Fluorites: Superionics and Conduction Processes
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1122-O05-01
Fluorites : Superionics and Conduction Processes Steve Hull The ISIS Facility, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom.
ABSTRACT The superionic conducting properties of fluorite structured compounds are discussed in this paper, starting with a brief historical overview of the topic. The current state of knowledge concerning the dynamic lattice disorder associated with the high values of ionic conductivity is summarized and illustrated using three examples taken from the author’s research program. These are chosen to highlight different aspects of the structure-property relationships within fluorite-structured superionics and, in particular, demonstrate the role of the neutron diffraction technique in investigations of compounds showing high levels of ionic conductivity. The paper concludes with an overview of some current developments within this research field. INTRODUCTION ‘Superionic conductors’ is a term used to describe compounds which exhibit exceptionally high values of ionic conductivity within the solid state. In many cases, their conductivities reach values of around 1Ω−1cm−1 and are comparable to the liquid state. The first experimental studies of superionic conductors were performed by Faraday in the 1830s. These include the ‘fluoride of lead’, for which, ‘When a piece of that substance, which had been fused and cooled, was introduced into the circuit of a voltaic battery, it stopped the current. Being heated, it acquired conducting powers before it was visibly red hot in daylight.....’ [1]. Later studies showed that the ionic conductivity of β-PbF2 increases continuously on heating, before leveling off at a value indistinguishable from that of the liquid state [2], and is predominantly due to the diffusion of anions through the crystalline lattice [3]. The gradual transition to the superionic state (a type-II transition in the notation of Boyce and Huberman [4]) is associated with a peak in the specific heat Cp, whose maximum is generally taken to define the superionic transition temperature, Tc, where Tc=711K in β-PbF2 [5]. It is now well established that β-PbF2 is only one member of a larger family of binary halides which possess the cubic fluorite crystal structure and display superionic behavior at elevated temperatures (these include CaF2, BaF2, and SrCl2 and, for a recent review, see [6]). The fluorite structure (space group Fm3m ) can be described as a face-centred cubic (f.c.c.) array of cations (in 4(a) sites at 0,0,0, etc.) in which all the tetrahedrally co-ordinated interstices (8(c) sites at 1/4,1/4,1/4, etc.) are filled with anions and the octahedrally co-ordinated ones (4(b) sites at 1 1 1 /2, /2, /2, etc.) are empty. However, an alternative view, which proves more convenient to describe the nature of the anion disorder, is to consider the ionic arrangement as a simple cubic array of anions with cations occupying alternate cube centers. At temperatures close to ambient
(i.e. well below Tc) the energetically favored defects
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