Simulation of Normal Rb₂ZnCl₄ Near the Incommensurate Transition

Authors

P. J. Edwardson, University of Nebraska - Lincoln
V. Katkanant, University of Nebraska - Lincoln
John R. Hardy, University of Nebraska - LincolnFollow
L. L. Boyer, U. S. Naval Research Laboratory, Washington, D.C.

Date of this Version

6-1-1987

Comments

Published in Physical Review B Volume 35, Number 16, 1 June 1987. Copyright © 1987 The American Physical Society. Used by permission.

Abstract

The purposes of the reported computer simulation of the normal (high-temperature) phase of rubidium tetrachlorozincate are to understand the disordered structure in that phase and to investigate the possibility that the transition, upon cooling, from the normal phase to one with an incommensurate modulation is associated with a change from the disordered structure to an ordered one. The simulation of the dynamics of 168 ions in a periodic structure begins from a slight perturbation of a structure that is determined by minimization of the potential energy within the constraints of the experimentally determined average symmetry. Rigid ions with short-range interactions described by the electron-gas model (with a qualification) are assumed. We find both zinc-induced and rubidium-induced instabilities in the chloride sublattices of the average experimental structure. The zinc-destabilized chloride ions move to a new sublattice in the simulation; however, a crude estimate indicates that this is caused by neglect of ionic polarizability and that these chlorides should either remain at their original sites or be disordered with chains of correlated positions. The rubidium-destabilized chloride ions form two-dimensional ordered networks in the disordered structure. We suggest that the inevitable freezing-out of disorder among the chains of zinc-destabilized chloride ions and among the networks of rubidium-destabilized chloride ions is the mechanism for the transition to the incommensurate phase.