Structural properties and lattice dynamics of 5d transition metal antifluorite crystals

This article contains a review of the physics of 5d transition metal antifluorite crystals with emphasis on the new insights that have resulted from the research of the past ten years. Section 2 contains a discussion of the lattice dynamics of these crystals. A rigid ion model is proposed and the necessary input data as obtained from various forms of spectroscopy considered. Force constants, normal mode frequencies and eigenvectors are deduced for the prototype crystal K2ReCl6. Nuclear magnetic resonance evidence for molecular reorientations is given. Section 3 presents a description of structural phase transitions in terms of the Landau theory. Evidence for several rotative type phase transitions is outlined; the soft modes are identified. A brief reference to central modes and cluster excitations is included. Data suggesting another type of structural transition, distortive in nature, are introduced. Section 4 provides an overview of our knowledge of the transferred hyperfine interaction and of the magnetic structure in the antiferromagnetic phase. In section 5 three topics considered peripheral to the main body of the article are mentioned briefly. These are the nature of the chemical bond, the study of mixed crystals and protonic conduction. (orig.)