S. G. Lushnikov, A. I. Fedoseev, L. A. Shuvalov, H. Shmidt, “Anomalous behavior of the velocity of hypersonic acoustic phonons in (NH$_4)_3$H(SO$_4)_2$ crystals”, Pis'ma v Zh. Èksper. Teoret. Fiz., 2003, Volume 78, Issue 2,Pages 95

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Anomalous behavior of the velocity of hypersonic acoustic phonons in (NH$_4)_3$H(SO$_4)_2$ crystals

S. G. Lushnikov^a, A. I. Fedoseev^a, L. A. Shuvalov^b, H. Shmidt^c

^a Ioffe Physico-Technical Institute, Russian Academy of Sciences, St. Petersburg
^b Institute of Cristallography Russian Academy of Sciences, Moscow
^c Montana State University

Abstract: The velocity of quasi-longitudinal acoustic (QLA) phonons in (NH$_4)_3$H(SO$_4)_2$ (TAHS) crystals was studied by Brillouin scattering in the temperature range from 295 to 430 K. It was found that the QLA phonon velocity in the vicinity of a superionic ferroelastic phase transition ($T_c=413\,$K) exhibits anomalous temperature dependence. At $T<T_c$, an additional contribution is separated from the temperature dependence of the hypersound velocity, the behavior of which is correlated with variations of the conductivity of a TAHS crystal. Based on an 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.

PACS: 63.20.-e, 64.60.Ht, 78.35.+c

Received: 24.06.2003