High-resolution broadband Brillouin scattering study of antiferroelectric phase transition in CsBi(MoO4)(2)

High-resolution Brillouin scattering studies of the antiferroelectric phase transition in CsBi(MoO4)(2) have been performed in the temperature range 360-150 K giving information about the temperature dependence of the acoustic phonons propagating in the [100], [010], and [001] directions as well as the lowest-frequency optic soft phonon. These studies show that the soft phonon remains underdamped up to the transition point where its frequency decreases to zero. Strong anomalies in sound velocity, attenuation, and intensity have been observed for sound waves corresponding to the c(11), c(22), and c(44) elastic constants. The coupling between the order parameter and elastic strains is linear quadratic for the c(11), c(22), and c(44) elastic constants. In case of the c(33) and c(55) elastic constants the coupling is biquadratic, giving rise to a frequency decrease and increase, respectively, of the corresponding acoustic modes below T-c. These results indicate that the phase transition is not purely antiferroelectric, as suggested previously, but occurs most likely to a monoclinic phase (point symmetry C-2h) which is simultaneously antiferroelectric and ferroelastic.