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Ionic conduction in solids has been a subject of interest as early as the beginning of the 19th century. It was established by Michael Faraday (1839) that the laws of electrolysis are also obeyed in ionic solids like PbF2 and Ag2S. There were, however, several important discoveries regarding ionic conduction phenomena in solids in the last century. For example, extraordinary high ionic conductivity in silver iodide was found by Tubandt et al (1921) is of special importance. It was observed during the conductivity measurement of AgI that above 147 °C it transforms into a phase that exhibits an ionic conductivity of ~ 1 -1cm−1, comparable to that of its liquid phase. Therefore, the high temperature phase of AgI is the first superionic conductor ever discovered. It was shown that a sublattice cationic disorder takes place in the highly conducting phase of AgI, which is now better known as -AgI. The liquid-like state of Ag+ ions, as proposed by Strock (1934, 1936) and later reinforced by others (Geller, 1977; Funke, 1976), consists of a cubic unit cell of iodine ions, in which 6 octahedral, 12 tetragonal and 24 trigonal bipyramidal, i.e., a total of 42 sites are available for 2 Ag+ ions, as shown in the Figure 1. O' Keeffe and Hyde (1976) have argued that this phase transition in AgI is dramatic and powerful, nothing less than the melting and also shown that the entropy change at the superionic transition is comparable to its value at the melting. Thus, in the -phase I− ions form a bcc lattice and the Ag+ ions are distributed in such a way that 42 crystallographic equivalent interstices are available for the two Ag+ ions.
Soon alpha-phases of various materials like Ag2S, Ag2Se, Ag2Te, etc. were discovered (Tubandt, 1932). By the early 1930s, it was demonstrated that these fast ionically conducting solids could be treated entirely the same as aqueous electrolytes from the viewpoint of chemical reactions and thermodynamics, hence these materials were also termed as solid electrolytes.
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