Elastic Instability in the Ferroelastic Phase Transition of TlH₂PO₄Studied by Brillouin Scattering

Abstract

Temperature dependence of the acoustic modes of TlH 2 PO 4 has been observed by the Brillouin scattering around its higher phase transition point at 357 K ( T tr ). The elastic constant c 55 obtained by the angular-dependent spectra shows softening with the Curie-Weiss law in both phases. The phase transition is concluded to be a ferroelastic one induced by an instability of the transverse acoustic mode which corresponds to the shear strain x 5 . The angular dependence of the spectra reveals a typical monoclinicity below T tr . The two-dimensionality of hydrogen-bonded network does not affect the dynamical property of the phase transition mechanism.