MOD process: ease of synthesis, decomposition without metal evaporation or carbon residues, and obtainment of metal oxides with phase purity [7]. In the precursor solution characterization, the thermogravimetric curves are important to determine the minimum pyrolysis temperature as well as to suggest the process decomposition steps. Fig. 1 shows the thermogravimetric/differential thermogravimetric (TG/DTG) curves of the precursor solutions. The minimum pyrolysis temperature observed was 325 ◦ C for Bi2 AlNbO7 and for 304 ◦ C for Bi2 InNbO7 . Vest [7] suggested that the metalorganic precursor decomposition mechanism involves the formation of free radicals by thermolysis, followed by a fragmentation of these radicals in a very fast oxidative chain reaction, suggesting a “domino” type mechanism. Thus, the decomposition temperature decreases as the chain length of R increases, as the oxygen partial pressure increases, or as the degree of branching of R increases [7]. Gravimetric analysis of the pyrolysed solution showed a difference lower than 1% from that expected for Bi2 MNbO7 (M = In or Al) compounds, showing that the oxides were obtained in the desired stoichiometry. Fig. 2 shows the diffraction pattern of the solids prepared at different temperatures, in comparison to the product obtained by the solid–solid reaction. Pyrochlore-type structures are related with high variations in the intensities of peaks observed in diffrac-
Fig. 2. Pyrochlore-type solids of (a) Bi2 InNbO7 (0.18 mol L−1 ) and (b) Bi2 AlNbO7 (0.09 mol L−1 ). Structural evolution by MOD process from 300 to 1000 ◦ C and solid–solid reaction (s–s).
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Fig. 3. SEM images: Bi2 InNbO7 powders obtained by (a) MOD at 600 ◦ C, (b) MOD at 900 ◦ C, and (c) by a solid-state reaction; Bi2 AlNbO7 powders obtained by (d) MOD at 600 ◦ C, (e) MOD at 900 ◦ C, and (f) by a solid-state reaction.
organized above 700 ◦ C. In the Bi2 InNbO7 compounds, we observed not only the peaks related to the pyrochlore structure, but also an additional peak at 30.4◦ , which is related to the maximum intensity peak of In2 O3 . It means that indium oxide as a phase impurity was detected, whereas Bi2 InNbO7 with defects in the BO6 site is a major phase. This peak totally disappears in treatments higher than 900 ◦ C. For the Bi2 AlNbO7 solids, additional peaks with smaller intensities are observed for compounds from both the solid-state reaction and from MOD. We also observed this same pattern in the literature diffraction results [21], but the authors did not suggest a phase impurity even after Rietveld analyses. Fig. 3 shows the SEM images of the samples. The EDS analysis of Bi2 InNbO7 showed that the solids are homogeneous, within the detection of this technique. For Bi2 AlNbO7 solids, however, we observed a higher density of aluminum and oxygen by EDS microanalysis (circled areas). This major impurity is not detectable by XRD most probably because of a low crystallinity that is commonly observed in aluminum oxides. The samples at 600 ◦ C show higher porosity, which is due to solvent evaporation and decomposition product during the pyrolysis step. The solids at 900 ◦ C present irregular rounded particles. Finally, we observed sintering characteristics for the solid-state reaction. In summary, the MOD process showed advantages for preparation with regards to lower time (6 h instead of 54 h) and temperature (900 ◦ C instead of 1100 ◦ C). Fig. 4 shows the XRD pattern of the films that were indexed to pyrochlore-type structures. The films were calcinated at 600 ◦ C, in which the substrate could be heated without damage. Moreover, a lower crystallization temperature is expected for films compared to powders, since there is a dimension reduction from bulk to one or two dimensions. Besides, a preferred orientation may occur in the substrate, which may induce phase purity. In the powder XRD characterization (Fig. 2), we observed at 600 ◦ C a peak at 30.54◦ , ascribed to an In2 O3 impurity, which had a relative intensity even higher than at 33.34◦ (4 0 0). The absence of a peak at 30.54◦ (Fig. 4) means the films were obtained with phase purity. The crystallite size estimate calculated by the Scherrer equation [22] from the diffractograms (Fig. 4) is shown in Table 1, showing that the films are nanostructured. SEM images of the samples (Fig. 5) show that the films are homogeneous with nano-sized clusters. A large number of thin films obtained by the MOD technique have porous morphology.
Fig. 4. X-ray diffraction patterns of (In) Bi2 InNbO7 and (Al) Bi2 AlNbO7 films deposited on vitreous substrates.
Table 1 Crystallite size estimate by the Scherrer equation for thin film samples. Thin film sample
Crystallite size (nm)
In1
In2
Al1
Al2
16.5 ± 2.1
20.7 ± 1.5
11.3 ± 5.0
12.0 ± 2.6
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Fig. 5. SEM images of (In) Bi2 InNbO7 and (Al) Bi2 AlNbO7 films.
Thin films of Cd2 SnO4 presented large porous or even dentrite ramifications depending on the withdrawal rates and concentration levels [23,24]. In another work, tin-doped indium oxide (ITO) showed non-uniformity at low pyrolysis temperatures (∼150 ◦ C), while the films obtained by a pyrolysis step at 550 ◦ C were uniform [25]. These authors suggested that at the precursor decomposition temperature, the viscosity is diminished. Consequently, a contact angle, depending on substrate wettability, turns into equilibrium on the substrate and the liquid may develop into discrete drops. Hence, a temperature above the precursor pyrolysis temperature after the coating process is important to obtain a homogeneous film. This detail suggests our films are uniform as the temperature for pyrolysis is sufficiently high to exclude drop or ramification formations. Further, the films (Fig. 5) have nano-sized clusters. In the SEM images for the Al samples, we observe that the thicker films have large grains. The possibility for graining growth is attributed to a change from two to three dimensions increasing the sample thickness. In films of ZnO obtained by the sol–gel technique, the authors also observed that the surface morphology showed that the grains become more uniform and bigger as the film thickness increased [26]. Fig. 6 shows the optical spectra in the UV–vis–NIR range. The films have interference fringes that are characteristic of homogeneity and low roughness as well as high transmittance in the vis–NIR range. The thickness (d), refractive index (n), and the optical gap were calculated from the transmission spectra as described previously [27,28]. Considering the thick substrate in the absence of a film, the substrate refractive index, s, is described as: 1 s= + Ts
1 Ts 2
fringes in the cut off region may not be used [27]. N = 2s
s2 + 1 TM − Tm + TM Tm 2
(3.3)
The film thicknesses may be calculated by: d=
1 2 2(1 n2 − 2 n1 )
(3.4)
where n1 and n2 are the refractive indices of two adjacent maxima or minima in the interference fringes with wavelengths 1 and 2 [27,28]. The optical band gap values were estimated by extrapolations of the straight regions of the absorption coefficient, (˛2 ), versus photon energy, h, by the least-squares method. The absorption coefficient ˛ was calculated by the equation: ˛ = ln
1 T
d
(3.5)
where T is the transmitance in the UV–vis–NIR spectra, and d is the thickness of the sample [29].
1/2 −1
(3.1)
where Ts is the transmittance of the substrate. The film refractive index, n, is described by: n = [N + (N 2 − s2 )
1/2 1/2
]
(3.2)
where N is obtained as a refractive index function as well as successive transmittance maximum (TM ) and minimum (Tm ) in the weak absorption region. In order to minor the errors, the interference
Fig. 6. UV–vis–NIR transmittance spectra of (In) Bi2 InNbO7 and (Al) Bi2 AlNbO7 film samples.
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Fig. 7. AFM three-dimensional images acquired from the Bi2 AlNbO7 thin film surfaces: (a) Al1; (b) Al2. Images recorded at higher magnification: (c) Al1 and (d) Al2.
AFM three-dimensional images were acquired from Bi2 AlNbO7 thin film surfaces. The results are shown in Fig. 7. The topographic images show an approximately spherical morphology: Al1 and Al2 particles present structures with 62 nm and 45 nm average diameters, respectively. To verify detectable topography changes, AFM images were acquired from the same sample used in this work but at a higher magnification, as shown in Fig. 7(c) and (d). This figure shows that the Al1 topographic surface is smoother than the Al2 surface. The surface roughness can be quantitatively identified by the root mean squared (rms) roughness. The rms roughness value (12.30 nm) obtained for the Al1 thin film is smaller than that of the Al2 sample (16.90 nm). The surface roughness results are very interesting because the optical properties are closely related to the microstructure of the thin films. The optical parameters, as well as the thicknesses and roughnesses measured by the perfilometer, are shown in Table 2. The values represent the medium ± S.D. for 10 measurements in the perfilometer on duplicate samples. The optical calculations for thickness, refractive index, and optical gap are represented as the average for duplicate samples ± S.D.
The low roughnesses and optical parameters such as high refractive index and low band gap values show interesting application possibilities of the films. A low roughness is strictly related to a low light scattering and consequently makes the films good candidates for optical application. For example, in the MgF2 thin films, obtained by atomic laser deposition (ALD), the roughness was dependent on the deposition temperature varying from 250 to 400 ◦ C. The film obtained at the highest temperature presented a high roughness that disables its use for optical devices [30]. Bi2 InNbO7 and Bi2 AlNbO7 solids have optical gaps at 2.7 and 2.9 eV, respectively [12], whereas in our films the optical gap is higher (Table 2) probably due to the nano-sized cluster film structures. Generally, films with low thicknesses show high values of optical band gap. In thin films of Bi3.25 La0.75 TiO3 O12 , prepared by the sol–gel technique, the band gap values were 4.05, 3.75, and 3.61 eV for thicknesses of 173, 415 and 740 nm, respectively [31]. These authors attributed these differences to stoichiometric changes in the annealing process. In V2 O5 films a red-shift from 2.40 to 2.25 eV was observed for samples with 150 and 400 nm thicknesses, respectively [32]. The XPS investigation of these samples
Table 2 Thickness, roughness and optical parameters for Bi2 InNbO7 and Bi2 AlNbO7 thin films. Sample
In1 In2 Al1 Al2
Thickness (d) (nm)
Roughness (nm)
Optical
Perfilometer
– 472 ± 41 – 463 ± 2
116 511 137 470
± ± ± ±
22 15 4 23
11.2 7.8 4.8 13.8
± ± ± ±
2.8 2.2 0.5 2.7
Refractive index (n) at 1000 nm
1.98 1.96 1.79 2.03
± ± ± ±
0.14 0.14 0.14 0.17
Optical gap (Eg ) (eV)
3.59 3.44 3.45 3.33
± ± ± ±
0.02 0.01 0.01 0.02
Z. Teixeira et al. / Materials Chemistry and Physics 124 (2010) 552–557
showed only the thicker film was stoichiometric, showing that the band gap variations were due to stoichiometric changes. Studies of V2 O5 , Nb2 O5 and Y2 O3 films obtained by magnetron sputtering (MS) or ion beam sputtering (IBS) with thicknesses between 50 and 400 nm, shown that the optical constants depended on the thicknesses. This was not only attributed to changes in the stoichiometry but also to changes in the band structures by quantum confinement [32]. The authors postulated that, independent on the material or preparation technique, with the thickness diminishing, there are increases in the absortivity and band gap, as well as a decrease in the refractive index. Thin films of Sn2 O–Fe2 O3 showed band gap increasing from 2.3 to 3.89 eV with decreasing the crystallite sizes, which was attributed to the quantum confinement as well [33]. Theories for the doping effect or quantum confinement may be explained by the Burstein–Moss shift [34,35] and the Brus theory [36], respectively. The Burstein–Moss shift is related to high doping effects of semiconductors, which results in a high carrier concentration, the so-called degenerate semiconductors [34,35,37]. It is worth mentioning that we tried to measure the sample resistances by the four probe method, which is widely employed to measure resistivity of semiconductor samples, but the samples did not show semiconductor properties. Thus, the Burstein–Moss shift theory may not be suitable for our films. The Brus model is related to the band gap shift as a function of particle sizes as: E =
h2 + 8R2
1 1 + ∗ m∗e mh
−
C1 e2 ∗ − C2 ERy εR
(3.6)
where R is the exciton radius, h is the Planck constant, m∗e and m∗h are the effective masses of electrons and holes, respectively. C1 and C2 are constants; e is the elementary charge, ε is the dielectric constant ∗ is the Rydberg effective constant. The of the semiconductor and ERy first term represents energy quantization by the particle-in-a-box quantum localization energy with 1/R2 dependence. The second term represents the Coulomb energy with 1/R dependence. In the limit of large R, the E values approach the energy, Eg , for the bulk [37,38]. In our work, we suggest that the main effect of the band gap shift is related to the Brus model due to the nano-sized clusters suggested by SEM and AFM microscopies and XRD. However, some possible contributions of a stoichiometry variation cannot be totally discarded. 4. Conclusion In this work, we successfully obtained powders and thin films of Bi2 InNbO7 and Bi2 AlNbO7 by the MOD process, presenting, as advantages, faster preparations and lower temperatures. In addition, the Bi2 AlNbO7 thin film compounds were obtained without impurities when compared to the solid-state reaction or to the MOD process for the powders. The films were uniform, with nanometric dimensions and high UV–vis transmitance. Film preparation makes
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feasible the use of these materials in technological applications, such as photocatalytic processes. Acknowledgments The authors acknowledge the financial support of the Fundac¸ão de Amparo à Pesquisa do Estado de São Paulo (FAPESP), the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and the Millennium Institute for Complex Materials (PADCT/MCT). References [1] M.A. Subramanian, G. Aravamudam, G.V. Subba Rao, Prog. Solid State Chem. 15 (1983) 55. [2] B. Li, J. Zhang, J. Am. Ceram. Soc. 72 (1989) 2377. [3] W.R. Cook, H. Jaffe, Phys. Rev. 89 (1953) 1297. [4] D. Huiling, Y. Xi, J. Phys. Chem. Solids 63 (2002) 2123. [5] J.C. Nino, I.M. Reaney, M.T. Lanagan, C.A. Randall, Mater. Lett. 57 (2002) 414. [6] Y.P. Hong, K.H. Ko, H.J. Lee, G.-K. Choi, S.H. Yoon, K.S. Hong, Thin Solid Films 516 (2008) 2195. [7] R.W. Vest, Ferroelectrics 102 (1990) 53. [8] O.L. Alves, C.M. Ronconi, A. Galembeck, Quim. Nova 25 (2002) 69. [9] C.M. Ronconi, O.L. Alves, Thin Solid Films 441 (2003) 121. [10] C.M. Ronconi, D. Gonc¸alves, N. Suvorova, O.L. Alves, E.A. Irene, J. Phys. Chem. Solids 70 (2009) 234. [11] J.Y. Kim, D.-W. Kim, H.S. Jung, K.S. Hong, J. Eur. Ceram. Soc. 26 (2002) 2161. [12] Z.G. Zou, J.H. Ye, H. Arakawa, Mater. Sci. Eng. B-Solid State Mater. Adv. Technol. 79 (2001) 83. [13] Z.G. Zou, J.H. Ye, H. Arakawa, J. Mater. Res. 15 (2000) 2073. [14] Z.G. Zou, H. Arakawa, J. Photochem. Photobiol. A 158 (2003) 145. [15] L.L. Garza-Tovar, L.M. Torres-Martínez, D.B. Rodríguez, R. Gómez, G. del Angel, J. Mol. Catal. A-Chem. 247 (2006) 283. [16] Q. Zhou, B.J. Kennedy, V. Ting, R.L. Withers, J. Solid State Chem. 178 (2005) 1575. [17] M.W. Lufaso, T.A. Vanderah, I.M. Pazos, I. Levina, R.S. Rotha, J.C. Nino, V. Provenzano, P.K. Schenck, J. Solid State Chem. 179 (2006) 3900. [18] W. Somphon, V. Ting, Y. Liu, R.L. Withers, Q. Zhou, B.J. Kennedy, J. Solid State Chem. 179 (2006) 2495. [19] G.M. Vest, S. Singaram, Mater. Res. Soc. Symp. Proc. 60 (1986) 35. [20] V.A. Burmistrov, D.G. Klenhchev, V.N. Konev, R.N. Pletnev, Zh. Neorg. Khim. 30 (1985) 1959. [21] Z. Zou, J. Ye, H. Arakawa, Chem. Phys. Lett. 333 (2001) 57. [22] B.D. Cullity, Elements in X-Ray Diffraction, Addison-Wesley, Boston, 1967. [23] C.M. Ronconi, O.L. Alves, Mol. Cryst. Liquid Cryst. 374 (2002) 275. [24] C.M. Ronconi, O.L. Alves, R.E. Bruns, Thin Solid Films 517 (2009) 2886. [25] J.J. Xu, A.S. Shaikh, R.W. Vest, Thin Solid Films 161 (1988) 273. [26] S. Mridha, D. Basak, Mater. Res. Bull. 42 (2007) 875. [27] R. Swanepoel, J. Phys. E-Sci. Instrum. 16 (1983) 1214. [28] J.C. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E-Sci. Instrum. 9 (1976) 1002. [29] S.B. Qadri, H. Kim, H.R. Khan, J. Mater. Res. 15 (2000) 21. [30] T. Pilvi, T. Hatanpää, E. Puukilainen, K. Arstila, M. Bischoff, U. Kaiser, N. Kaiser, M. Leskelä, M. Ritala, J. Mater. Chem. 17 (2007) 5077. [31] S.H. Hu, Z.G. Hu, G.S. Wang, X.J. Meng, J.A. Chu, N. Dai, Mater. Res. Bull. 39 (2004) 1223. [32] M.G. Krishna, A.K. Bhattacharya, Mater. Sci. Eng. B-Solid State Mater. Adv. Technol. 86 (2001) 41. [33] M.B. Sahana, C. Sudakar, G. Setzler, A. Dixit, J.S. Thakur, G. Lawes, R. Naik, Appl. Phys. Lett. 93 (2008) 231909. [34] E. Burstein, Phys. Rev. 93 (1954) 632. [35] T.S. Moss, Proc. Phys. Soc. Lond. B 67 (1954) 775. [36] L.E. Brus, J. Chem. Phys. 80 (1984) 4403. [37] C.H. Ong, H. Gong, Thin Solid Films 445 (2003) 299. [38] B. Pejova, I. Grozdanov, Mater. Lett. 58 (2004) 666.
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Preparation and characterization of powders and thin films of Bi2 AlNbO7 and Bi2 InNbO7 pyrochlore oxides Zaine Teixeira ∗ , Larissa Otubo, Rubia Figueredo Gouveia, Oswaldo Luiz Alves Solid State Chemistry Laboratory, Institute of Chemistry, University of Campinas (UNICAMP), P.O. Box 6154, 13083-970. Campinas, SP, Brazil
a r t i c l e
i n f o
Article history: Received 2 November 2009 Received in revised form 8 January 2010 Accepted 8 July 2010 Keywords: Thin films Atomic force microscopy (AFM) Oxides Optical properties
a b s t r a c t Powders and thin films of Bi2 InNbO7 and Bi2 AlNbO7 with pyrochlore-type structures were prepared by metalorganic decomposition processes (MOD). The structural evolution of the powders was characterized by X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). The powders were obtained at lower temperatures and faster than with conventional solidstate reactions. The films were further characterized by ultraviolet–visible–near-infrared (UV–vis–NIR) absorbance spectroscopy and atomic force microscopy (AFM). A pyrochlore-type phase in the films was obtained at even lower temperatures than the powders. These presented high transmittances in the vis–NIR range, uniform thicknesses and low roughnesses. A blue shift in the optical gap was observed as the thickness decreased. This was mainly attributed to the Brus effect, since the films showed the presence of nano-sized clusters. The film properties make them very interesting for technological applications such as in photocatalytic processes. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Pyrochlore-type structures have been the subject of numerous studies due to their wide spectrum of characteristics, such as their electrical, magnetic, dielectric, optical, and catalytic properties [1]. The general formula of pyrochlore oxides is described as A2 B2 O6 O, with four crystallographically non-equivalent kinds of atoms, a Fd3m space group and eight molecules per unit cell. The pyrochlore oxides present mainly ionic character, which allows a number of crystallographic substitutions as long as ionic radii and neutrality are respected. Many phases with A and B elements in higher oxidation states exhibit dielectric, piezo, and ferroelectric properties [1]. The pyrochlores show several technological applications such as: gas sensors (Cd2 Sb2 O7 ), transistors (Cd2 Nb2 O7 ), thermistors (Bi2 Ru2 O7 ), switching elements and thin film resistors (Cd2 Os2 O7 and Ca2 Os2 O7 ) [2,3]. Furthermore, the Bi2 O3 –ZnO–Nb2 O5 (BZN) pyrochlore systems are promising materials for electronic devices in microwave communications [4,5]. Recently, thin films of PbO–ZnO–Nb2 O5 pyrochlore have shown high-permittivity and low-loss dielectric tunable that make this material promising for wireless communication [6]. Pyrochloretype compounds are obtained mainly by solid-state reactions [1]. An alternative technique is the metalorganic decomposition pro-
∗ Corresponding author. E-mail addresses: [email protected] (Z. Teixeira), [email protected] (O.L. Alves). URL: http://lqes.iqm.unicamp.br (Z. Teixeira). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.07.009
cess (MOD), which produces films and powders faster and at lower temperatures compared to conventional methods. In this technique, an organic solution containing a stoichiometric mixture of metalorganic components is pyrolysed, generating the oxide [7,8]. Some examples of thin films of pyrochlore oxides obtained by this method are Cd2 Nb2 O7 that presented high electrical resistivity [9,10], and Bi–Zn–Ti–Nb–O (BZTN) with pronounced dielectric properties [11]. The Bi2 MNbO7 (M = Al, In, Fe, Ga) pyrochlores obtained by solid-state reactions presented photocatalytic properties in the production of H2 and O2 from aqueous methanol and cerium sulfate solutions [12–14]. In these papers, the authors suggested that M3+ and Nb5+ , doping B2 site in A2 3+ B2 4+ O7 might cause an increase in hole (carrier) concentrations and might provide a change in magnetic, electrical transport and photophysical properties [14]. A sol–gel route for the preparation of Bi2 MNbO7 (M = Al, In, Fe, Sn) showed that the solids prepared at 400 ◦ C presented higher photocatalytic activity for methylene blue degradation than similar compounds prepared by solid-state reaction or even TiO2 P25 [15]. Recent papers with regards to these Bi2 MNbO7 solids have mainly focused on structural investigations and dielectric properties [16–18]. In this work, powder and thin films preparations by MOD processes of Bi2 InNbO7 and Bi2 AlNbO7 pyrochlore-type oxides are described. The optical properties of the films were also investigated. 2. Experimental details Bismuth(2-ethylhexanoate), Bi(hex)3 and niobium ethoxide were purchased from Strem Chemicals. The 2-ethyl-hexanoic acid, H(hex), was purchased from Across Organics. Indium chloride, aluminum chloride and ammonium hydroxide
Z. Teixeira et al. / Materials Chemistry and Physics 124 (2010) 552–557 were purchased from Merck. Commercially available reagents were used without further purification. Niobium tri-(ethoxy)-di-(2-ethylhexanoate) precursor, Nb(OEt)3 (hex)2 , was prepared by a metathesis reaction described elsewhere [9,19]. Aluminum(2-ethylhexanoate), AlOH(hex)2 , and indium(2-ethylhexanoate), In(hex)3 , precursors were obtained by double-decomposition from the ammonium salts as described by Vest and Singaram [19]. The precursor solutions for the MOD process were obtained by dissolution of the individual precursors for Bi2 InNbO7 in xylene and for Bi2 AlNbO7 in dried tetrahydrofuran. The final concentrations were chosen to obtain 0.09 and 0.18 mol L−1 Bi2 InNbO7 solutions, which were labeled In1 and In2, respectively. The Bi2 AlNbO7 final concentrations were 0.045 and 0.09 mol L−1 , which were labeled Al1 and Al2, respectively. Two concentrations were used to evaluate how this factor affects film optical properties, thickness, crystallite size and average transmission. In MOD powder preparation, an aliquot (3 mL) of the precursor solutions (In1 and Al1) was placed into a platinum crucible and pyrolyzed in a muffle using a heating rate of 10 ◦ C min−1 , from room temperature to a plateau, ranging from 400 to 1100 ◦ C for each sample, each being held at the final temperature for 4 h. For comparison, solids were also obtained by solid-state reactions as described by Zou and co-workers [12]. The metal oxides were weighted in the stoichiometric ratio, mixed in an agate mortar, pressed into pellets, placed into a platinum crucible and heated in air at 1100 ◦ C for 2 h. The solids were re-mixed and re-pressed for 3 times following this procedure. In the final step, the pellets were maintained at 1100 ◦ C for 48 h. The films were obtained by the dip-coating method from the two concentrations for each composition. The solution precursors were deposited on borosilicate glass slides at a 2 cm min−1 rate. After coating, the film was removed from one of the slide faces with a piece of cotton. The samples were placed in a quartz tube furnace preheated to 400 ◦ C for 15 min for the pyrolysis step. The coating process was repeated five times in order to get the desired thickness. The samples were finally annealed at 600 ◦ C under an ambient atmosphere for 8 h. The individual precursors were investigated by thermogravimetric analysis (TGA), infrared spectroscopy (IR), X-ray diffraction (XRD) and gravimetric characterizations (data not shown). The precursor solutions were investigated by TGA (Shimadzu 50-WS) using a heating rate of 10 ◦ C min−1 under synthetic air (50 cm3 min−1 ). The powders and film samples were characterized by X-ray diffraction (Shimadzu XRD-6000, Cu K␣, = 0.154 nm), and scanning electron microscopy (SEM) (Jeol JSM-6063LV) with secondary electrons or energy dispersive spectroscopic (EDS) detection. Optical characterization of the films was carried out by UV–vis–NIR absorbance (Cary 5G spectrophotometer, Varian). The film thicknesses and roughnesses were measured by a perfilometer (Alpha Step 200). Atomic force microscopy (AFM) experiments were performed on a Shimadzu WET-SPM 9500J3 instrument. The noncontact AFM mode was used to obtain topography information about the thin film surface. Image processing was analyzed in a PC microcomputer using Shimadzu software.
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Fig. 1. (—) TG and (- - -) DTG curves of stoichiometric precursor solutions used in the preparation of (a) Bi2 InNbO7 (0.18 mol L−1 ) and (b) Bi2 AlNbO7 (0.09 mol L−1 ).
tograms, mainly the (1 1 1), (3 1 1), and (2 2 2) ones. The atoms in structural positions B (16d) and A (16c) for the space group Fd3m have diffraction on opposite faces for the Miller index [20]. Heavy atoms in A sites results in a reduction of intensity of the peaks which have an odd sum of their Miller index. Thus, in the present compounds, we propose that the octahedral BO6 are first organized, given that the (3 1 1) intensity is higher than the (2 2 2) intensity in the solids treated at lower temperatures. At 700 ◦ C, an inversion in the intensity of (3 1 1) and (2 2 2) is observed, which may be related to A site organization. Thus, the pyrochlore-type structure is
3. Results and discussion The individual precursors showed the required characteristics for the MOD process: ease of synthesis, decomposition without metal evaporation or carbon residues, and obtainment of metal oxides with phase purity [7]. In the precursor solution characterization, the thermogravimetric curves are important to determine the minimum pyrolysis temperature as well as to suggest the process decomposition steps. Fig. 1 shows the thermogravimetric/differential thermogravimetric (TG/DTG) curves of the precursor solutions. The minimum pyrolysis temperature observed was 325 ◦ C for Bi2 AlNbO7 and for 304 ◦ C for Bi2 InNbO7 . Vest [7] suggested that the metalorganic precursor decomposition mechanism involves the formation of free radicals by thermolysis, followed by a fragmentation of these radicals in a very fast oxidative chain reaction, suggesting a “domino” type mechanism. Thus, the decomposition temperature decreases as the chain length of R increases, as the oxygen partial pressure increases, or as the degree of branching of R increases [7]. Gravimetric analysis of the pyrolysed solution showed a difference lower than 1% from that expected for Bi2 MNbO7 (M = In or Al) compounds, showing that the oxides were obtained in the desired stoichiometry. Fig. 2 shows the diffraction pattern of the solids prepared at different temperatures, in comparison to the product obtained by the solid–solid reaction. Pyrochlore-type structures are related with high variations in the intensities of peaks observed in diffrac-
Fig. 2. Pyrochlore-type solids of (a) Bi2 InNbO7 (0.18 mol L−1 ) and (b) Bi2 AlNbO7 (0.09 mol L−1 ). Structural evolution by MOD process from 300 to 1000 ◦ C and solid–solid reaction (s–s).
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Fig. 3. SEM images: Bi2 InNbO7 powders obtained by (a) MOD at 600 ◦ C, (b) MOD at 900 ◦ C, and (c) by a solid-state reaction; Bi2 AlNbO7 powders obtained by (d) MOD at 600 ◦ C, (e) MOD at 900 ◦ C, and (f) by a solid-state reaction.
organized above 700 ◦ C. In the Bi2 InNbO7 compounds, we observed not only the peaks related to the pyrochlore structure, but also an additional peak at 30.4◦ , which is related to the maximum intensity peak of In2 O3 . It means that indium oxide as a phase impurity was detected, whereas Bi2 InNbO7 with defects in the BO6 site is a major phase. This peak totally disappears in treatments higher than 900 ◦ C. For the Bi2 AlNbO7 solids, additional peaks with smaller intensities are observed for compounds from both the solid-state reaction and from MOD. We also observed this same pattern in the literature diffraction results [21], but the authors did not suggest a phase impurity even after Rietveld analyses. Fig. 3 shows the SEM images of the samples. The EDS analysis of Bi2 InNbO7 showed that the solids are homogeneous, within the detection of this technique. For Bi2 AlNbO7 solids, however, we observed a higher density of aluminum and oxygen by EDS microanalysis (circled areas). This major impurity is not detectable by XRD most probably because of a low crystallinity that is commonly observed in aluminum oxides. The samples at 600 ◦ C show higher porosity, which is due to solvent evaporation and decomposition product during the pyrolysis step. The solids at 900 ◦ C present irregular rounded particles. Finally, we observed sintering characteristics for the solid-state reaction. In summary, the MOD process showed advantages for preparation with regards to lower time (6 h instead of 54 h) and temperature (900 ◦ C instead of 1100 ◦ C). Fig. 4 shows the XRD pattern of the films that were indexed to pyrochlore-type structures. The films were calcinated at 600 ◦ C, in which the substrate could be heated without damage. Moreover, a lower crystallization temperature is expected for films compared to powders, since there is a dimension reduction from bulk to one or two dimensions. Besides, a preferred orientation may occur in the substrate, which may induce phase purity. In the powder XRD characterization (Fig. 2), we observed at 600 ◦ C a peak at 30.54◦ , ascribed to an In2 O3 impurity, which had a relative intensity even higher than at 33.34◦ (4 0 0). The absence of a peak at 30.54◦ (Fig. 4) means the films were obtained with phase purity. The crystallite size estimate calculated by the Scherrer equation [22] from the diffractograms (Fig. 4) is shown in Table 1, showing that the films are nanostructured. SEM images of the samples (Fig. 5) show that the films are homogeneous with nano-sized clusters. A large number of thin films obtained by the MOD technique have porous morphology.
Fig. 4. X-ray diffraction patterns of (In) Bi2 InNbO7 and (Al) Bi2 AlNbO7 films deposited on vitreous substrates.
Table 1 Crystallite size estimate by the Scherrer equation for thin film samples. Thin film sample
Crystallite size (nm)
In1
In2
Al1
Al2
16.5 ± 2.1
20.7 ± 1.5
11.3 ± 5.0
12.0 ± 2.6
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Fig. 5. SEM images of (In) Bi2 InNbO7 and (Al) Bi2 AlNbO7 films.
Thin films of Cd2 SnO4 presented large porous or even dentrite ramifications depending on the withdrawal rates and concentration levels [23,24]. In another work, tin-doped indium oxide (ITO) showed non-uniformity at low pyrolysis temperatures (∼150 ◦ C), while the films obtained by a pyrolysis step at 550 ◦ C were uniform [25]. These authors suggested that at the precursor decomposition temperature, the viscosity is diminished. Consequently, a contact angle, depending on substrate wettability, turns into equilibrium on the substrate and the liquid may develop into discrete drops. Hence, a temperature above the precursor pyrolysis temperature after the coating process is important to obtain a homogeneous film. This detail suggests our films are uniform as the temperature for pyrolysis is sufficiently high to exclude drop or ramification formations. Further, the films (Fig. 5) have nano-sized clusters. In the SEM images for the Al samples, we observe that the thicker films have large grains. The possibility for graining growth is attributed to a change from two to three dimensions increasing the sample thickness. In films of ZnO obtained by the sol–gel technique, the authors also observed that the surface morphology showed that the grains become more uniform and bigger as the film thickness increased [26]. Fig. 6 shows the optical spectra in the UV–vis–NIR range. The films have interference fringes that are characteristic of homogeneity and low roughness as well as high transmittance in the vis–NIR range. The thickness (d), refractive index (n), and the optical gap were calculated from the transmission spectra as described previously [27,28]. Considering the thick substrate in the absence of a film, the substrate refractive index, s, is described as: 1 s= + Ts
1 Ts 2
fringes in the cut off region may not be used [27]. N = 2s
s2 + 1 TM − Tm + TM Tm 2
(3.3)
The film thicknesses may be calculated by: d=
1 2 2(1 n2 − 2 n1 )
(3.4)
where n1 and n2 are the refractive indices of two adjacent maxima or minima in the interference fringes with wavelengths 1 and 2 [27,28]. The optical band gap values were estimated by extrapolations of the straight regions of the absorption coefficient, (˛2 ), versus photon energy, h, by the least-squares method. The absorption coefficient ˛ was calculated by the equation: ˛ = ln
1 T
d
(3.5)
where T is the transmitance in the UV–vis–NIR spectra, and d is the thickness of the sample [29].
1/2 −1
(3.1)
where Ts is the transmittance of the substrate. The film refractive index, n, is described by: n = [N + (N 2 − s2 )
1/2 1/2
]
(3.2)
where N is obtained as a refractive index function as well as successive transmittance maximum (TM ) and minimum (Tm ) in the weak absorption region. In order to minor the errors, the interference
Fig. 6. UV–vis–NIR transmittance spectra of (In) Bi2 InNbO7 and (Al) Bi2 AlNbO7 film samples.
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Fig. 7. AFM three-dimensional images acquired from the Bi2 AlNbO7 thin film surfaces: (a) Al1; (b) Al2. Images recorded at higher magnification: (c) Al1 and (d) Al2.
AFM three-dimensional images were acquired from Bi2 AlNbO7 thin film surfaces. The results are shown in Fig. 7. The topographic images show an approximately spherical morphology: Al1 and Al2 particles present structures with 62 nm and 45 nm average diameters, respectively. To verify detectable topography changes, AFM images were acquired from the same sample used in this work but at a higher magnification, as shown in Fig. 7(c) and (d). This figure shows that the Al1 topographic surface is smoother than the Al2 surface. The surface roughness can be quantitatively identified by the root mean squared (rms) roughness. The rms roughness value (12.30 nm) obtained for the Al1 thin film is smaller than that of the Al2 sample (16.90 nm). The surface roughness results are very interesting because the optical properties are closely related to the microstructure of the thin films. The optical parameters, as well as the thicknesses and roughnesses measured by the perfilometer, are shown in Table 2. The values represent the medium ± S.D. for 10 measurements in the perfilometer on duplicate samples. The optical calculations for thickness, refractive index, and optical gap are represented as the average for duplicate samples ± S.D.
The low roughnesses and optical parameters such as high refractive index and low band gap values show interesting application possibilities of the films. A low roughness is strictly related to a low light scattering and consequently makes the films good candidates for optical application. For example, in the MgF2 thin films, obtained by atomic laser deposition (ALD), the roughness was dependent on the deposition temperature varying from 250 to 400 ◦ C. The film obtained at the highest temperature presented a high roughness that disables its use for optical devices [30]. Bi2 InNbO7 and Bi2 AlNbO7 solids have optical gaps at 2.7 and 2.9 eV, respectively [12], whereas in our films the optical gap is higher (Table 2) probably due to the nano-sized cluster film structures. Generally, films with low thicknesses show high values of optical band gap. In thin films of Bi3.25 La0.75 TiO3 O12 , prepared by the sol–gel technique, the band gap values were 4.05, 3.75, and 3.61 eV for thicknesses of 173, 415 and 740 nm, respectively [31]. These authors attributed these differences to stoichiometric changes in the annealing process. In V2 O5 films a red-shift from 2.40 to 2.25 eV was observed for samples with 150 and 400 nm thicknesses, respectively [32]. The XPS investigation of these samples
Table 2 Thickness, roughness and optical parameters for Bi2 InNbO7 and Bi2 AlNbO7 thin films. Sample
In1 In2 Al1 Al2
Thickness (d) (nm)
Roughness (nm)
Optical
Perfilometer
– 472 ± 41 – 463 ± 2
116 511 137 470
± ± ± ±
22 15 4 23
11.2 7.8 4.8 13.8
± ± ± ±
2.8 2.2 0.5 2.7
Refractive index (n) at 1000 nm
1.98 1.96 1.79 2.03
± ± ± ±
0.14 0.14 0.14 0.17
Optical gap (Eg ) (eV)
3.59 3.44 3.45 3.33
± ± ± ±
0.02 0.01 0.01 0.02
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showed only the thicker film was stoichiometric, showing that the band gap variations were due to stoichiometric changes. Studies of V2 O5 , Nb2 O5 and Y2 O3 films obtained by magnetron sputtering (MS) or ion beam sputtering (IBS) with thicknesses between 50 and 400 nm, shown that the optical constants depended on the thicknesses. This was not only attributed to changes in the stoichiometry but also to changes in the band structures by quantum confinement [32]. The authors postulated that, independent on the material or preparation technique, with the thickness diminishing, there are increases in the absortivity and band gap, as well as a decrease in the refractive index. Thin films of Sn2 O–Fe2 O3 showed band gap increasing from 2.3 to 3.89 eV with decreasing the crystallite sizes, which was attributed to the quantum confinement as well [33]. Theories for the doping effect or quantum confinement may be explained by the Burstein–Moss shift [34,35] and the Brus theory [36], respectively. The Burstein–Moss shift is related to high doping effects of semiconductors, which results in a high carrier concentration, the so-called degenerate semiconductors [34,35,37]. It is worth mentioning that we tried to measure the sample resistances by the four probe method, which is widely employed to measure resistivity of semiconductor samples, but the samples did not show semiconductor properties. Thus, the Burstein–Moss shift theory may not be suitable for our films. The Brus model is related to the band gap shift as a function of particle sizes as: E =
h2 + 8R2
1 1 + ∗ m∗e mh
−
C1 e2 ∗ − C2 ERy εR
(3.6)
where R is the exciton radius, h is the Planck constant, m∗e and m∗h are the effective masses of electrons and holes, respectively. C1 and C2 are constants; e is the elementary charge, ε is the dielectric constant ∗ is the Rydberg effective constant. The of the semiconductor and ERy first term represents energy quantization by the particle-in-a-box quantum localization energy with 1/R2 dependence. The second term represents the Coulomb energy with 1/R dependence. In the limit of large R, the E values approach the energy, Eg , for the bulk [37,38]. In our work, we suggest that the main effect of the band gap shift is related to the Brus model due to the nano-sized clusters suggested by SEM and AFM microscopies and XRD. However, some possible contributions of a stoichiometry variation cannot be totally discarded. 4. Conclusion In this work, we successfully obtained powders and thin films of Bi2 InNbO7 and Bi2 AlNbO7 by the MOD process, presenting, as advantages, faster preparations and lower temperatures. In addition, the Bi2 AlNbO7 thin film compounds were obtained without impurities when compared to the solid-state reaction or to the MOD process for the powders. The films were uniform, with nanometric dimensions and high UV–vis transmitance. Film preparation makes
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feasible the use of these materials in technological applications, such as photocatalytic processes. Acknowledgments The authors acknowledge the financial support of the Fundac¸ão de Amparo à Pesquisa do Estado de São Paulo (FAPESP), the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and the Millennium Institute for Complex Materials (PADCT/MCT). References [1] M.A. Subramanian, G. Aravamudam, G.V. Subba Rao, Prog. Solid State Chem. 15 (1983) 55. [2] B. Li, J. Zhang, J. Am. Ceram. Soc. 72 (1989) 2377. [3] W.R. Cook, H. Jaffe, Phys. Rev. 89 (1953) 1297. [4] D. Huiling, Y. Xi, J. Phys. Chem. Solids 63 (2002) 2123. [5] J.C. Nino, I.M. Reaney, M.T. Lanagan, C.A. Randall, Mater. Lett. 57 (2002) 414. [6] Y.P. Hong, K.H. Ko, H.J. Lee, G.-K. Choi, S.H. Yoon, K.S. Hong, Thin Solid Films 516 (2008) 2195. [7] R.W. Vest, Ferroelectrics 102 (1990) 53. [8] O.L. Alves, C.M. Ronconi, A. Galembeck, Quim. Nova 25 (2002) 69. [9] C.M. Ronconi, O.L. Alves, Thin Solid Films 441 (2003) 121. [10] C.M. Ronconi, D. Gonc¸alves, N. Suvorova, O.L. Alves, E.A. Irene, J. Phys. Chem. Solids 70 (2009) 234. [11] J.Y. Kim, D.-W. Kim, H.S. Jung, K.S. Hong, J. Eur. Ceram. Soc. 26 (2002) 2161. [12] Z.G. Zou, J.H. Ye, H. Arakawa, Mater. Sci. Eng. B-Solid State Mater. Adv. Technol. 79 (2001) 83. [13] Z.G. Zou, J.H. Ye, H. Arakawa, J. Mater. Res. 15 (2000) 2073. [14] Z.G. Zou, H. Arakawa, J. Photochem. Photobiol. A 158 (2003) 145. [15] L.L. Garza-Tovar, L.M. Torres-Martínez, D.B. Rodríguez, R. Gómez, G. del Angel, J. Mol. Catal. A-Chem. 247 (2006) 283. [16] Q. Zhou, B.J. Kennedy, V. Ting, R.L. Withers, J. Solid State Chem. 178 (2005) 1575. [17] M.W. Lufaso, T.A. Vanderah, I.M. Pazos, I. Levina, R.S. Rotha, J.C. Nino, V. Provenzano, P.K. Schenck, J. Solid State Chem. 179 (2006) 3900. [18] W. Somphon, V. Ting, Y. Liu, R.L. Withers, Q. Zhou, B.J. Kennedy, J. Solid State Chem. 179 (2006) 2495. [19] G.M. Vest, S. Singaram, Mater. Res. Soc. Symp. Proc. 60 (1986) 35. [20] V.A. Burmistrov, D.G. Klenhchev, V.N. Konev, R.N. Pletnev, Zh. Neorg. Khim. 30 (1985) 1959. [21] Z. Zou, J. Ye, H. Arakawa, Chem. Phys. Lett. 333 (2001) 57. [22] B.D. Cullity, Elements in X-Ray Diffraction, Addison-Wesley, Boston, 1967. [23] C.M. Ronconi, O.L. Alves, Mol. Cryst. Liquid Cryst. 374 (2002) 275. [24] C.M. Ronconi, O.L. Alves, R.E. Bruns, Thin Solid Films 517 (2009) 2886. [25] J.J. Xu, A.S. Shaikh, R.W. Vest, Thin Solid Films 161 (1988) 273. [26] S. Mridha, D. Basak, Mater. Res. Bull. 42 (2007) 875. [27] R. Swanepoel, J. Phys. E-Sci. Instrum. 16 (1983) 1214. [28] J.C. Manifacier, J. Gasiot, J.P. Fillard, J. Phys. E-Sci. Instrum. 9 (1976) 1002. [29] S.B. Qadri, H. Kim, H.R. Khan, J. Mater. Res. 15 (2000) 21. [30] T. Pilvi, T. Hatanpää, E. Puukilainen, K. Arstila, M. Bischoff, U. Kaiser, N. Kaiser, M. Leskelä, M. Ritala, J. Mater. Chem. 17 (2007) 5077. [31] S.H. Hu, Z.G. Hu, G.S. Wang, X.J. Meng, J.A. Chu, N. Dai, Mater. Res. Bull. 39 (2004) 1223. [32] M.G. Krishna, A.K. Bhattacharya, Mater. Sci. Eng. B-Solid State Mater. Adv. Technol. 86 (2001) 41. [33] M.B. Sahana, C. Sudakar, G. Setzler, A. Dixit, J.S. Thakur, G. Lawes, R. Naik, Appl. Phys. Lett. 93 (2008) 231909. [34] E. Burstein, Phys. Rev. 93 (1954) 632. [35] T.S. Moss, Proc. Phys. Soc. Lond. B 67 (1954) 775. [36] L.E. Brus, J. Chem. Phys. 80 (1984) 4403. [37] C.H. Ong, H. Gong, Thin Solid Films 445 (2003) 299. [38] B. Pejova, I. Grozdanov, Mater. Lett. 58 (2004) 666.