A priori theory of incommensurate behavior in Rb₂ZnCl₄

Abstract

Abstract We have performed a study of the potential energy, normal modes, static relaxation, and molecular dynamics of Rb2ZnCl4 using ab initio Gordon-Kim interionic potentials for intermolecular and interionic potentials. We find that the Pnam–Pna21 transition in this, and almost certainly many isomorphous systems, is entropy driven and shows no soft mode behavior. This is shown to arise from the presence in this system of a large equipotential volume of phase space between the Pnam and the tripled Pna21 low temperature structure. This region arises from a major lattice instability involving large rotations and displacements and having approximately six-fold screw symmetry along the a axis. This concealed imperfect symmetry which we call latent symmetry is the basic cause of the existence of a near-tripled incommensurate phase over a range of 113 K (302 K–189 K) between the Pnam and Pna21 phases. In this region the helicity is incommensurate with the lattice, being largely determined by the relief of intrahelical stresses. In addition, we have demonstrated by molecular dynamics that the ZnCl4 2- tetrahedra are orientationally disorderd, but, despite this, the helical instability remains. These findings are also related to anomalies in the observed Raman spectra and to X-ray data which confirm the presence of dynamically correlated disorder.