The authors review the development of extended x-ray absorption fine structure (EXAFS) within the last decade. Advances in experimental techniques have been largely stimulated by the availability of synchrotron radiation. The theory of EXAFS has also matured to the point where quantitative comparison with experiments can be made. The authors review in some detail the analysis of EXAFS data, starting from the treatment of raw data to the extraction of distances and amplitude information, and they also discuss selected examples of applications of EXAFS chosen to illustrate both the strength and limitations of EXAFS as a structural tool.

Extended x-ray absorption fine structure—its strengths and limitations as a structural tool

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: Transitions, structures, dynamics

The present theoretical understanding of various properties of superionic conductors is reviewed. Emphasis is put on their treatment as classical many-particle systems and on the analysis of their dynamic behaviour. Different kinds of approaches pertaining to the low frequency dynamics are considered in detail. They include stochastic models, like hopping or Fokker-Planck models as well as a hydrodynamic theory. The high frequency (phonon-) dynamics and the information obtained from computer simulations is also analysed. As far as possible, the relevance of the different approaches with respect to experiments on specific materials is discussed. Possible directions for future investigations are outlined.

Theoretical models for superionic conductors

Publisher Summary This chapter describes extended x-ray absorption fine structure spectroscopy (EXAFS) spectroscopy with a breadth sufficient to communicate to the non-expert potential of this technique. It discusses the theoretical underpinnings and experimental state of the art with depth sufficient to delineate clearly to a practitioner the capabilities and limitations of this spectroscopy. A especially useful expression is derived for the EXAFS from basic considerations, delineating explicitly the approximations required at each step. The more significant of these approximations are examined in the context of both theory and experiment. The calculation of EXAFS spectra is also discussed. The structural information content of the EXAFS is set forth explicitly and compared with the information available from a diffraction measurement. Finally, a survey is presented of the wide and growing variety of the applications of this technique, with particular emphasis on those in condensed matter physics.

Extended X-Ray Absorption Fine Structure Spectroscopy

An excluded volume model is proposed for the cation-anion interaction in superionic conductors. The resulting cation-anion correlations are in excellent quantitative agreement with the pair distribution function derived from EXAFS measurements on AgI in both the normal beta and superionic alpha phases. Further, from the EXAFS data alone, the model predicts the substantial increase in ionic conductivity at the phase transition as well as the modest temperature dependence above it.

https://iopscience.iop.org/article/10.1088/0022-3719/11/14/016/pdf

J B Boyce

Papers

A structural model for superionic conduction