Brillouin-scattering studies of ferroelectric [NH3(CH3)](5)Bi2Cl11

High-resolution Brillouin spectroscopy was used to investigate the elastic properties of [NH3(CH3)](5)Bi2Cl11. The temperature dependences of nine pho non modes propagating in the directions [100], [010], [001], [110], [101], and [011] were obtained in the temperature range from 120 to 330 K. Strong correlations were found with the electric polarization mode which exhibits anomalous behavior at 307 and 170 K. A theoretical explanation of the exper imental observations was given in terms of Landau-type free energy expansio n for two inequivalent sublattices. Discussion was provided regarding the p ossible types of couplings between strain tensor components and spontaneous polarization. [S0163-1829(98)07345-7].