Brillouin scattering by transverse acoustic waves in NH4 LiSO4 near the ferroelectric transition

The elastic constant C55 and the combinations (C44 + C55)/2 and (C55 + C66)/2 have been investigated as function of temperature from Brillouin scattering spectra by transverse acoustic waves. The main feature is an increase for every constant below the transition. A broadening of the Brillouin peak corresponding to (C44+ C55)/2 is also observed. The results are discussed on the basis of a biquadratic coupling between the shear strains and the order parameter.