Superionics: crystal structures and conduction processes

Abstract

Superionic conductors are compounds that exhibit exceptionally high values of ionic conductivity within the solid state. Indeed, their conductivities often reach values of the order of 1 −1 cm −1 , which are comparable to those observed in the molten state. Following Faraday's first observation of high ionic conductivity within the solids β-PbF 2 and Ag 2 S in 1836, a fundamental understanding of the nature of the superionic state has provided one of the major challenges in the field of condensed matter science. However, experimental and theoretical approaches to their study are often made difficult by the extensive dynamic structural disorder which characterizes superionic conduction and the inapplicability of many of the commonly used approximations in solid state physics. Nevertheless, a clearer picture of the nature of the superionic state at the ionic level has emerged within the past few decades. Many different techniques have contributed to these advances, but the most significant insights have been provided by neutron scattering experiments and molecular dynamics simulations. This review will summarize the state of current knowledge concerning the crystal structures and conduction processes of superionic conductors, beginning with a comparison of the behaviour of two of the most widely studied binary compounds, AgI and β-PbF 2 . Each can be considered a parent of two larger families of highly conducting compounds which are related by either chemical or structural means. These include perovskite-structured oxides and Li + containing spinel-structured compounds, which have important commercial applications in fuel cells and lightweight batteries, respectively. In parallel with these discussions, the relative importance of factors such as bonding character and the properties of the mobile and immobile ions (charge, size, polarizability, etc) in promoting the extensive lattice disorder which characterizes superionic behaviour will be assessed and the possibilities for predicting a priori which compounds will display high ionic conductivity discussed.

A study of high-energy proton induced damage in cerium fluoride in comparison with measurements in lead tungstate calorimeter crystals

“…It is also paramagnetic, a characteristics which influences the Faraday rotation in a magnetic field [17]. Fluorine ions labelled F 1 in literature [18] account for most of the Cerium Fluoride conductivity and hence their mobility might help repair defects. There is a vast literature on this and in particular on doping [19] with Barium and Strontium, which have been observed to increase the conductivity even further [20].…”

High-temperature thermal expansion and structural behaviour of stromeyerite, AgCuS

“…For a long time, copper and silver based chalcogenides have attracted considerable attention due to their promising ion transport properties [1,2]. Recently, the high ionic conductivity for cations renewed the interest in silver copper sulfide, AgCuS (mineral name stromeyerite).…”

Giant Mechanocaloric Effects in Fluorite-Structured Superionic Materials

“…12,13 In fact, the critical temperature in superionic materials can be reduced significantly by means of nanopatterning and chemical substitution strategies. 2,23 Aimed at alleviating the technical shortcomings found in CaF 2 , we investigated the same class of superionic and mechanocaloric phenomena in PbF 2 , a related fluorite-structured FIC with a much lower transition temperature of T s (0) ∼ 700 K. 24 (We note that by minimally doping PbF 2 with potassium ions it is possible to reduce the corresponding critical temperature practically down to ambient. 25 ) We adopted a rigid-ion BMH interatomic potential to describe the interactions between atoms in PbF 2 .…”