The Raman spectrum of hexagonal BaTiO 3 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.
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Yamaguchi et al. (1987) studied this question.
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