- Email: [email protected]
The role of the Eu3+ ions in structure and photoluminescence properties of SrBi2Nb2O9 powders
Optical Materials 31 (2009) 995–999
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
The role of the Eu3+ ions in structure and photoluminescence properties of SrBi2Nb2O9 powders Diogo P. Volanti a,*, Ieda L.V. Rosa b, Elaine C. Paris c, Carlos A. Paskocimas d, Paulo S. Pizani e, José A. Varela a, Elson Longo a a
Institute of Chemistry, São Paulo State University, UNESP, 14800-900 Araraquara, SP, Brazil Department of Chemistry, Federal University of São Carlos, UFSCar, 13565-905 São Carlos, SP, Brazil c Department of Chemistry, Federal University of Paraíba, UFPB, 58059-900 João Pessoa, PB, Brazil d Department of Mechanical Engineering, Federal University of Rio Grande do Norte, UFRN, 59072-970 Natal, RN, Brazil e Department of Physics, Federal University of São Carlos, UFSCar, 13565-905 São Carlos, SP, Brazil b
a r t i c l e
i n f o
Article history: Received 8 July 2008 Received in revised form 15 October 2008 Accepted 7 November 2008 Available online 24 December 2008 Keywords: Photoluminescence Strontium bismuth niobate Europium Polymeric precursor method
a b s t r a c t This work rationalized for the first time the key role of Eu3+ ion in the doping process of SrBi2Nb2O9 (SBN) compound. This process allows us to understand the role of Eu3+ as lattice modifier as well as to obtain information on the crystalline structure surrounding. Therefore, the SBN and Eu3+ doped SBN (SBN:Eu) were synthesized using the polymeric precursor method (PPM). Scanning electron microscope (SEM) images reveal the strong influence of europium on the SBN microstructure. Thermogravimetry (TG) and differential thermal analysis (DTA) techniques were used to determine the weight loss and changes associated with phase transitions in SBN during thermal evolution. The behavior of the Eu3+ lattice modifier was followed in a long-range order by X-ray diffraction (XRD), while Fourier transform Raman (FTRaman) spectroscopy was used to analyze the short-range order. To this end, the SBN orthorhombic phase was observed for all samples heat treated from 400 to 700 °C for 2 h. In addition, photoluminescence measurements were employed to study the structural modifications in SBN lattice. The characteristic red emission of the Eu3+ using the 488 nm exciting wavelength of an argon–ion laser was distinctly observed for SBN:Eu samples heat treated from 550 to 700 °C. Europium characteristic emission bands are related to 5D1 ? 7FJ (J = 0–2) transitions at 538 and 555 nm, as well as the 5D0 ? 7FJ (J = 0–4) ones at 580, 592, 615, 653 and 695 nm. By means of the emission spectra analyses it was possible to predict that the Eu3+ ions are located at sites of higher symmetry, since the relative area of the (5D0 ? 7F2)/ (5D0 ? 7F1) transitions for the SBN:Eu samples decrease from 3.82 to 2.60 with increasing temperature from 550 to 700 °C. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Photoluminescence (PL) technique is a powerful tool to understand the energy levels of materials, providing fundamental information that allows associating the structural properties to the band gap of a given material [1–5]. The spectroscopic properties of Eu3+ doped materials have been widely studied based on the crystal field splitting of the 2S+1LJ manifolds of its 4f6 configuration [6,7]. Eu3+ presents some characteristic properties enabling it to be frequently used as a probe. The excited states 5DJ (J = 0–3) are well separated from the ground terms 7FJ0 (J0 = 0–6). The main emitting level, 5D0 and the ground state 7F0 are non-degenerated; therefore the 5D0 ? 7F0 transition usually appears as a single peak in the photoluminescence spectra as the Eu3+ ions occupy identical site * Corresponding author. Tel.: +55 16 3301 6643. E-mail address: [email protected] (D.P. Volanti). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.11.006
symmetries. These properties promote the interpretation of the spectral data and provide information on the eventual existence of at least two different symmetry sites occupied by the Eu3+ ions [8]. 5D0 ? 7F1 transition is usually taken as a reference. It is allowed by magnetic dipole and its intensity is not considerably altered by the perturbing crystal field. The perovskite-like layered SrBi2Nb2O9 (SBN) is a member of the Aurivillius phases and can be represented by (Bi2O2)2+ (Am1BmO3m+1)2, where A is a divalent ion such as Sr2+, Ba2+ and Ca2+; B is a metal of valence (5+) generally using Nb5+ or Ta5+, and m is the number of the perovskite unit cells between the Bi2O2 layers [9]. The SBN orthorhombic phase is crystallized via a metastable fluorite-like phase in a cubic structure [10,11], and in this system stoichiometric deviations can occur producing the pyrochlore and fluorite intermediary phases [12]. In the literature, the luminescence properties were studied in the rareearth doped layered niobate and Aurivillius compounds [13–15]. Additionally, the polymeric precursor method (PPM) is a useful soft
996
D.P. Volanti et al. / Optical Materials 31 (2009) 995–999
ment. This approach renders a plausible description of the PL properties of SBN:Eu compound under laser excitation, as well as an interesting correlation between the Fourier transform Raman (FTRaman) spectroscopy, X-ray diffraction (XRD) and UV–visible (UV–vis) absorption. The present paper is organized as follows. In Section 2 the experimental methods of the Eu3+ doped SBN powder preparation are described, together with measurement techniques. The results are discussed proposing to explain the Eu3+ influence in the SBN structure in Section 3. Finally, Section 4 describes the main lines of the order–disorder behavior for SBN:Eu system.
2. Experimental 2.1. Synthesis
Fig. 1. TG/DTA profiles of the SBN and SBN:Eu powders: (a) SBN–TG, (b) SBN:Eu– TG, (c) SBN–DTA and (d) SBN:Eu–DTA.
chemical method for the preparation of several luminescence compounds as YNbO4 [16], Eu3+ or Pr3+ doped Gd3Ga5O12 [17] and Eu2+ doped calcium aluminates [18]. In a previous work it was established that the PL property in the SBN powders occur due to the presence of niobium complex clusters and b-Bi2O3 phase on the pure SBN lattice [19]. However, in the case of europium doped SBN, described in the present work, the rare-earth ion favors the arrangement of the orthorhombic phase to the detriment of the b-Bi2O3 one. The PL properties are discussed in relation to the dependence of the structural changes improved by thermal treat-
SBN and SBN:Eu powders were synthesized by the polymeric precursor method (PPM); and the experimental procedure is described as follows: NH4H2NbO(C2O4)33H2O (99.5%, CBMM) was dissoluted in H2O and Nb was precipitated as Nb(OH)5 by the addition of NH4OH. After filtration, Nb(OH)5 was complexed in aqueous citric acid solution. SrCO3 (99.9%, Aldrich), Bi2O3 (99.9%, Aldrich) and Eu2O3 (99.9%, Aldrich) were dissolved in concentrated H2NO3, and were added to the reactional mixture. The molar ratio between (Sr:Bi:Nb) and (Sr:Eu:Bi:Nb) was (1:2:2) and (0.99:0.01: 2:2), respectively. Ethylenediamine was dripped into the solution with constant stirring until pH 9. After homogenization, ethylene glycol was added to the solution. The molar ratio of citric acid related to the metal was fixed at 3.95 and the mass ratio citric acid/ ethylene glycol was 60/40. SBN and SBN:Eu resins were placed in a furnace and heat treated at 350 °C for 4 h to obtain polymer pyro-
Fig. 2. SEM images of (a) SBN and (b)SBN:Eu powders.
D.P. Volanti et al. / Optical Materials 31 (2009) 995–999
997
Fig. 1 illustrates the thermal analyses for the SBN and SBN:Eu powders, annealed at 350 °C for 4 h. The weight loss in approximately 100 °C is due to the complete dehydration of the powders as shown in Fig. 1a and b. The TG analysis in Fig. 1a presents a weight loss of 64.2% between 260 and 690 °C for the SBN and 36.5% in the range of 330–591 °C for the SBN:Eu (Fig. 1b). This fact can be attributed to the elimination of CO and CO2. Weight variation is not observed from 690 °C in TG (a) and 591 °C in TG (b). In the SBN–DTA curve (Fig. 1c) there are two exothermic peaks at approximately 460 and 600 °C that can be attributed to the formation of the b-Bi2O3 and SrBi2Nb2O9 phases, respectively. On the other hand, in SBN:Eu–DTA (Fig. 1d) for SBN:Eu a single exother-
mic peak at 475 °C can be related to the formation of the orthorhombic phase. The SEM micrographs (Fig. 2) reveal the strong influence of europium on the powder microstructures processed at 400 °C in oxygen atmosphere. Although both micrographs indicate a dense structure, the SBN sample (Fig. 2a) shows a different microstructure with the formation of spherical grains. These spherical grains are ascribed to the intermediate b-Bi2O3 phase detected in the XRD pattern and EDX analysis with quantitative results. However, in Fig. 2b it was not possible to observe the spherical grains (bBi2O3 phase). These spherical grains are not present probably due to the formation of the orthorhombic phase in SBN:Eu powders due to the influence of the Eu3+ doping. XRD analysis was used to follow the long-range order and the crystallization process of the samples with increasing of the annealing temperature. Fig. 3 shows the XRD patterns of the SBN:Eu powders heat treated from 400 to 700 °C for 2 h under oxygen atmosphere. The europium oxide, with cubic structure, appears in the (1 1 1) direction according to JCPDS card N°32-0380. On the other hand, the orthorhombic phase was observed through the assigned peaks according to JCPDS card N°49-0607, for all samples. In the short-range, the SBN:Eu structural behavior was analyzed by the FT-Raman spectra (Fig. 4). According to Nelis et al. [10] and Kojima et al. [20], the SBN orthorhombic phase was observed for all samples. However, the pure SBN orthorhombic phase is observed only for the material heat treated at temperatures up to 550 °C [19]. The modes located at 178, 210, 570, 687 and 848 cm1 are related to SBN orthorhombic phase [19]. In addition, the shift at 570 cm1 corresponds to an unbending sublattice mode and indicates that the equivalent and opposite displacements of positive and negative ions are slightly changed [21]. The peak located at 840 cm1 can be attributed to a symmetric stretching of the octahedral BO6 (B = Nb or Ta) [22]. The presence of NbO6 octahedral, which appears for the first time in the sample heat treated at 400 °C (see Fig. 4f–h) is in agreement with the XRD patterns (Fig. 3a–g). Therefore, the presence of Eu3+ in the SBN structure promotes the appearance of the orthorhombic phase at a lower temperature, since the presence of this phase in the pure material takes place only at around 600 °C. Thus, the Eu3+ ion is a lattice modifier since it promotes the premature formation of the orthorhombic phase without the intermediate b-Bi2O3 one. The modes at 98 and 206 cm1 can be attributed to the vibrations of A (Bi3+ or Eu3+) site ions. This result is in agreement with the Raman scat-
Fig. 3. XRD patterns of SBN:Eu powders heat treated at (a) 400, (b) 450, (c) 500, (d) 550, (e) 600, (f) 650 and (g) 700 °C for 2 h under oxygen atmosphere.
Fig. 4. FT-Raman spectra of the SBN:Eu powders heat treated at (a) 400, (b) 450, (c) 500, (d) 550, (e) 600, (f) 650 and (g) 700 °C for 2 h under oxygen atmosphere.
lysis. After that, these powders were heat treated at 400, 450, 500, 550, 600, 650 and 700 °C for 2 h under oxygen atmosphere at a heating rate of 5 °C min1. 2.2. Instruments After the preliminary heat treatment at 350 °C for 4 h, SBN and SBN:Eu powders were analyzed by differential thermal analysis (DTA) and thermogravimetry (TG) in a Netzsch STA 409 equipment. DTA and TG analyses were carried out using 10 mg of the samples at a heating rate of 5 °C min1 up to 750 °C, in synthetic air under a flow of 30 cm3 min1. The microstructure was examined with a scanning electron microscope (SEM), Zeiss DSM 940A and the powder compositions were determined by EDX analysis. The diffraction patterns of the crystalline powders were recorded in a Rigaku DMax 2500PC diffractometer. The power tube used was 6000 W in a 2h interval from 10° to 75°, using Cu-Ka radiation with a graphite monochromator. FT-Raman spectra were recorded in a Bruker RFS 100/S spectrometer, excited by a Nd:YAG laser at 1064 nm with a spectral resolution of 4.0 cm1. The spectral dependence of the optical reflectance, UV–vis, of the SBN and SBN:Eu samples was taken in the total reflection mode, using a Cary 5G equipment in the wavelength range from 300 to 800 nm. To measure the PL properties a U1000 Jobin-Yvon double monochromator coupled to a cooled Ga–As photomultiplier was used and a conventional photon counting system. The 488 nm exciting wavelength of an argon–ion laser was used, with the laser maximum output power kept within 30 mW. All the measurements were recorded at room temperature. 3. Results and discussion
998
D.P. Volanti et al. / Optical Materials 31 (2009) 995–999
Table 1 Band gap energies for the SBN and SBN:Eu powders. Band gap (eV)
SBN SBN:Eu
400 (°C)
450 (°C)
500 (°C)
550 (°C)
600 (°C)
650 (°C)
700 (°C)
1.87 2.52
2 2.55
2.15 2.66
2.17 2.67
2.2 2.67
2.7 2.7
2.72 2.72
tering study of La-doped SrBi2Nb2O9 prepared by the solid-state reaction [23]. The values of the gap energies can be obtained by the Wood and Tauc method [24]. The gap energies of the SBN and SBN:Eu powders treated at different temperatures are shown in Table 1. As the annealing temperature is increased, the gap value increases and reduces the delocalized states, organizing the crystalline system. This behavior indicates that the density of localized states in the band gap of the powders heat treated at a lower temperature is greater than those of the higher temperatures due to structural defects. Daldosso et al. [25] reported the local structure at around SrxBa1xNb2O6 doped with different Ln3+ ions by Optical spectroscopy. In this work, by means of EXAFS (Extended X-ray Absorption Fine Structure) investigation, it was verified that Eu3+ appears to enter mainly sites with coordination number 12 in SrxBa1xNb2O6 single crystals, indicating that this ion occupies only A1 site-types. Consequently, the presence of europium can favor the reduction of tensions in the SBN lattice, promoting an increase of the band gap values in the SBN:Eu samples. Fig. 5 depicts the spectral dependence of the absorbance for the SBN and SBN:Eu annealed at 400 °C, which is more disordered than SBN and SBN:Eu powders heat treated at 700 °C. The SBN-700 °C and SBN:Eu-700 °C powders show one well-defined absorption front while the SBN-400 °C and SBN:Eu-400 °C powders exhibit a typically continuous smooth absorption that increases as a function of the energy, suggesting the presence of localized states in the band gap. The values of the gap energies for these powders are depicted in Table 1. Fig. 6 illustrates PL spectra recorded at room temperature for the SBN:Eu powders heat treated at 400, 450, 500, 550, 600, 650 and 700 °C for 2 h in oxygen flow. The exciting wavelength of an argon–ion laser was 488.0 nm. The profile of the emission band is typical of a multiphonon process, i.e., a system in which relaxation occurs by means of several paths, involving the participation of numerous states within the band gap of the material. This
Fig. 5. UV–vis absorption spectra of (a) SBN-400 °C, (b) SBN:Eu-400 °C, (c) SBN700 °C and (d) SBN:Eu-700 °C, at room temperature.
Fig. 6. Room temperature PL spectra of SBN:Eu powders heat treated at (a) 400, (b) 450, (c) 500, (d) 550, (e) 600, (f) 650 and (g) 700 °C. The insert shows PL spectra of SBN heat treated at (h) 400 (i) 450 and (j) 500 °C, kexc = 488 nm.
behavior is related to the structural disorder of SBN:Eu and indicates the presence of additional electronic levels in the forbidden band of the material. The general aspects of the spectra are a broad band covering a large part of the visible spectra, from 490 to 725 nm. As can be seen, the structurally disordered powder annealed at 400 °C presents PL emission centered at 550 nm, and PL emission centered at 540 nm if annealed at 500 °C. The PL emission vanishes when the crystallization process takes place at 550 °C. Thus, the interesting PL properties of structural ordered–disordered SBN:Eu powders are in the range of 400 to 500 °C. We attributed that an increase in the lattice disorder is associated to the presence of [NbO5V zO ] or [SrO11V zO ] in the complex clusters, where V zO ¼ V xO , V O or V O [26,27]. With increasing of the lattice order, this complex vacancies vanishes and also the PL emission. According to the hypothesis proposed by Korzhik et al. [28], there are vacant localized states associated to defects, such as oxygen vacancies in the band gap. With the temperature increase, these vacancies are reduced, in which an electron from the conduction band loses its energy and reoccupies the energy levels of electrons (e0 )/hole (h) in the valence band. Therefore, most of the oxygen vacancies are vacancy complexes in the ordered–disordered structure. In Fig. 6a–c the broad intense luminescence band with a maximum PL at about 560 nm in the green region of the PL spectra is observed, hence the charge transference in the [NbO6]0 /[NbO5V O ] complex clusters may be responsible for the PL behavior resulting from the structural order–disorder in the SBN lattice. However, the increase of the number of [NbO6]0 clusters affects the structural order in the system, reducing the PL intensity [19,26,27]. We related the PL emission in the SBN:Eu lattice as a consequence of its structural evolution in terms of the structural order–disorder. Fig. 7 shows the emission spectra of the SBN:Eu samples heat treated at 550, 600, 650 and 700 °C for 2 h, excited at 488 nm using an argon–ion laser, performed at room temperature. The weak emission bands observed at 538 and 555 nm are ascribed to the Eu3+ transitions 5D1 ? 7FJ (J = 0–2). In these spectra the presence of the bands at 580, 592, 615, 653 and 695 nm were also observed, which are respectively attributed to the 5D0 ? 7FJ (J = 0–4) transitions of the Eu3+. The presence of two bands at 578 and 580 nm are related to the Eu3+. Thus, the 5D0 ? 7F0 transition indicates that the Eu3+ ions are located in at least two different chemical surroundings in the SBN lattice. The Eu3+ luminescence spectra show emission lines that extend from visible to near infrared range of the electromagnetic spectrum. The ratio of the (5D0 ? 7F2)/(5D0 ? 7F1) emission bands can be taken as the relative area of these bands
D.P. Volanti et al. / Optical Materials 31 (2009) 995–999
999
tic red emission of the Eu3+ is clearly observed for SBN:Eu where the annealed temperatures are higher than 550 °C. As the temperature increased the system became more symmetric in a shortrange order, as it is followed by the relative area decrease of the (5D0 ? 7F2)/(5D0 ? 7F1) transitions. In addition, it was possible concluding that the presence of Eu3+ collaborates to the formation of the orthorhombic phase in detriment of the b-Bi2O3 one. Thus, the presence of europium favors the reduction of tensions in the lattice of SBN, which favors the increase of the experimental values of the SBN:Eu band gap. Acknowledgements The authors gratefully acknowledge the agencies CNPq, CAPES, FAPESQ and FAPESP/CEPID 98/14324-0.
Fig. 7. Emission spectra of the SBN:Eu annealed at (a) 550, (b) 600, (c) 650 and (d) 700 °C for 2 h under oxygen atmosphere recorded at room temperature, kexc = 488 nm.
Table 2 Relative area of the (5D0 ? 7F2)/(5D0 ? 7F1) Eu3+ transitions in SBN:Eu samples heat treated from 550 to 700 °C. Temperature (°C)
Relative area
550 600 650 700
3.82 2.81 2.65 2.60
which are dependent on the environment around the Eu3+, since the 5D0 ? 7F2 transition exhibits a magnetic dipole character and is very sensible to the Eu3+ surrounding, while the 5D0 ? 7F1 transition has a magnetic dipole character in low symmetry systems, therefore its intensity has a low dependence of the Eu3+ environment [29]. The relative area of the (5D0 ? 7F2)/(5D0 ? 7F1), obtained from the emission spectra of the SBN:Eu samples heat treated at 550, 600, 650 and 700 °C are depicted in Table 2. By analysis of these data it is possible to suppose that the Eu3+ environment changes to a higher symmetry site when the annealed temperature is increased, since the (5D0 ? 7F2)/(5D0 ? 7F1) relative area decreases with the heat treatment. 4. Conclusions The soft chemical synthesis such as the PPM is a promising route for studying the crystallization process of the SBN:Eu system. Thermal analyses (TG/DTA) expose the phase changes in SBN that were confirmed by SEM. It was observed that the Eu3+ ion allows the characterization of the transformations of ordered–disordered states in the SBN lattice. The PL is a complementary technique for XRD and FT-Raman spectroscopy on the characterization of this material in a short, medium and long-range order. The characteris-
References [1] E.C. Paris, J.W.M. Espinosa, S. de Lazaro, R.C. Lima, M.R. Joya, P.S. Pizani, E.R. Leite, A.G. Souza, J.A. Varela, E. Longo, Chem. Phys. 335 (2007) 7. [2] R.C. Lima, J.W.M. Espinosa, M.F.C. Gurgel, E.C. Paris, E.R. Leite, M.R. Joya, P.S. Pizani, J.A. Varela, E. Longo, J. Appl. Phys. 100 (2006). [3] L.S. Cavalcante, M.F.C. Gurgel, A.Z. Simões, E. Longo, J.A. Varela, M.R. Joya, P.S. Pizani, Appl. Phys. Lett. 90 (2007). [4] A.T. de Figueiredo, S. de Lazaro, E. Longo, E.C. Paris, J.A. Varela, M.R. Joya, P.S. Pizani, Chem. Mater. 18 (2006) 2904. [5] M.F.C. Gurgel, J.W.M. Espinosa, A.B. Campos, I.L.V. Rosa, M.R. Joya, A.G. Souza, M.A. Zaghete, P.S. Pizani, E.R. Leite, J.A. Varela, E. Longo, J. Lumin. 126 (2007) 771. [6] W.T. Carnall, G.L. Goodman, K. Rajnak, R.S. Rana, J. Chem. Phys. 90 (1989) 3443. [7] J. Holsa, P. Porcher, J. Chem. Phys. 75 (1981) 2108. [8] O.L. Malta, H.F. Brito, J.F.S. Menezes, F. Silva, S. Alves, F.S. Farias, A.V.M. deAndrade, J. Lumin. 75 (1997) 255. [9] C. Pirovano, M.S. Islam, R.-N. Vannier, G. Nowogrocki, G. Mairesse, Sol. Stat. Ionics 140 (2001) 115. [10] D. Nelis, J.M. Calderon-Moreno, M. Popa, M.K. van Bael, J. Mullens, L.C. van Poucke, J. Eur. Ceram. Soc. 26 (2006) 409. [11] N.S. Prasad, K.B.R. Varma, Y. Takahashi, Y. Benino, T. Fujiwara, T. Komatsu, J. Solid State Chem. 173 (2003) 209. [12] T. Asai, E.R. Camargo, M. Kakihana, M. Osada, J. Alloys Compd. 309 (2000) 113. [13] M.A. Bizeto, V.R.L. Constantino, H.F. Brito, J. Alloys Compd. 311 (2000) 159. [14] V.R.L. Constantino, M.A. Bizeto, H.F. Brito, J. Alloys Compd. 278 (1998) 142. [15] M.E. Villafuerte-Castrejon, F. Camacho-Alanis, F. Gonzalez, A. Ibarra-Palos, G. Gonzalez, L. Fuentes, L. Bucio, J. Eur. Ceram. Soc. 27 (2007) 545. [16] O.A. Serra, S.A. Cicillini, R.R. Ishiki, J. Alloys Compd. 303 (2000) 316. [17] M. Daldosso, D. Falcomer, A. Speghini, P. Ghigna, M. Bettinelli, Opt. Mater. 30 (2008) 1162. [18] T. Aitasalo, J. Holsa, H. Jungner, M. Lastusaari, J. Niittykoski, M. Parkkinen, R. Valtonen, Opt. Mat. 26 (2004) 113. [19] D.P. Volanti, L.S. Cavalcante, E.C. Paris, A.Z. Simões, D. Keyson, V.M. Longo, A.T. de Figueiredo, E. Longo, J.A. Varela, F.S. De Vicente, A.C. Hernandes, Appl. Phys. Lett. 90 (2007). [20] S. Kojima, I. Saitoh, T. Yamamoto, IEEE Inter. Symp. Appl. Ferroelectron. (1998) 471. [21] H. Qiu, G.Z. Liu, M. He, H.S. Gu, T.S. Zhou, Physica B 400 (2007) 134. [22] P.S. Dobal, R.S. Katiyar, J. Raman Spectrosc. 33 (2002) 405. [23] G.Z. Liu, C. Wang, H.S. Gu, H.B. Lu, J. Phys. D 40 (2007) 7817. [24] D.L. Wood, J. Tauc, Phys. Rev. B 5 (1972) 3144. [25] M. Daldosso, A. Speghini, P. Ghigna, M.D. Ramirez, D. Jaque, L.E. Bausá, J.G. Solé, M. Bettinelli, J. Alloys Compd. 451 (2008) 12. [26] V.M. Longo, E. Orhan, L.S. Cavalcante, S.L. Porto, J.W.M. Espinosa, J.A. Varela, E. Longo, Chem. Phys. 334 (2007) 180. [27] M. Anicete-Santos, F.C. Picon, M.T. Escote, E.R. Leite, P.S. Pizani, J.A. Varela, E. Longo, Appl. Phys. Lett. 88 (2006). [28] M.V. Korzhik, V.B. Pavlenko, T.N. Timoschenko, V.A. Katchanov, A.V. Singovskii, A.N. Annenkov, V.A. Ligun, I.M. Solskii, J.P. Peigneux, Phys. Status Solidi A 154 (1996) 779. [29] I.L.V. Rosa, A.P. Maciel, E. Longo, E.R. Leite, J.A. Varela, Mater. Res. Bull. 41 (2006) 1791.
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
The role of the Eu3+ ions in structure and photoluminescence properties of SrBi2Nb2O9 powders Diogo P. Volanti a,*, Ieda L.V. Rosa b, Elaine C. Paris c, Carlos A. Paskocimas d, Paulo S. Pizani e, José A. Varela a, Elson Longo a a
Institute of Chemistry, São Paulo State University, UNESP, 14800-900 Araraquara, SP, Brazil Department of Chemistry, Federal University of São Carlos, UFSCar, 13565-905 São Carlos, SP, Brazil c Department of Chemistry, Federal University of Paraíba, UFPB, 58059-900 João Pessoa, PB, Brazil d Department of Mechanical Engineering, Federal University of Rio Grande do Norte, UFRN, 59072-970 Natal, RN, Brazil e Department of Physics, Federal University of São Carlos, UFSCar, 13565-905 São Carlos, SP, Brazil b
a r t i c l e
i n f o
Article history: Received 8 July 2008 Received in revised form 15 October 2008 Accepted 7 November 2008 Available online 24 December 2008 Keywords: Photoluminescence Strontium bismuth niobate Europium Polymeric precursor method
a b s t r a c t This work rationalized for the first time the key role of Eu3+ ion in the doping process of SrBi2Nb2O9 (SBN) compound. This process allows us to understand the role of Eu3+ as lattice modifier as well as to obtain information on the crystalline structure surrounding. Therefore, the SBN and Eu3+ doped SBN (SBN:Eu) were synthesized using the polymeric precursor method (PPM). Scanning electron microscope (SEM) images reveal the strong influence of europium on the SBN microstructure. Thermogravimetry (TG) and differential thermal analysis (DTA) techniques were used to determine the weight loss and changes associated with phase transitions in SBN during thermal evolution. The behavior of the Eu3+ lattice modifier was followed in a long-range order by X-ray diffraction (XRD), while Fourier transform Raman (FTRaman) spectroscopy was used to analyze the short-range order. To this end, the SBN orthorhombic phase was observed for all samples heat treated from 400 to 700 °C for 2 h. In addition, photoluminescence measurements were employed to study the structural modifications in SBN lattice. The characteristic red emission of the Eu3+ using the 488 nm exciting wavelength of an argon–ion laser was distinctly observed for SBN:Eu samples heat treated from 550 to 700 °C. Europium characteristic emission bands are related to 5D1 ? 7FJ (J = 0–2) transitions at 538 and 555 nm, as well as the 5D0 ? 7FJ (J = 0–4) ones at 580, 592, 615, 653 and 695 nm. By means of the emission spectra analyses it was possible to predict that the Eu3+ ions are located at sites of higher symmetry, since the relative area of the (5D0 ? 7F2)/ (5D0 ? 7F1) transitions for the SBN:Eu samples decrease from 3.82 to 2.60 with increasing temperature from 550 to 700 °C. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Photoluminescence (PL) technique is a powerful tool to understand the energy levels of materials, providing fundamental information that allows associating the structural properties to the band gap of a given material [1–5]. The spectroscopic properties of Eu3+ doped materials have been widely studied based on the crystal field splitting of the 2S+1LJ manifolds of its 4f6 configuration [6,7]. Eu3+ presents some characteristic properties enabling it to be frequently used as a probe. The excited states 5DJ (J = 0–3) are well separated from the ground terms 7FJ0 (J0 = 0–6). The main emitting level, 5D0 and the ground state 7F0 are non-degenerated; therefore the 5D0 ? 7F0 transition usually appears as a single peak in the photoluminescence spectra as the Eu3+ ions occupy identical site * Corresponding author. Tel.: +55 16 3301 6643. E-mail address: [email protected] (D.P. Volanti). 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.11.006
symmetries. These properties promote the interpretation of the spectral data and provide information on the eventual existence of at least two different symmetry sites occupied by the Eu3+ ions [8]. 5D0 ? 7F1 transition is usually taken as a reference. It is allowed by magnetic dipole and its intensity is not considerably altered by the perturbing crystal field. The perovskite-like layered SrBi2Nb2O9 (SBN) is a member of the Aurivillius phases and can be represented by (Bi2O2)2+ (Am1BmO3m+1)2, where A is a divalent ion such as Sr2+, Ba2+ and Ca2+; B is a metal of valence (5+) generally using Nb5+ or Ta5+, and m is the number of the perovskite unit cells between the Bi2O2 layers [9]. The SBN orthorhombic phase is crystallized via a metastable fluorite-like phase in a cubic structure [10,11], and in this system stoichiometric deviations can occur producing the pyrochlore and fluorite intermediary phases [12]. In the literature, the luminescence properties were studied in the rareearth doped layered niobate and Aurivillius compounds [13–15]. Additionally, the polymeric precursor method (PPM) is a useful soft
996
D.P. Volanti et al. / Optical Materials 31 (2009) 995–999
ment. This approach renders a plausible description of the PL properties of SBN:Eu compound under laser excitation, as well as an interesting correlation between the Fourier transform Raman (FTRaman) spectroscopy, X-ray diffraction (XRD) and UV–visible (UV–vis) absorption. The present paper is organized as follows. In Section 2 the experimental methods of the Eu3+ doped SBN powder preparation are described, together with measurement techniques. The results are discussed proposing to explain the Eu3+ influence in the SBN structure in Section 3. Finally, Section 4 describes the main lines of the order–disorder behavior for SBN:Eu system.
2. Experimental 2.1. Synthesis
Fig. 1. TG/DTA profiles of the SBN and SBN:Eu powders: (a) SBN–TG, (b) SBN:Eu– TG, (c) SBN–DTA and (d) SBN:Eu–DTA.
chemical method for the preparation of several luminescence compounds as YNbO4 [16], Eu3+ or Pr3+ doped Gd3Ga5O12 [17] and Eu2+ doped calcium aluminates [18]. In a previous work it was established that the PL property in the SBN powders occur due to the presence of niobium complex clusters and b-Bi2O3 phase on the pure SBN lattice [19]. However, in the case of europium doped SBN, described in the present work, the rare-earth ion favors the arrangement of the orthorhombic phase to the detriment of the b-Bi2O3 one. The PL properties are discussed in relation to the dependence of the structural changes improved by thermal treat-
SBN and SBN:Eu powders were synthesized by the polymeric precursor method (PPM); and the experimental procedure is described as follows: NH4H2NbO(C2O4)33H2O (99.5%, CBMM) was dissoluted in H2O and Nb was precipitated as Nb(OH)5 by the addition of NH4OH. After filtration, Nb(OH)5 was complexed in aqueous citric acid solution. SrCO3 (99.9%, Aldrich), Bi2O3 (99.9%, Aldrich) and Eu2O3 (99.9%, Aldrich) were dissolved in concentrated H2NO3, and were added to the reactional mixture. The molar ratio between (Sr:Bi:Nb) and (Sr:Eu:Bi:Nb) was (1:2:2) and (0.99:0.01: 2:2), respectively. Ethylenediamine was dripped into the solution with constant stirring until pH 9. After homogenization, ethylene glycol was added to the solution. The molar ratio of citric acid related to the metal was fixed at 3.95 and the mass ratio citric acid/ ethylene glycol was 60/40. SBN and SBN:Eu resins were placed in a furnace and heat treated at 350 °C for 4 h to obtain polymer pyro-
Fig. 2. SEM images of (a) SBN and (b)SBN:Eu powders.
D.P. Volanti et al. / Optical Materials 31 (2009) 995–999
997
Fig. 1 illustrates the thermal analyses for the SBN and SBN:Eu powders, annealed at 350 °C for 4 h. The weight loss in approximately 100 °C is due to the complete dehydration of the powders as shown in Fig. 1a and b. The TG analysis in Fig. 1a presents a weight loss of 64.2% between 260 and 690 °C for the SBN and 36.5% in the range of 330–591 °C for the SBN:Eu (Fig. 1b). This fact can be attributed to the elimination of CO and CO2. Weight variation is not observed from 690 °C in TG (a) and 591 °C in TG (b). In the SBN–DTA curve (Fig. 1c) there are two exothermic peaks at approximately 460 and 600 °C that can be attributed to the formation of the b-Bi2O3 and SrBi2Nb2O9 phases, respectively. On the other hand, in SBN:Eu–DTA (Fig. 1d) for SBN:Eu a single exother-
mic peak at 475 °C can be related to the formation of the orthorhombic phase. The SEM micrographs (Fig. 2) reveal the strong influence of europium on the powder microstructures processed at 400 °C in oxygen atmosphere. Although both micrographs indicate a dense structure, the SBN sample (Fig. 2a) shows a different microstructure with the formation of spherical grains. These spherical grains are ascribed to the intermediate b-Bi2O3 phase detected in the XRD pattern and EDX analysis with quantitative results. However, in Fig. 2b it was not possible to observe the spherical grains (bBi2O3 phase). These spherical grains are not present probably due to the formation of the orthorhombic phase in SBN:Eu powders due to the influence of the Eu3+ doping. XRD analysis was used to follow the long-range order and the crystallization process of the samples with increasing of the annealing temperature. Fig. 3 shows the XRD patterns of the SBN:Eu powders heat treated from 400 to 700 °C for 2 h under oxygen atmosphere. The europium oxide, with cubic structure, appears in the (1 1 1) direction according to JCPDS card N°32-0380. On the other hand, the orthorhombic phase was observed through the assigned peaks according to JCPDS card N°49-0607, for all samples. In the short-range, the SBN:Eu structural behavior was analyzed by the FT-Raman spectra (Fig. 4). According to Nelis et al. [10] and Kojima et al. [20], the SBN orthorhombic phase was observed for all samples. However, the pure SBN orthorhombic phase is observed only for the material heat treated at temperatures up to 550 °C [19]. The modes located at 178, 210, 570, 687 and 848 cm1 are related to SBN orthorhombic phase [19]. In addition, the shift at 570 cm1 corresponds to an unbending sublattice mode and indicates that the equivalent and opposite displacements of positive and negative ions are slightly changed [21]. The peak located at 840 cm1 can be attributed to a symmetric stretching of the octahedral BO6 (B = Nb or Ta) [22]. The presence of NbO6 octahedral, which appears for the first time in the sample heat treated at 400 °C (see Fig. 4f–h) is in agreement with the XRD patterns (Fig. 3a–g). Therefore, the presence of Eu3+ in the SBN structure promotes the appearance of the orthorhombic phase at a lower temperature, since the presence of this phase in the pure material takes place only at around 600 °C. Thus, the Eu3+ ion is a lattice modifier since it promotes the premature formation of the orthorhombic phase without the intermediate b-Bi2O3 one. The modes at 98 and 206 cm1 can be attributed to the vibrations of A (Bi3+ or Eu3+) site ions. This result is in agreement with the Raman scat-
Fig. 3. XRD patterns of SBN:Eu powders heat treated at (a) 400, (b) 450, (c) 500, (d) 550, (e) 600, (f) 650 and (g) 700 °C for 2 h under oxygen atmosphere.
Fig. 4. FT-Raman spectra of the SBN:Eu powders heat treated at (a) 400, (b) 450, (c) 500, (d) 550, (e) 600, (f) 650 and (g) 700 °C for 2 h under oxygen atmosphere.
lysis. After that, these powders were heat treated at 400, 450, 500, 550, 600, 650 and 700 °C for 2 h under oxygen atmosphere at a heating rate of 5 °C min1. 2.2. Instruments After the preliminary heat treatment at 350 °C for 4 h, SBN and SBN:Eu powders were analyzed by differential thermal analysis (DTA) and thermogravimetry (TG) in a Netzsch STA 409 equipment. DTA and TG analyses were carried out using 10 mg of the samples at a heating rate of 5 °C min1 up to 750 °C, in synthetic air under a flow of 30 cm3 min1. The microstructure was examined with a scanning electron microscope (SEM), Zeiss DSM 940A and the powder compositions were determined by EDX analysis. The diffraction patterns of the crystalline powders were recorded in a Rigaku DMax 2500PC diffractometer. The power tube used was 6000 W in a 2h interval from 10° to 75°, using Cu-Ka radiation with a graphite monochromator. FT-Raman spectra were recorded in a Bruker RFS 100/S spectrometer, excited by a Nd:YAG laser at 1064 nm with a spectral resolution of 4.0 cm1. The spectral dependence of the optical reflectance, UV–vis, of the SBN and SBN:Eu samples was taken in the total reflection mode, using a Cary 5G equipment in the wavelength range from 300 to 800 nm. To measure the PL properties a U1000 Jobin-Yvon double monochromator coupled to a cooled Ga–As photomultiplier was used and a conventional photon counting system. The 488 nm exciting wavelength of an argon–ion laser was used, with the laser maximum output power kept within 30 mW. All the measurements were recorded at room temperature. 3. Results and discussion
998
D.P. Volanti et al. / Optical Materials 31 (2009) 995–999
Table 1 Band gap energies for the SBN and SBN:Eu powders. Band gap (eV)
SBN SBN:Eu
400 (°C)
450 (°C)
500 (°C)
550 (°C)
600 (°C)
650 (°C)
700 (°C)
1.87 2.52
2 2.55
2.15 2.66
2.17 2.67
2.2 2.67
2.7 2.7
2.72 2.72
tering study of La-doped SrBi2Nb2O9 prepared by the solid-state reaction [23]. The values of the gap energies can be obtained by the Wood and Tauc method [24]. The gap energies of the SBN and SBN:Eu powders treated at different temperatures are shown in Table 1. As the annealing temperature is increased, the gap value increases and reduces the delocalized states, organizing the crystalline system. This behavior indicates that the density of localized states in the band gap of the powders heat treated at a lower temperature is greater than those of the higher temperatures due to structural defects. Daldosso et al. [25] reported the local structure at around SrxBa1xNb2O6 doped with different Ln3+ ions by Optical spectroscopy. In this work, by means of EXAFS (Extended X-ray Absorption Fine Structure) investigation, it was verified that Eu3+ appears to enter mainly sites with coordination number 12 in SrxBa1xNb2O6 single crystals, indicating that this ion occupies only A1 site-types. Consequently, the presence of europium can favor the reduction of tensions in the SBN lattice, promoting an increase of the band gap values in the SBN:Eu samples. Fig. 5 depicts the spectral dependence of the absorbance for the SBN and SBN:Eu annealed at 400 °C, which is more disordered than SBN and SBN:Eu powders heat treated at 700 °C. The SBN-700 °C and SBN:Eu-700 °C powders show one well-defined absorption front while the SBN-400 °C and SBN:Eu-400 °C powders exhibit a typically continuous smooth absorption that increases as a function of the energy, suggesting the presence of localized states in the band gap. The values of the gap energies for these powders are depicted in Table 1. Fig. 6 illustrates PL spectra recorded at room temperature for the SBN:Eu powders heat treated at 400, 450, 500, 550, 600, 650 and 700 °C for 2 h in oxygen flow. The exciting wavelength of an argon–ion laser was 488.0 nm. The profile of the emission band is typical of a multiphonon process, i.e., a system in which relaxation occurs by means of several paths, involving the participation of numerous states within the band gap of the material. This
Fig. 5. UV–vis absorption spectra of (a) SBN-400 °C, (b) SBN:Eu-400 °C, (c) SBN700 °C and (d) SBN:Eu-700 °C, at room temperature.
Fig. 6. Room temperature PL spectra of SBN:Eu powders heat treated at (a) 400, (b) 450, (c) 500, (d) 550, (e) 600, (f) 650 and (g) 700 °C. The insert shows PL spectra of SBN heat treated at (h) 400 (i) 450 and (j) 500 °C, kexc = 488 nm.
behavior is related to the structural disorder of SBN:Eu and indicates the presence of additional electronic levels in the forbidden band of the material. The general aspects of the spectra are a broad band covering a large part of the visible spectra, from 490 to 725 nm. As can be seen, the structurally disordered powder annealed at 400 °C presents PL emission centered at 550 nm, and PL emission centered at 540 nm if annealed at 500 °C. The PL emission vanishes when the crystallization process takes place at 550 °C. Thus, the interesting PL properties of structural ordered–disordered SBN:Eu powders are in the range of 400 to 500 °C. We attributed that an increase in the lattice disorder is associated to the presence of [NbO5V zO ] or [SrO11V zO ] in the complex clusters, where V zO ¼ V xO , V O or V O [26,27]. With increasing of the lattice order, this complex vacancies vanishes and also the PL emission. According to the hypothesis proposed by Korzhik et al. [28], there are vacant localized states associated to defects, such as oxygen vacancies in the band gap. With the temperature increase, these vacancies are reduced, in which an electron from the conduction band loses its energy and reoccupies the energy levels of electrons (e0 )/hole (h) in the valence band. Therefore, most of the oxygen vacancies are vacancy complexes in the ordered–disordered structure. In Fig. 6a–c the broad intense luminescence band with a maximum PL at about 560 nm in the green region of the PL spectra is observed, hence the charge transference in the [NbO6]0 /[NbO5V O ] complex clusters may be responsible for the PL behavior resulting from the structural order–disorder in the SBN lattice. However, the increase of the number of [NbO6]0 clusters affects the structural order in the system, reducing the PL intensity [19,26,27]. We related the PL emission in the SBN:Eu lattice as a consequence of its structural evolution in terms of the structural order–disorder. Fig. 7 shows the emission spectra of the SBN:Eu samples heat treated at 550, 600, 650 and 700 °C for 2 h, excited at 488 nm using an argon–ion laser, performed at room temperature. The weak emission bands observed at 538 and 555 nm are ascribed to the Eu3+ transitions 5D1 ? 7FJ (J = 0–2). In these spectra the presence of the bands at 580, 592, 615, 653 and 695 nm were also observed, which are respectively attributed to the 5D0 ? 7FJ (J = 0–4) transitions of the Eu3+. The presence of two bands at 578 and 580 nm are related to the Eu3+. Thus, the 5D0 ? 7F0 transition indicates that the Eu3+ ions are located in at least two different chemical surroundings in the SBN lattice. The Eu3+ luminescence spectra show emission lines that extend from visible to near infrared range of the electromagnetic spectrum. The ratio of the (5D0 ? 7F2)/(5D0 ? 7F1) emission bands can be taken as the relative area of these bands
D.P. Volanti et al. / Optical Materials 31 (2009) 995–999
999
tic red emission of the Eu3+ is clearly observed for SBN:Eu where the annealed temperatures are higher than 550 °C. As the temperature increased the system became more symmetric in a shortrange order, as it is followed by the relative area decrease of the (5D0 ? 7F2)/(5D0 ? 7F1) transitions. In addition, it was possible concluding that the presence of Eu3+ collaborates to the formation of the orthorhombic phase in detriment of the b-Bi2O3 one. Thus, the presence of europium favors the reduction of tensions in the lattice of SBN, which favors the increase of the experimental values of the SBN:Eu band gap. Acknowledgements The authors gratefully acknowledge the agencies CNPq, CAPES, FAPESQ and FAPESP/CEPID 98/14324-0.
Fig. 7. Emission spectra of the SBN:Eu annealed at (a) 550, (b) 600, (c) 650 and (d) 700 °C for 2 h under oxygen atmosphere recorded at room temperature, kexc = 488 nm.
Table 2 Relative area of the (5D0 ? 7F2)/(5D0 ? 7F1) Eu3+ transitions in SBN:Eu samples heat treated from 550 to 700 °C. Temperature (°C)
Relative area
550 600 650 700
3.82 2.81 2.65 2.60
which are dependent on the environment around the Eu3+, since the 5D0 ? 7F2 transition exhibits a magnetic dipole character and is very sensible to the Eu3+ surrounding, while the 5D0 ? 7F1 transition has a magnetic dipole character in low symmetry systems, therefore its intensity has a low dependence of the Eu3+ environment [29]. The relative area of the (5D0 ? 7F2)/(5D0 ? 7F1), obtained from the emission spectra of the SBN:Eu samples heat treated at 550, 600, 650 and 700 °C are depicted in Table 2. By analysis of these data it is possible to suppose that the Eu3+ environment changes to a higher symmetry site when the annealed temperature is increased, since the (5D0 ? 7F2)/(5D0 ? 7F1) relative area decreases with the heat treatment. 4. Conclusions The soft chemical synthesis such as the PPM is a promising route for studying the crystallization process of the SBN:Eu system. Thermal analyses (TG/DTA) expose the phase changes in SBN that were confirmed by SEM. It was observed that the Eu3+ ion allows the characterization of the transformations of ordered–disordered states in the SBN lattice. The PL is a complementary technique for XRD and FT-Raman spectroscopy on the characterization of this material in a short, medium and long-range order. The characteris-
References [1] E.C. Paris, J.W.M. Espinosa, S. de Lazaro, R.C. Lima, M.R. Joya, P.S. Pizani, E.R. Leite, A.G. Souza, J.A. Varela, E. Longo, Chem. Phys. 335 (2007) 7. [2] R.C. Lima, J.W.M. Espinosa, M.F.C. Gurgel, E.C. Paris, E.R. Leite, M.R. Joya, P.S. Pizani, J.A. Varela, E. Longo, J. Appl. Phys. 100 (2006). [3] L.S. Cavalcante, M.F.C. Gurgel, A.Z. Simões, E. Longo, J.A. Varela, M.R. Joya, P.S. Pizani, Appl. Phys. Lett. 90 (2007). [4] A.T. de Figueiredo, S. de Lazaro, E. Longo, E.C. Paris, J.A. Varela, M.R. Joya, P.S. Pizani, Chem. Mater. 18 (2006) 2904. [5] M.F.C. Gurgel, J.W.M. Espinosa, A.B. Campos, I.L.V. Rosa, M.R. Joya, A.G. Souza, M.A. Zaghete, P.S. Pizani, E.R. Leite, J.A. Varela, E. Longo, J. Lumin. 126 (2007) 771. [6] W.T. Carnall, G.L. Goodman, K. Rajnak, R.S. Rana, J. Chem. Phys. 90 (1989) 3443. [7] J. Holsa, P. Porcher, J. Chem. Phys. 75 (1981) 2108. [8] O.L. Malta, H.F. Brito, J.F.S. Menezes, F. Silva, S. Alves, F.S. Farias, A.V.M. deAndrade, J. Lumin. 75 (1997) 255. [9] C. Pirovano, M.S. Islam, R.-N. Vannier, G. Nowogrocki, G. Mairesse, Sol. Stat. Ionics 140 (2001) 115. [10] D. Nelis, J.M. Calderon-Moreno, M. Popa, M.K. van Bael, J. Mullens, L.C. van Poucke, J. Eur. Ceram. Soc. 26 (2006) 409. [11] N.S. Prasad, K.B.R. Varma, Y. Takahashi, Y. Benino, T. Fujiwara, T. Komatsu, J. Solid State Chem. 173 (2003) 209. [12] T. Asai, E.R. Camargo, M. Kakihana, M. Osada, J. Alloys Compd. 309 (2000) 113. [13] M.A. Bizeto, V.R.L. Constantino, H.F. Brito, J. Alloys Compd. 311 (2000) 159. [14] V.R.L. Constantino, M.A. Bizeto, H.F. Brito, J. Alloys Compd. 278 (1998) 142. [15] M.E. Villafuerte-Castrejon, F. Camacho-Alanis, F. Gonzalez, A. Ibarra-Palos, G. Gonzalez, L. Fuentes, L. Bucio, J. Eur. Ceram. Soc. 27 (2007) 545. [16] O.A. Serra, S.A. Cicillini, R.R. Ishiki, J. Alloys Compd. 303 (2000) 316. [17] M. Daldosso, D. Falcomer, A. Speghini, P. Ghigna, M. Bettinelli, Opt. Mater. 30 (2008) 1162. [18] T. Aitasalo, J. Holsa, H. Jungner, M. Lastusaari, J. Niittykoski, M. Parkkinen, R. Valtonen, Opt. Mat. 26 (2004) 113. [19] D.P. Volanti, L.S. Cavalcante, E.C. Paris, A.Z. Simões, D. Keyson, V.M. Longo, A.T. de Figueiredo, E. Longo, J.A. Varela, F.S. De Vicente, A.C. Hernandes, Appl. Phys. Lett. 90 (2007). [20] S. Kojima, I. Saitoh, T. Yamamoto, IEEE Inter. Symp. Appl. Ferroelectron. (1998) 471. [21] H. Qiu, G.Z. Liu, M. He, H.S. Gu, T.S. Zhou, Physica B 400 (2007) 134. [22] P.S. Dobal, R.S. Katiyar, J. Raman Spectrosc. 33 (2002) 405. [23] G.Z. Liu, C. Wang, H.S. Gu, H.B. Lu, J. Phys. D 40 (2007) 7817. [24] D.L. Wood, J. Tauc, Phys. Rev. B 5 (1972) 3144. [25] M. Daldosso, A. Speghini, P. Ghigna, M.D. Ramirez, D. Jaque, L.E. Bausá, J.G. Solé, M. Bettinelli, J. Alloys Compd. 451 (2008) 12. [26] V.M. Longo, E. Orhan, L.S. Cavalcante, S.L. Porto, J.W.M. Espinosa, J.A. Varela, E. Longo, Chem. Phys. 334 (2007) 180. [27] M. Anicete-Santos, F.C. Picon, M.T. Escote, E.R. Leite, P.S. Pizani, J.A. Varela, E. Longo, Appl. Phys. Lett. 88 (2006). [28] M.V. Korzhik, V.B. Pavlenko, T.N. Timoschenko, V.A. Katchanov, A.V. Singovskii, A.N. Annenkov, V.A. Ligun, I.M. Solskii, J.P. Peigneux, Phys. Status Solidi A 154 (1996) 779. [29] I.L.V. Rosa, A.P. Maciel, E. Longo, E.R. Leite, J.A. Varela, Mater. Res. Bull. 41 (2006) 1791.