Department of Physics and Astronomy: Publications and Other Research
Date of this Version
1992
Abstract
Structural relaxations, molecular-dynamics simulations, and lattice-dynamics calculations were performed to study the phase transitions in Rb2ZnCl4, using intermolecular and intramolecular potentials generated from ab initio quantum-chemistry calculations for the whole molecular ion ZnCl4 2-. Compared with an earlier treatment of the system by a polarizable-ion model, the present approach emphasizes the static effect of the electron covalency within the molecular ions that affects strongly both the intermolecular and intramolecular interactions. The calculations gave a close agreement with experiment on the static structures of the Pnam and the Pna21 phases and the transition temperature from the former to the latter. For the lower-temperature, monoclinic phase of Rb2ZnCl4, the detailed structure of which is unknown, our simulations predict a structure with C1c1 space-group symmetry, which doubles the Pna21 structure along both the b and c axes and thus has 48 formula units per unit cell. The lattice-dynamics calculations for the Pna21 structure clearly revealed the lattice instability responsible for the Pna21-monoclinic transition and provided a more convincing explanation of a previous Raman measurement. We have shown that the potential-energy surface in Rb2ZnCl4 pertinent to the phase transitions contains a double-well structure, very similar to that of K2SeO4, except that the double well is much deeper, causing the much more severe disordering in the Pnam structure of Rb2ZnCl4 observed experimentally.
Comments
Published in Phys. Rev. B 45, 7609 - 7620 (1992). Copyright © 1992 The American Physical Society. Used by permission.