Lattice- and Molecular-Dynamics Studies of RbLiSO₄

Authors

V. Katkanant, California State University, Fresno, California
John R. Hardy, University of Nebraska - LincolnFollow

Date of this Version

1995

Comments

Published in Phys. Rev. B Physical Review B 51, 137 - 145 (1995). Copyright © 1995 The American Physical Society. Used by permission.

Abstract

Using our previously developed method for calculating parameter-free potential-energy surfaces for ionic molecular crystals, specifically sulfates, we study phase transitions in RbLiSO₄ by means of lattice and molecular dynamics. We found that the high-temperature phase I (Pnam) is highly unstable and transforms to the observed lower temperature phase VI (P1121/n) at about 475–525 K. Compared with isomorphous CsLiSO₄, there are more branches of unstable modes in the Pnam phase for RbLiSO₄. The maximum instability of these modes occurs away from the zone center, q=(0.118a*,0,0), which implies that a high-order incommensurate phase could form during the phase transformation. The driving mechanism of these phase transitions is directly related to the rotational ordering of tetrahedral SO₄ ^2- groups. The rms values of the deviations of the S-O bonds from their orientations in the Pnam phase to those in the P112₁/n phase were found to be approximately ±23–25°.