High-Pressure Studies of Ionic Conductivity in Solids

Abstract

The chapter presents a discussion on high-pressure studies of ionic conductivity in solids. The chapter presents a review and discusses hydrostatic pressure studies performed on a variety of ionic conductors. The chapter presents discussion on the experimental results and their interpretation in terms of relevant concepts and theory. The chapter presents examples, which illustrate special features or show systematic trends or both. Much of the available pressure work has been on relatively simple materials and crystal structures because these are amenable to theoretical treatment. Some of the materials investigated (for example, PbF2 and the thallous halides) are especially interesting because, among other properties, they possess large dielectric constants and also exhibit relatively soft, low-lying phonon modes. These factors are important in relation to ionic conduction because the larger the dielectric constant of an ionic crystal, the lower the energy of formation of lattice defects. Also, physically, ionic transport occurs by hopping motion across an energy barrier, and this barrier might be expected to become smaller the “softer” the lattice. The chapter presents analysis on the evidence for the connection between these properties and the transport properties. The chapter presents a brief theoretical background with some of the concepts and results necessary for the analysis and interpretation. For this purpose it was found most convenient to divide the substances of interest into several groupings as follows: alkali halides (both NaCl and CsCl types), silver halides, thallium halides, fluorites and related structures, and fast ion conductors.