Structures of disordered alkali chlorides in normal and compressed states: An isothermal-isobaric molecular-dynamics study.

Abstract

Isothermal-isobaric molecular-dynamics simulations have been performed to investigate the glassy and liquid structures of two alkali chloride systems [mixture (LiCl${)}_{0.50}$(KCl${)}_{0.40}$(CsCl${)}_{0.10}$ and pure LiCl]. With the use of an ionic interaction model, the basic thermodynamic properties of the crystalline, liquid, and glassy states are successfully reproduced in the simulations. At normal pressure (101.3 MPa), it is found that the predominant short-range order in both systems is the ${\mathrm{LiCl}}_{4}$ tetrahedral units, each pair of which is mutually connected by sharing not only the vertices but also the edges of the tetrahedra. In the glassy and liquid states of LiCl-KCl-CsCl, the network structure is formed by polytetrahedral medium-range order (${\mathrm{LiLi}}_{4}$) consisting of five connected ${\mathrm{LiCl}}_{4}$ tetrahedra. Some portions of this network are truncated by the ${\mathrm{K}}^{+}$ and ${\mathrm{Cs}}^{+}$ ions adjacent to the vertex ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ ions. On the other hand, liquid and glassy LiCl has a disordered structure approximately analogous to zinc-blende structure, also including the wurtzitelike ionic arrangement. For LiCl in the amorphous and crystalline states, we performed isothermal-compression simulations up to 30 GPa. As in the experimental findings, no structural transformation occurs for the rocksalt LiCl crystal. The compression causes the crystallization of amorphous LiCl into rocksalt structure at more than 3--10 GPa with continuous structural change. This structural transformation is analogous to the pressure-induced polymorphic transition (zinc blende\ensuremath{\rightarrow}rocksalt) that occurs in the crystals of more covalent compounds such as CdS.