Temperature Raman scattering study of CaAl0.5Ta0.5O3 crystal
|Tomasz Runka 1, Marek Berkowski 2, Miroslaw Drozdowski 1|
1. Poznan University of Technology (PUT), Nieszawska 13A, Poznań 60-965, Poland
The temperature Raman scattering study of CaAl0.5Ta0.5O3 (abbreviated as CAT) perovskite crystal grown by the Czochralski method was carried out. The CAT crystal belongs to family of ordered perovskites with general formula. At room temperature, the crystal possesses orthorhombic Pbnm structure with small deviation to monoclinic P21/n structure, which results from octahedral distortions of simple cubic perovskite structure (b-b-c+ tilt system in Glazer notation) and B-site cation ordering. [1-2].
The Raman spectra were recorded in spectral range 70 – 1000 cm-1 for Z(XX)Y and Z(XZ)Y scattering geometry. The number of observed Raman modes for investigated crystal is lower than theoretically predicted for monoclinic P21/n structure. The site symmetry group analysis predicts 24 Raman-active modes (12Ag + 12Bg) associated with octahedral tilting and cation ordering. In room temperature Raman spectrum, we observe characteristic features (internal vibrations of F2g, Eg and A1g symmetry) for cubic perovskite crystals with 1:1 B-site cations ordering . It suggests the existence of regions with 1:1 B-site order in crystal structure. However, triply degenerate cubic-like mode F2g(2) splits into three nondegenerate modes F2g(2)→2Ag+Bg in monoclinic P21/n structure. What is interest, no splitting of doubly degenerate cubic-like mode Eg is observed. The numerous Raman bands observed below 300 cm-1 can be attributed to lattice vibrations. They can be assigned to translational vibrations of Ca2+ and B’/B”O6 ions and tilt modes of B’/B”O6 ions. The triply degenerate cubic-like mode F2g(1) attributed to translational motions of Ca2+ cations splits into three nondegenerate, distinctly separated modes F2g(1)→2Ag+Bg appearing in spectral range 180 – 240 cm-1.
The temperature investigations were carried out in wide temperature range from 77 K to 630 K. All bands recorded in spectra were fitted considering Böse-Einstein occupation factor. Significant temperature changes in spectral parameters of Raman bands, especially in the lattice modes region have been revealed. Some changes are connected with temperature induced distortions of oxygen octahedra, and on the other hand, with lattice parameter changes.
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. T. Runka, R. Aleksiyko, M. Berkowski, M. Drozdowski, Cryst. Res. Technol. 40 (2005) 453.
Presentation: Poster at Joint Fith International Conference on Solid State Crystals & Eighth Polish Conference on Crystal Growth, by Tomasz Runka
See On-line Journal of Joint Fith International Conference on Solid State Crystals & Eighth Polish Conference on Crystal Growth
Submitted: 2007-01-15 11:58 Revised: 2009-06-07 00:44