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 Ω−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-PbF2 and Ag2S 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-PbF2. 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.

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Superionics: crystal structures and conduction processes

Superionic conductors are materials which exhibit exceptionally high ionic conductivity whilst in the solid state. The manner in which certain structures accommodate superionic conduction has preoccupied many scientists throughout the latter part of the last century, beginning with the early debate about the disordered structure of the superionic α-phase of silver iodide. In this review, the key methods that have been used to deduce structural disorder in superionic conductors are described, and the important results summarized. The review focuses on simple archetypal systems, since these have dominated the literature, concentrating on more recent work and including emerging methodologies for deducing structural disorder. In most cases, the interpretation of diffuse scattering, as observed in scattering measurements, has played a crucial role in the understanding of these highly disordered systems and this is considered in some detail.

https://iopscience.iop.org/article/10.1088/0953-8984/14/32/201/pdf

Disordering phenomena in superionic conductors

Crystal structures and ionic conductivities of ternary derivatives of the silver and copper monohalides-II: ordered phases within the (AgX)x-(MX)1-x and (CuX)x-(MX)1-x (M = K, Rb and Cs; X = Cl, Br and I) systems

Silver and copper halides generally display an abrupt (first-order) transition to the superionic state. However, powder diffraction studies and molecular dynamics (MD) simulations of AgI under hydrostatic pressure both indicate that a continuous superionic transition occurs on heating. The gradual onset of the highly conducting state is accompanied by an increasing fraction of dynamic Frenkel defects, a peak in the specific heat and anomalous behavior of the lattice expansion. Similar methods have been employed to investigate the proposed continuous superionic transition between the two ambient pressure face centered cubic phases of CuI. This is difficult to examine experimentally, because the hexagonal $\ensuremath{\beta}$ phase exists over a narrow temperature range between the $\ensuremath{\gamma}$ (cation ordered) and $\ensuremath{\alpha}$ (cation disordered) phases. MD simulations performed with the simulation box constrained to remain cubic at all temperatures show that, although limited ${\mathrm{Cu}}^{+}$ Frenkel disorder occurs within $\ensuremath{\gamma}$-CuI, CuI undergoes an abrupt superionic transition at 670 K to the superionic $\ensuremath{\alpha}$ phase. This is supported by powder neutron diffraction studies of CuI lightly doped with ${\mathrm{Cs}}^{+}$ to prevent stabilization of the $\ensuremath{\beta}$ phase. The implications of these results on the phase transitions of other copper and silver halide superionic conductors are discussed.

Nature of the superionic transition in Ag+ and Cu+ halides

Silver mercury iodide thin films (Ag2HgI4) were successively deposited onto pre-cleaned glass substrates at room temperature by chemical bath deposition at different deposition times producing films of different thicknesses. The elemental composition of the as-deposited films measured by energy dispersive X-ray analysis shows the formation of stoichiometric Ag2HgI4 thin films. X-ray diffraction patterns reveal that Ag2HgI4 films are single phase of a polycrystalline structure, and have a dominant diffraction along [111] direction. The structure of the films was further investigated by transmission electron microscope verifying the polycrystalline nature of samples and nearly matching with X-ray diffraction data. The FESEM images reveal dense and well-defined grains. The optical properties of as-deposited films have been calculated from the recorded transmission and reflection data in the spectral range 400–2500 nm. The analysis of the optical absorption coefficient indicates the presence of direct and indirect optical transition whose values decrease with the increase of thickness, while the values Urbach energy follow is a reverse behavior. The study of optic parameters of 558 nm thick film heated at 330 K was also investigated. The electrical conductivity was measured by four-probe experiment in the range from 300 to 500 K. The activation energy was found to nearly equal half of the direct band gap indicating the intrinsic conduction of studied films is dominant.

Synthesis and characterization of thermochromic Ag 2 HgI 4 thin films

The nature of the superionic transition in Ag2 HgI4 and Cu2 HgI4 has been investigated using temper dependent powder neutron diffraction and impedance spectroscopy techniques. In the case of Ag2 HgI4 , the superionic transition occurs at Tc = 326(2) K and is accompanied by a 50-fold increase in the ionic conductivity. In the Cu+ analogue, which has a lower conductivity for a given temperature, the corresponding values are Tc = 338(4) K and / ~ 6. The ambient temperature crystal structures of the two compounds are different (space group I for -Ag2 HgI4 and I 2m for -Cu2 HgI4 ) but, in contrast to the most recent study, the high temperature polymorphs are found to be isostructural (space group F 3m ). Possible explanations for the different behaviour of the ionic conductivity of the two compounds are given.

https://iopscience.iop.org/article/10.1088/0953-8984/12/16/302/pdf

D A Keen

Papers

Structural characterization of the superionic transition in Ag2HgI4 and Cu2HgI4