  The interaction between soft optic and acoustic phonons was investigated for
Sn2P2(Se0.28S0.72)6 proper uniaxial ferroelectrics by Brillouin scattering and
ultrasonic pulse-echo techniques. The elastic softening of hypersound velocity
of transverse acoustic phonons and for both longitudinal and transverse
ultrasound waves which propagate near direction of the modulation wave vector
(in the incommensurate phase at x>xLP) was found at cooling to the Lifshitz
point in the paraelectric phase. The strong increase of the ultrasound
attenuation have also been observed. Such phenomena are related to the linear
interaction of the soft optic and acoustic branches in the region of relatively
short-range hypersound waves and to the strongly developed long-range order
parameter fluctuations in the ultrasound frequency range. The hypersound
velocity temperature dependence was described within the Landau-Khalatnikov
approximation for the ferroelectric phase.
