Raman-Scattering Study of the Soft Phonon Modes in Hexagonal Barium Titanate

Abstract

The Raman spectrum of hexagonal BaTiO₃ has been investigated in the temperature range from 2 K to room temperature. Below the structural-phase-transition temperature of T_a(∼222 K), we find the corresponding underdamped soft-phonon modes; the mode frequencies are proportional to (T_a−T)^β with β∼1⁄3 in a wide temperature range (0.5<T⁄T_a<0.95). Although, in the ferroelectric phase below T_c(∼74 K), several new features are observed, no soft mode which correlates with the ferroelectric transition is found.