Superionics: crystal structures and conduction …

1 INSTITUTE OFPHYSICSPUBLISHINGREPORTS ONPROGRESS INPHYSICSRep. Prog. (2004) 1233 1314 PII: S0034-4885(04)93704-9 superionics : crystal structures and conductionprocessesStephen HullThe ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX, UKReceived 16 February 2004 Published 14 June 2004 Online conductors are compounds that exhibit exceptionally high values of ionicconductivity within the solid state. Indeed, their conductivities often reach values of theorder of 1 1cm 1, which are comparable to those observed in the molten state. FollowingFaraday s first observation of high ionic conductivity within the solids -PbF2and Ag2 Sin1836, a fundamental understanding of the nature of the superionic state has provided oneof the major challenges in the field of condensed matter science.

2 However, experimentaland theoretical approaches to their study are often made difficult by the extensive dynamicstructural disorder which characterizes superionic conduction and the inapplicability of manyof the commonly used approximations in solid state physics. Nevertheless, a clearer pictureof the nature of the superionic state at the ionic level has emerged within the past few different techniques have contributed to these advances, but the most significant insightshave been provided by neutron scattering experiments and molecular dynamics review will summarize the state of current knowledge concerning the crystal structuresand conduction processes of superionic conductors, beginning with a comparison of thebehaviour of two of the most widely studied binary compounds, AgI and -PbF2.

3 Eachcan be considered a parent of two larger families of highly conducting compounds which arerelated by either chemical or structural means. These include perovskite-structured oxides andLi+containing spinel-structured compounds, which have important commercial applicationsin fuel cells and lightweight batteries, respectively. In parallel with these discussions, therelative importance of factors such as bonding character and the properties of the mobile andimmobile ions (charge, size, polarizability, etc) in promoting the extensive lattice disorderwhich characterizes superionic behaviour will be assessed and the possibilities for predictinga prioriwhich compounds will display high ionic conductivity discussed.

4 (Some figures in this article are in colour only in the electronic version)0034-4885/04/071233+82$ 2004 IOP Publishing LtdPrinted in the UK12331234S HullContentsPage1. Introduction12362. Two superionic compounds: AgI and Silver iodide: Factors promoting superionic Lead fluoride: Type-I and type-II superionic High pressure studies12453. -AgI: chemical and structural The family of Ag+and Cu+halide The preferred cation sites in The role of the immobile Ion ion correlations: the Ag+ Stabilizing the -AgI phase at ambient The room temperature superionic: The role of dopant cations: Ag2MI4,Ag3MI5and The -AgI and perovskite structures : Perovskite-structured Distortions of the cubic perovskite structure : halide Superionicity in the lower mantle: Selection criteria for high conductivity perovskites: BaCeO3and Anion-deficient perovskites: Perovskite intergrowth structures .

5 The BiMeVOx compounds deficient perovskites: tysonite-structured deficient perovskites: La2/3 TiO3,La1/3 NbO3and Double perovskites: cryolite-structured Na3 AlF612744. -PbF2: chemical and structural The concentration of Frenkel The role of cation Ternary layered fluorite: Cuboctahedral defects: (K1 xBix)F1+2xand (Rb1 xBix)F1+ Non-stoichiometric Anion-excess fluorite: (Ca1 xYx)F2+ Anion-deficient fluorite: (Zr1 xYx)O2 Pyrochlore-structured superionics : Heavily defective fluorite: Oxide fluorites: superionicity in The effect of ionic size: anti-fluorite The paddle-wheel and percolation mechanisms in Spinel-structured superionics : Lithium battery applications.

6 LixMn2O41297 crystal structures and conduction superionics based on an hcp sublattice: the The tetragonal-packed sublattice: Li4 SiO4and Li4 GeO412985. Summary and conclusions1300 Acknowledgments1301 References13021236S Hull1. IntroductionSodium chloride, NaCl, is a typical example of a normal ionic solid. At ambient temperature,nominally pure NaCl has an ionic conductivity iless than 10 8 1cm 1, the finite valuebeing primarily due to the presence of extrinsic defects associated with cation impurities(see, [1]). As illustrated in figure 1, iincreases with temperature above 500 K as ionictransport becomes dominated by an increasing concentration of intrinsic, thermally activatedSchottky defects, reaching a value of 10 5 1cm 1immediately below the melting pointof 1074 K.

7 On melting , iincreases abruptly by around five orders of magnitude, to a value 3 1cm 1[2].The first reports of compounds that possess exceptionally high ( liquid-like ) values ofionic conductivity within the solid state were given by Faraday in the first half of the 19thcentury. In the case of the fluoride of lead , the extraordinary nature of this behaviour is clearfrom the original text [3]: When a piece of that substance, which had been fused and cooled, was introducedinto the circuit of a voltaic battery, it stopped the current. Being heated, it acquiredconducting powers before it was visibly red hot in -PbF2is just one example of a collection of highly conducting solids, which subsequentlybecome known as superionic or fast-ionic shown in figure 1, thereis a continuous and rapid increase in the ionic conductivity of -PbF2with , ilevels off above 700 K at a value of 4 1cm 1and, remarkably, showsno measurable change on melting at 1158 K [4 6].

8 The behaviour of another widely studiedsuperionic compound, AgI, is rather different. At 420 K it undergoes an abrupt increasein iof over three orders of magnitude associated with a solid solid phase transition(see figure 1). In the high temperature superionic phase (labelled -AgI) iis again very high( i 2 1cm 1), increases only slowly with temperature and actually decreases by 10%on melting at 829 K [7].The next section of this review will provide a more detailed description of both AgIand -PbF2, including a comparison of their behaviour, followed by two sections giving adiscussion of the properties of many other highly conducting compounds.

9 The primary aim hereis to summarize the current state of knowledge concerning the crystal structures of the varioussuperionic compounds and the associated thermally induced lattice disorder that underlies thehigh macroscopic ionic conductivity. A comprehensive survey of the entire literature relatingto superionic conductors would be a mammoth undertaking more appropriate to a dedicatedtextbook than a review inReports on Progress in Physics. Inevitably, the descriptions will berather general in nature and restricted to systems in which the ionic migration occurs in threedimensions throughout the bulk of the reviews of this subject have categorized the known superionic conductorsaccording to their technological function [23], the nature of the transition to the highlyconducting state (abrupt or continuous) [24], their constituent chemical species [25] or crystalstructures [26].

10 The rather haphazard scheme suggested by the Contents list of this reviewhas been devised to highlight the similarities, rather than differences, between the variouscompounds. This provides a convenient approach when assessing the relative importance ofthe crystal structure and the properties of the ions themselves (size, charge, polarizability, etc)1As a consequence, a number of interesting systems in which the ionic motion is restricted to essentially isolatedone-dimensional channels or two-dimensional planes will be ignored. These include hollandite-structured oxides[8, 9] and the -aluminas [10, 11], respectively.