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Structural and IR-spectroscopic characterization of some new Sr2LnSbO6 perovskites
Journal of Alloys and Compounds 460 (2008) 152–154
Structural and IR-spectroscopic characterization of some new Sr2LnSbO6 perovskites Araceli E. Lavat a , Enrique J. Baran b,∗ a
Departamento de Ingenier´ıa Qu´ımica, Facultad de Ingenier´ıa, Universidad Nacional del Centro de la Provincia de Buenos Aires, 7400 Olavarr´ıa, Argentina b Centro de Qu´ımica Inorg´ anica (CEQUINOR/CONICET, UNLP), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, C. Correo 962, 1900 La Plata, Argentina Received 17 May 2007; accepted 5 June 2007 Available online 7 June 2007
Abstract A series of ternary perovskite-type oxides of composition Sr2 LnSbO6 (with Ln = La, Nd, Sm, Gd, Dy, Er, Yb and Y), have been prepared and their unit cell parameters determined by X-ray powder diffractometry. The infrared spectra of these materials were also recorded and briefly discussed on the basis of their structural peculiarities and by comparison with those of related oxides. © 2007 Elsevier B.V. All rights reserved. Keywords: Mixed oxides; Perovskite structure; X-ray diffraction; IR spectroscopy
1. Introduction Materials belonging to the perovskite structural type and some closely related structures present important and valuable technological applications derived from their interesting physicochemical properties [1–4]. The ideal ABO3 perovskite is cubic, with space group Pm3m—Oh 1 and Z = 1, being A the larger and B the smaller cation. The B cation is 6-fold coordinated and the A cation is 12-fold coordinated by the oxygen anions [2]. As a measure of the deviation from the ideal structure a tolerance factor (t) defined by the following equation has been introduced [2,3,5]: rA + r O t=√ 2(rB + rO ) where rA , rB and rO are the constituent ionic radii. Although for an ideal perovskite t is unity, this structure is also found for lower t-values (0.75 < t ≤ 1.0) [5]. In such cases, the structure distorts to tetragonal, rhombohedral, orthorhombic or monoclinic symmetry [4,5]. The simple perovskite structure may be appropriately modified by incorporating two types of B ions with suitable different size and charge. The most frequent sub∗
Corresponding author. Tel.: +54 221 4259485; fax: +54 221 4259485. E-mail address: [email protected] (E.J. Baran).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.06.003
stitutions are the equiatomic proportions of the two cations at the B-site, for which the general formula is A2 BB O6 (or AB0.5 B 0.5 O3 ) and a great number of double perovskites of this type has so far been described [2]. The resulting cell may be viewed as doubled along the three axis, regarding the primitive ABO3 cell. If the charges of B and B are different, in the ordered structure the oxygen ions are slightly displaced toward the more charged cation although the octahedral symmetry of the BO6 and B O6 units is preserved [4]. Double perovskites of the type Ba2 LnSbO6 (Ln = trivalent lanthanide) have recently been described [6–8], whereas for the case of similar materials containing Sr(II) instead of Ba(II) only one oxide, namely Sr2 YSbO6 , has so far been reported [9]. In order to extend our knowledge on this type of perovskites we have now prepared and characterized a wider number of Sr2 LnSbO6 materials, containing a series of lanthanide cations. 2. Experimental Samples of Sr2 LnSbO6 (with Ln = La, Nd, Sm, Gd, Dy, Er, Yb and Y) were prepared by standard solid-state reactions, in air, using stoichiometric amounts of SrCO3 , Sb2 O3 and the respective Ln2 O3 oxides. The mixtures were first reacted at 800 ◦ C for a few hours, to avoid Sb2 O3 losses and then fired at 1200 ◦ C, during 24 h, with some intermediate grindings to ensure homogeneity and reaction completion. The samples were finally furnace cooled to room temperature.
A.E. Lavat, E.J. Baran / Journal of Alloys and Compounds 460 (2008) 152–154 The obtained mixed oxides were characterized by X-ray diffractometry and infrared spectroscopic analysis. The X-ray powder diagrams were recorded using a continuous step scanning procedure (step size: 0.020◦ (in 2 θ); time per step: 0.5 s), with a Philips PW 3710 diffractometer and monochromatic Cu K␣ radia˚ using NaCl as an external calibration standard. The powder tion (λ = 1.54060 A), diagrams of all samples could be indexed as single-phase materials and the unit cell parameters were calculated using a locally modified version of the program PIRUM of Werner [10]. The infrared spectra were recorded with a NicoletMagna 550 FTIR instrument, using the KBr pellet technique. The results were also confirmed employing the Nujol-mull technique. Spectral resolution was ±4 cm−1 .
3. Results and discussion 3.1. Structural aspects A recent structural study of SrYSbO6 and SrInSbO6 by means of X-ray and neutron powder diffraction has established the monoclinic P21 /n symmetry [9], in agreement with the departure of the tolerance factor from unity. Notwithstanding, it is interesting to note that this monoclinic distortion is rather small, with the β-angle very close to 90◦ and with practically identical a- and b-unit cell constants [9]. As a consequence of the difference in charge and ionic radii the Ln3+ and Sb5+ cations adopt an ordered distribution over the B-site of the ideal cubic perovskite structure [2,9]. Besides, in order to optimize the Sr O bonds, both the LnO6 and SbO6 octahedra are tilted [5,9]. The X-ray diffraction patterns of the eight materials prepared, in this study, are quite similar and are dominated by the strong lines characteristic of a primitive perovskite cell, in the same way as previously found for SrYSbO6 and SrInSbO6 [9]. Notwithstanding, our diffraction patterns could not be fitted using a monoclinic unit cell and instead the best results were obtained with an orthorhombic symmetry. This can probably be explained by the fact that the very subtle distortion due to the octahedral tilting was not observed in the powder diagrams. As mentioned above, in the previously investigated cases, the refined β-values obtained by neutron diffraction analysis are very close to 90◦ , suggesting that the monoclinic distortion is very small and that the unit cells are almost orthorhombic. The structural parameters of the investigated materials are presented in Table 1. The unit cell parameters determined for SrYSbO6 are in good agreement with the previously reported ˚ with β = 89.92◦ [9]). data (a = 5.812, b = 5.838 and c = 8.239 A,
153
From the reported data, some interesting correlations can be detected, as follows: - The unit cell parameters of the complete series follow the sequence a ≤ b < c, except for the two larger lanthanide cations, La(III) and Nd(III), in which a is slightly larger than b. - The general trend correlates fairly well with the ionic size of the Ln(III) cations as the unit cell parameters and unit cell volumes decrease regularly from Sm(III) to Yb(III), in agreement with their respective ionic radii. The variation is particularly smooth in the case of the c-parameter. - In the case of SrLaSbO6 and SrNdSbO6 the trends for the unit cell parameters as well as for the unit cell volumes are not clearly fulfilled. 3.2. Infrared spectra Similarly, as in the case of other previously investigated A2 BB O6 perovskites [11] the FTIR spectra of the now investigated materials are rather simple, showing two strong and well-defined absorption bands, eventually with some weak satellite shoulders. Such a spectral pattern is typical of perovskite oxides [1,11–13] and the two characteristic bands are mainly associated to the antisymmetric F1u stretching and deformational modes of the octahedral BO6 moieties [11]. Because in the present case both, the Ln(III) and Sb(V) cations are present as octahedral BO6 building blocks, strong vibrational coupling between them may be expected. On the other hand, the B O bonds of these units, involving metal cations with charges +3 and +5 are undoubtedly stronger than those belonging to the 12 coordinated Sr(II)–O units. Therefore, on the basis of simple arguments one may predict that the BO6 units behave as approximately “isolated” groupings that dominate the spectroscopic behaviour [11]. Besides, taking into account that Sb(V) is lighter and most highly charged than the Ln(III) cations, it becomes clear that the SbO6 units must dominate the vibrational spectra of these materials. Nevertheless, the band broadening and weak splitting suggest that couplings with the other structural units should not be totally neglected. The IR spectra of the eight compounds are rather similar. As a typical example, that obtained for Sr2 ErSbO6 is shown in Fig. 1. The measured band positions for all oxides are presented in Table 2. Some interesting aspects Table 2 IR-spectroscopic data for the investigated mixed oxides
Table 1 Unit cell parameters of the investigated mixed oxides Oxide
˚ a (A)
˚ b (A)
˚ c (A)
˚ 3) Volume (A
Sr2 LaSbO6 Sr2 NdSbO6 Sr2 SmSbO6 Sr2 GdSbO6 Sr2 DySbO6 Sr2 ErSbO6 Sr2 YbSbO6 Sr2 YSbO6
5.887(1) 5.849(1) 5.817(6) 5.812(1) 5.805(1) 5.802(5) 5.767(6) 5.807(4)
5.820(1) 5.784(2) 5.887(4) 5.890(6) 5.824(9) 5.822(3) 5.795(4) 5.829(8)
8.325(7) 8.363(4) 8.291(2) 8.271(2) 8.144(6) 8.106(9) 8.121(3) 8.215(3)
285.234 282.925 283.922 283.138 275.335 273.814 271.402 278.069
Oxide
Band positions (cm−1 )
Sr2 LaSbO6 Sr2 NdSbO6 Sr2 SmSbO6 Sr2 GdSbO6 Sr2 DySbO6 Sr2 ErSbO6 Sr2 YbSbO6 Sr2 YSbO6
671 sh, 647 vs 625 sh, 602 vs 620 sh, 613 vs 672 sh, 622 vs 672 sh, 630 vs 670 sh, 657 vs, 636 sh 672 vs, 643 sh 665 sh, 643 vs
vs: very strong; sh: shoulder.
390 vs, 380 sh 380 sh, 355 vs 365 vs 380 sh, 377 vs 370 vs, 330 sh 376 vs 368 vs, 333 sh 377 vs, 338 sh, 310 sh
154
A.E. Lavat, E.J. Baran / Journal of Alloys and Compounds 460 (2008) 152–154
Fig. 1. FTIR spectra of Sr2 ErSbO6 .
reinforcement of the Sb O bonds in this material. This abnormality can probably be related with the fact that the size difference between the cations located in the octahedral voids is the greatest for the pair La(III)/Sb(V). Therefore, the polarizing effect of the Sb(V) cation over the oxide anions has a stronger effect in this case than in all the others, in which the Ln(III) presents a smaller size and, concomitantly, a somewhat stronger polarizing power. This polarizing effects, which affect the Sb O La bonds, can eventually also impact on the relative tilting of the SbO6 and LaO6 octahedra and may also be responsible for the commented differences in the behaviour of the unit cell of Sr2 LaSbO6 , i.e., in this case a > b. A similar effect probably occurs in the case of Sr2 NdSbO6 which also shows the same inversion in unit cell parameters. But in this case the overall effects over the Sb O Nd bonds do not have the same impact over the spectroscopic behaviour as in the case of the lanthanum compound. - The fact that the lower energy band does not show a clearly defined trend demonstrates that this band originates in more complex motions which not only involve the antisymmetric SbO6 deformation but probably also vibrations related to LnO6 modes. Acknowledgements This work has been supported by the Universidad Nacional de La Plata and the Consejo Nacional de Investigaciones Cient´ıficas y T´ecnicas de la Rep´ublica Argentina. E.J.B. is a member of the Research Career from this organization. References
Fig. 2. Dependence of the highest energy IR-band (ν1 ) of the Sr2 LnSbO6 materials from the ionic radii (ri ) of the Ln(III) cations.
of these spectroscopic data merits further comments, as follows: - When the frequency of the strongest component of the higher energy band is plotted against the ionic radii of the Ln(III) cations (Shannon and Prewitt radii for octahedral coordination [14]) a practically linear trend is observed for all the materials, except that containing La(III), as depicted in Fig. 2. This behaviour may be interpreted as a direct consequence of the reinforcement of the Sb O bonds, which parallels the overall diminution of the unit cell volumes towards Yb(III). A similar behaviour has been often observed in other oxidic materials (cf. for example [11,15,16]). - The lighter and more voluminous La(III) presents an anomalous behaviour, pointing to a peculiar and unexpected
[1] E.J. Baran, Catal. Today 8 (1990) 133. [2] F.S. Galaso, Perovskites and High Tc Superconductors, Gordon and Breach, New York, 1990. [3] L.J. Tejuca, J.L.G. Fierro, Properties and Applications of Perovskite Type Oxides, Marcel Dekker, New York, 1993. [4] M.A. Pe˜na, J.L.G. Fierro, Chem. Rev. 101 (2001) 1981. [5] C.N.R. Rao, J. Gopalakrishnan, New Directions in Solid State Chemistry, Cambridge University Press, Cambridge, 1989. [6] J. Kurian, J. Koshi, P.R.S. Wariar, Y.P. Yadava, A.D. Damodaran, J. Solid State Chem. 116 (1995) 193. [7] W.T. Fu, D.J.W. Ijdo, J. Solid State Chem. 178 (2005) 2363. [8] P.J. Saines, B.J. Kennedy, M.M. Elcombe, J. Solid State Chem. 180 (2007) 401. [9] W.T. Fu, D.J.W. Ijdo, Solid State Comm. 134 (2005) 177. [10] P.E. Werner, Ark. Kemi 31 (1969) 51. [11] A.E. Lavat, E.J. Baran, Vibrat. Spectr. 32 (2003) 167. [12] J.T. Last, Phys. Rev. 105 (1957) 1740. [13] S.D. Ross, Inorganic Infrared and Raman Spectra, McGraw-Hill, London, 1972. [14] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B 36 (1970) 1076. [15] A.E. Lavat, E.J. Baran, R. S´aez-Puche, A. Salinas-S´anchez, J.M. MartinLlorente, Vibrat. Spectr. 3 (1992) 291. [16] R. Pis-Diez, E.J. Baran, A.E. Lavat, M.C. Grasselli, J. Phys. Chem. Solids 56 (1995) 135.
Structural and IR-spectroscopic characterization of some new Sr2LnSbO6 perovskites Araceli E. Lavat a , Enrique J. Baran b,∗ a
Departamento de Ingenier´ıa Qu´ımica, Facultad de Ingenier´ıa, Universidad Nacional del Centro de la Provincia de Buenos Aires, 7400 Olavarr´ıa, Argentina b Centro de Qu´ımica Inorg´ anica (CEQUINOR/CONICET, UNLP), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, C. Correo 962, 1900 La Plata, Argentina Received 17 May 2007; accepted 5 June 2007 Available online 7 June 2007
Abstract A series of ternary perovskite-type oxides of composition Sr2 LnSbO6 (with Ln = La, Nd, Sm, Gd, Dy, Er, Yb and Y), have been prepared and their unit cell parameters determined by X-ray powder diffractometry. The infrared spectra of these materials were also recorded and briefly discussed on the basis of their structural peculiarities and by comparison with those of related oxides. © 2007 Elsevier B.V. All rights reserved. Keywords: Mixed oxides; Perovskite structure; X-ray diffraction; IR spectroscopy
1. Introduction Materials belonging to the perovskite structural type and some closely related structures present important and valuable technological applications derived from their interesting physicochemical properties [1–4]. The ideal ABO3 perovskite is cubic, with space group Pm3m—Oh 1 and Z = 1, being A the larger and B the smaller cation. The B cation is 6-fold coordinated and the A cation is 12-fold coordinated by the oxygen anions [2]. As a measure of the deviation from the ideal structure a tolerance factor (t) defined by the following equation has been introduced [2,3,5]: rA + r O t=√ 2(rB + rO ) where rA , rB and rO are the constituent ionic radii. Although for an ideal perovskite t is unity, this structure is also found for lower t-values (0.75 < t ≤ 1.0) [5]. In such cases, the structure distorts to tetragonal, rhombohedral, orthorhombic or monoclinic symmetry [4,5]. The simple perovskite structure may be appropriately modified by incorporating two types of B ions with suitable different size and charge. The most frequent sub∗
Corresponding author. Tel.: +54 221 4259485; fax: +54 221 4259485. E-mail address: [email protected] (E.J. Baran).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.06.003
stitutions are the equiatomic proportions of the two cations at the B-site, for which the general formula is A2 BB O6 (or AB0.5 B 0.5 O3 ) and a great number of double perovskites of this type has so far been described [2]. The resulting cell may be viewed as doubled along the three axis, regarding the primitive ABO3 cell. If the charges of B and B are different, in the ordered structure the oxygen ions are slightly displaced toward the more charged cation although the octahedral symmetry of the BO6 and B O6 units is preserved [4]. Double perovskites of the type Ba2 LnSbO6 (Ln = trivalent lanthanide) have recently been described [6–8], whereas for the case of similar materials containing Sr(II) instead of Ba(II) only one oxide, namely Sr2 YSbO6 , has so far been reported [9]. In order to extend our knowledge on this type of perovskites we have now prepared and characterized a wider number of Sr2 LnSbO6 materials, containing a series of lanthanide cations. 2. Experimental Samples of Sr2 LnSbO6 (with Ln = La, Nd, Sm, Gd, Dy, Er, Yb and Y) were prepared by standard solid-state reactions, in air, using stoichiometric amounts of SrCO3 , Sb2 O3 and the respective Ln2 O3 oxides. The mixtures were first reacted at 800 ◦ C for a few hours, to avoid Sb2 O3 losses and then fired at 1200 ◦ C, during 24 h, with some intermediate grindings to ensure homogeneity and reaction completion. The samples were finally furnace cooled to room temperature.
A.E. Lavat, E.J. Baran / Journal of Alloys and Compounds 460 (2008) 152–154 The obtained mixed oxides were characterized by X-ray diffractometry and infrared spectroscopic analysis. The X-ray powder diagrams were recorded using a continuous step scanning procedure (step size: 0.020◦ (in 2 θ); time per step: 0.5 s), with a Philips PW 3710 diffractometer and monochromatic Cu K␣ radia˚ using NaCl as an external calibration standard. The powder tion (λ = 1.54060 A), diagrams of all samples could be indexed as single-phase materials and the unit cell parameters were calculated using a locally modified version of the program PIRUM of Werner [10]. The infrared spectra were recorded with a NicoletMagna 550 FTIR instrument, using the KBr pellet technique. The results were also confirmed employing the Nujol-mull technique. Spectral resolution was ±4 cm−1 .
3. Results and discussion 3.1. Structural aspects A recent structural study of SrYSbO6 and SrInSbO6 by means of X-ray and neutron powder diffraction has established the monoclinic P21 /n symmetry [9], in agreement with the departure of the tolerance factor from unity. Notwithstanding, it is interesting to note that this monoclinic distortion is rather small, with the β-angle very close to 90◦ and with practically identical a- and b-unit cell constants [9]. As a consequence of the difference in charge and ionic radii the Ln3+ and Sb5+ cations adopt an ordered distribution over the B-site of the ideal cubic perovskite structure [2,9]. Besides, in order to optimize the Sr O bonds, both the LnO6 and SbO6 octahedra are tilted [5,9]. The X-ray diffraction patterns of the eight materials prepared, in this study, are quite similar and are dominated by the strong lines characteristic of a primitive perovskite cell, in the same way as previously found for SrYSbO6 and SrInSbO6 [9]. Notwithstanding, our diffraction patterns could not be fitted using a monoclinic unit cell and instead the best results were obtained with an orthorhombic symmetry. This can probably be explained by the fact that the very subtle distortion due to the octahedral tilting was not observed in the powder diagrams. As mentioned above, in the previously investigated cases, the refined β-values obtained by neutron diffraction analysis are very close to 90◦ , suggesting that the monoclinic distortion is very small and that the unit cells are almost orthorhombic. The structural parameters of the investigated materials are presented in Table 1. The unit cell parameters determined for SrYSbO6 are in good agreement with the previously reported ˚ with β = 89.92◦ [9]). data (a = 5.812, b = 5.838 and c = 8.239 A,
153
From the reported data, some interesting correlations can be detected, as follows: - The unit cell parameters of the complete series follow the sequence a ≤ b < c, except for the two larger lanthanide cations, La(III) and Nd(III), in which a is slightly larger than b. - The general trend correlates fairly well with the ionic size of the Ln(III) cations as the unit cell parameters and unit cell volumes decrease regularly from Sm(III) to Yb(III), in agreement with their respective ionic radii. The variation is particularly smooth in the case of the c-parameter. - In the case of SrLaSbO6 and SrNdSbO6 the trends for the unit cell parameters as well as for the unit cell volumes are not clearly fulfilled. 3.2. Infrared spectra Similarly, as in the case of other previously investigated A2 BB O6 perovskites [11] the FTIR spectra of the now investigated materials are rather simple, showing two strong and well-defined absorption bands, eventually with some weak satellite shoulders. Such a spectral pattern is typical of perovskite oxides [1,11–13] and the two characteristic bands are mainly associated to the antisymmetric F1u stretching and deformational modes of the octahedral BO6 moieties [11]. Because in the present case both, the Ln(III) and Sb(V) cations are present as octahedral BO6 building blocks, strong vibrational coupling between them may be expected. On the other hand, the B O bonds of these units, involving metal cations with charges +3 and +5 are undoubtedly stronger than those belonging to the 12 coordinated Sr(II)–O units. Therefore, on the basis of simple arguments one may predict that the BO6 units behave as approximately “isolated” groupings that dominate the spectroscopic behaviour [11]. Besides, taking into account that Sb(V) is lighter and most highly charged than the Ln(III) cations, it becomes clear that the SbO6 units must dominate the vibrational spectra of these materials. Nevertheless, the band broadening and weak splitting suggest that couplings with the other structural units should not be totally neglected. The IR spectra of the eight compounds are rather similar. As a typical example, that obtained for Sr2 ErSbO6 is shown in Fig. 1. The measured band positions for all oxides are presented in Table 2. Some interesting aspects Table 2 IR-spectroscopic data for the investigated mixed oxides
Table 1 Unit cell parameters of the investigated mixed oxides Oxide
˚ a (A)
˚ b (A)
˚ c (A)
˚ 3) Volume (A
Sr2 LaSbO6 Sr2 NdSbO6 Sr2 SmSbO6 Sr2 GdSbO6 Sr2 DySbO6 Sr2 ErSbO6 Sr2 YbSbO6 Sr2 YSbO6
5.887(1) 5.849(1) 5.817(6) 5.812(1) 5.805(1) 5.802(5) 5.767(6) 5.807(4)
5.820(1) 5.784(2) 5.887(4) 5.890(6) 5.824(9) 5.822(3) 5.795(4) 5.829(8)
8.325(7) 8.363(4) 8.291(2) 8.271(2) 8.144(6) 8.106(9) 8.121(3) 8.215(3)
285.234 282.925 283.922 283.138 275.335 273.814 271.402 278.069
Oxide
Band positions (cm−1 )
Sr2 LaSbO6 Sr2 NdSbO6 Sr2 SmSbO6 Sr2 GdSbO6 Sr2 DySbO6 Sr2 ErSbO6 Sr2 YbSbO6 Sr2 YSbO6
671 sh, 647 vs 625 sh, 602 vs 620 sh, 613 vs 672 sh, 622 vs 672 sh, 630 vs 670 sh, 657 vs, 636 sh 672 vs, 643 sh 665 sh, 643 vs
vs: very strong; sh: shoulder.
390 vs, 380 sh 380 sh, 355 vs 365 vs 380 sh, 377 vs 370 vs, 330 sh 376 vs 368 vs, 333 sh 377 vs, 338 sh, 310 sh
154
A.E. Lavat, E.J. Baran / Journal of Alloys and Compounds 460 (2008) 152–154
Fig. 1. FTIR spectra of Sr2 ErSbO6 .
reinforcement of the Sb O bonds in this material. This abnormality can probably be related with the fact that the size difference between the cations located in the octahedral voids is the greatest for the pair La(III)/Sb(V). Therefore, the polarizing effect of the Sb(V) cation over the oxide anions has a stronger effect in this case than in all the others, in which the Ln(III) presents a smaller size and, concomitantly, a somewhat stronger polarizing power. This polarizing effects, which affect the Sb O La bonds, can eventually also impact on the relative tilting of the SbO6 and LaO6 octahedra and may also be responsible for the commented differences in the behaviour of the unit cell of Sr2 LaSbO6 , i.e., in this case a > b. A similar effect probably occurs in the case of Sr2 NdSbO6 which also shows the same inversion in unit cell parameters. But in this case the overall effects over the Sb O Nd bonds do not have the same impact over the spectroscopic behaviour as in the case of the lanthanum compound. - The fact that the lower energy band does not show a clearly defined trend demonstrates that this band originates in more complex motions which not only involve the antisymmetric SbO6 deformation but probably also vibrations related to LnO6 modes. Acknowledgements This work has been supported by the Universidad Nacional de La Plata and the Consejo Nacional de Investigaciones Cient´ıficas y T´ecnicas de la Rep´ublica Argentina. E.J.B. is a member of the Research Career from this organization. References
Fig. 2. Dependence of the highest energy IR-band (ν1 ) of the Sr2 LnSbO6 materials from the ionic radii (ri ) of the Ln(III) cations.
of these spectroscopic data merits further comments, as follows: - When the frequency of the strongest component of the higher energy band is plotted against the ionic radii of the Ln(III) cations (Shannon and Prewitt radii for octahedral coordination [14]) a practically linear trend is observed for all the materials, except that containing La(III), as depicted in Fig. 2. This behaviour may be interpreted as a direct consequence of the reinforcement of the Sb O bonds, which parallels the overall diminution of the unit cell volumes towards Yb(III). A similar behaviour has been often observed in other oxidic materials (cf. for example [11,15,16]). - The lighter and more voluminous La(III) presents an anomalous behaviour, pointing to a peculiar and unexpected
[1] E.J. Baran, Catal. Today 8 (1990) 133. [2] F.S. Galaso, Perovskites and High Tc Superconductors, Gordon and Breach, New York, 1990. [3] L.J. Tejuca, J.L.G. Fierro, Properties and Applications of Perovskite Type Oxides, Marcel Dekker, New York, 1993. [4] M.A. Pe˜na, J.L.G. Fierro, Chem. Rev. 101 (2001) 1981. [5] C.N.R. Rao, J. Gopalakrishnan, New Directions in Solid State Chemistry, Cambridge University Press, Cambridge, 1989. [6] J. Kurian, J. Koshi, P.R.S. Wariar, Y.P. Yadava, A.D. Damodaran, J. Solid State Chem. 116 (1995) 193. [7] W.T. Fu, D.J.W. Ijdo, J. Solid State Chem. 178 (2005) 2363. [8] P.J. Saines, B.J. Kennedy, M.M. Elcombe, J. Solid State Chem. 180 (2007) 401. [9] W.T. Fu, D.J.W. Ijdo, Solid State Comm. 134 (2005) 177. [10] P.E. Werner, Ark. Kemi 31 (1969) 51. [11] A.E. Lavat, E.J. Baran, Vibrat. Spectr. 32 (2003) 167. [12] J.T. Last, Phys. Rev. 105 (1957) 1740. [13] S.D. Ross, Inorganic Infrared and Raman Spectra, McGraw-Hill, London, 1972. [14] R.D. Shannon, C.T. Prewitt, Acta Crystallogr. B 36 (1970) 1076. [15] A.E. Lavat, E.J. Baran, R. S´aez-Puche, A. Salinas-S´anchez, J.M. MartinLlorente, Vibrat. Spectr. 3 (1992) 291. [16] R. Pis-Diez, E.J. Baran, A.E. Lavat, M.C. Grasselli, J. Phys. Chem. Solids 56 (1995) 135.