Effect of Ca-Substitution on CdTiO₃ Studied by Raman Scattering and First Principles Calculations

Abstract

An effect of Ca-substitution on ferroelectricity of CdTiO₃, whose paraelectric structure involves rotated oxygen octahedra, is studied by Raman scattering and first-principle calculations from a viewpoint of a relation between covalency and instability of lattice vibrations. Raman scattering experiments reveal that the Ca-substitution suppresses softening of a soft mode and reduces the phase transition temperature. An origin of the soft mode is elucidated by first principles calculations to be an asymmetric stretching of O-Cd-O bonds. The mechanism of Ca-induced suppression of the soft mode softening is clarified as follows: off-centering of Cd, which stems from freezing of the O-Cd-O asymmetric stretching vibration, is suppressed by the Ca-substitution through weakening of the covalency between Cd and O. The present results indicate that the covalency of ions plays a key role even in the perovskite-type oxides with pre-existing octahedral rotations.

Keywords:

• Ferroelectricity
• perovskite
• phase transition
• Raman scattering
• first principles calculation

Acknowledgment

Part of this work was financially supported by KAKENHI, Nos. 20246098, 23246113, and 21560708, from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.