Low Frequency Optical Phonons in Bi_4-xSm_xTi₃O₁₂

Abstract

A 180° Raman scattering experiment has been performed in Bi_4-x Sm _x Ti₃O₁₂ (BSmT-x) single crystals with x = 0, 0.15, 0.3, 0.46, and 0.61 in the temperature range between room temperature and ∼ 600°C. These materials were reported to undergo successive phase transitions at T _H and T _L (≤ T _H). In every BSmT-x, we have found the mode that has been assigned to a ferroelectric soft mode in Bi₄Ti₃O₁₂. There is quite a possibility that this mode is not a ferroelectric soft mode but an antiferrodistortive soft mode associated with T _L. The squared frequency ω_A ² of the mode can be scaled with a temperature difference (T - T _L) regardless of x, that is, the temperature T dependence of ω_A ² can be expressed by a straight line of −α (T- T_L ) + β, where α = 0.60 ± 0.01 cm⁻²/°C and β = 360 ± 10 cm⁻².

Keywords:

• Bi_4-x Sm _x Ti₃O₁₂
• layered perovskite
• Raman scattering
• ferroelectric soft mode
• antiferrodistortive soft mode

Acknowledgments

The authors express their appreciation to Mr. Yasushi Hirata for his production of the high temperature sample cell. The present work has been supported in part by a Grant-in-Aid for Scientific Research (C) from the Ministry of Education, Science, Sports and Culture of Japan, No. 22540323.