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Temperature Raman scattering study of CaAl_0.5Ta_0.5O₃ crystal

Tomasz Runka ¹, Marek Berkowski ², Miroslaw Drozdowski ¹

1. Poznan University of Technology (PUT), Nieszawska 13A, Poznań 60-965, Poland
2. Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Poland

Abstract

The temperature Raman scattering study of CaAl_0.5Ta_0.5O₃ (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 (12A_g + 12B_g) associated with octahedral tilting and cation ordering. In room temperature Raman spectrum, we observe characteristic features (internal vibrations of F_2g, E_g and A_1g symmetry) for cubic perovskite crystals with 1:1 B-site cations ordering [3]. It suggests the existence of regions with 1:1 B-site order in crystal structure. However, triply degenerate cubic-like mode F_2g(2) splits into three nondegenerate modes F_2g(2)→2A_g+B_g in monoclinic P2₁/n structure. What is interest, no splitting of doubly degenerate cubic-like mode E_g 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 F_2g(1) attributed to translational motions of Ca²⁺ cations splits into three nondegenerate, distinctly separated modes F_2g(1)→2A_g+B_g 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.

[1]. M. Berkowski, R. Aleksiyko, J. Fink-Finowicki, R. Diduszko, P. Byszewski, R. Kikalejshvili- Domukhowska, J. Cryst. Growth 269 (2004) 512.

[2]. I. Levin, J.Y. Chan, J.E. Maslar, T.A. Vanderah, S.M. Bell, J. Apply Phys. 90 (2001) 904.

[3]. T. Runka, R. Aleksiyko, M. Berkowski, M. Drozdowski, Cryst. Res. Technol. 40 (2005) 453.

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