Phenomenological theory of the reconstructive phase transition between the NaCl and CsCl structure types

A phenomenological model of the reconstructive transformation between the NaCl and CsCl structure types is described. It consists of two consecutive displacive mechanisms, coupled to tensile and shear strains. These mechanisms give rise to intermediate orthorhombic structures, including the B16 and B33 structure types. The model is shown to explain consistently the experimental observations in different groups of binary compounds as monochalcogenides, monopnictides, and monohalides. The isostructural transitions found in rare-earth chalcogenides are interpreted by the coupling of the displacive order parameters to the change in compressibility related to the valence transitions. The previous models proposed for the NaCl-CsCl transformation are critically analyzed.