A Discrete Two-Sublattice Model of the Lattice Instability in A₂BX₄-Type Crystals

Abstract

The discrete four-sublattice model previously proposed for reproducing the lattice instabilities in A₂BX₄-type crystals is reduced to a two-sublattice model. Eigenfrequencies and the associated eigenvectors of the dynamical matrix are given analytically for general k values. The conditions on the interaction parameters for the appearance of incommensurate phase transitions are clarified.