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].