Acousto-ionic interaction in sound velocity behavior at a superionic phase transition in Me3H(AO4)2-type crystals

The velocities of quasi-longitudinal acoustic (QLA) phonons in (NH4)3H(SO4)2-(NHS) and Rb3H(SeO4)2-(RHS) crystals were studied by Brillouin scattering in the temperature range from 295 to 450 K. It has been found that the QLA phonon velocity in the vicinity of a superionic ferroelastic phase transition (Tc) exhibits an anomalous temperature dependence. Another characteristic feature of these crystals is a nontrivial frequency dispersion of the sound velocity manifesting itself in the temperature dependence of the QLA phonon velocity in the vicinity of Tc of RHS. At T < Tc, an additional contribution into the temperature dependence of the hypersound velocity of NHS has been revealed. Its behavior correlates with variations in conductivity. Based on analysis of the experimental data, a thermoactivation mechanism explaining the anomalous behavior of the QLA phonon velocity is proposed, and the corresponding activation energy is estimated. The results of our investigation provide direct evidence for the existence of the acousto-ionic interaction in the crystals featuring no piezoelectric effect. The anomalous behavior of the QLA phonon in the NHS crystal at T > Tc confirms the hypothesis that there exists an intermediate phase between 413 and 425 K.