Raman Scattering Study of the Soft Mode in Ferroelectric (CH₃NHCH₂COOH)₃CaCl₂

Abstract

Raman spectra of the soft modes have been measured in the very vicinity of the transition temperature T_c=131.7 K in the ferroelectric phase of (CH₃NHCH₂COOH)₃CaCl₂(TSCC) with scattering geometries b−c(b+c, b−c)b+c and c−b(c+b, c−b)c+b. The longitudinal soft mode is still observed as an underdamped mode even just below T_c, whereas the transverse one is overdamped. The temperature dependence of dielectric constant described with the small Curie-Weiss constant is quantitatively explained by those of the two modes in terms of the Lyddane-Sachs-Teller relation. The effective relaxation time of the overdamped mode is the same order of magnitude as the relaxation time obtained by dielectric dispersion. The temperature dependence of the intensity of the soft mode agrees with the theoretical prediction based on the fact that this mode becomes Raman active below T_c.