The crystal structures of the four phases of the 'anti-perovskite' superionic conductor Ag 3 SI have been investigated by powder neutron diffraction and complex impedance spectroscopy. The high-temperature α-phase is characterized by a random distribution of the two cation species S 2− and I − over the 2(a) positions at 0, 0, 0 and 1/2, 1/2, 1/2 within space group I m3m. The Ag + are found to preferentially occupy the 24(h) trigonal interstices within the anion sublattice, though with significant anisotropy in their thermal vibrations. On quenching from high temperature the disordered cation array is retained, forming the metastable α * -phase, but there is a significant change in the cation distribution. The Ag + are predominantly located in 24(g) sites between the octahedral and tetrahedral cavities, though a significant proportion are found in 48(i) sites close to the 1/4, 1/4, 1/4 position midway between two anions. The latter are displaced by ∼0.7 Å in 110 directions to avoid short cation-anion contacts. Slow cooling from the α-phase followed by prolonged annealing at modest temperatures (∼473 K) stabilizes the β-phase of Ag 3 SI, in which longrange ordering of the two cation species lowers the symmetry to P m3m. The Ag + now occupy 12(h) positions close to half the octahedral sites, such that they do not form close contacts with the larger I − species. The displacements are in the four 100 directions and towards the tetrahedral sites, with a single Ag + randomly occupying one of these four 'split' positions. On cooling, the ionic conductivity is found to drop abruptly by ∼4× at T = 156(2) K at the β → γ transition. High-resolution powder neutron diffraction studies indicate that γ -Ag 3 SI possesses a very small rhombohedral distortion of the unit cell. The crystal structure of γ -Ag 3 SI (space group R3) can be derived from that of the β-phase by long-range ordering of the Ag + onto a subset of the displaced octahedral positions and an associated small displacement of the S 2− in a 111 direction.

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The crystal structure and ionic conductivity of polycrystalline samples of with are investigated using neutron diffraction and impedance spectroscopy techniques. The maximum solid solubility of KF in the fluorite structured is found to be x = 0.013(6). However, the introduction of and the associated charge compensating anion vacancies has a dramatic effect on the ambient temperature ionic conductivity, which increases by a factor for an x = 0.01 sample at 350 K. At higher dopant levels the ambient temperature conductivity falls steadily in the range due to the presence of an increasing volume fraction of relatively poorly conducting additional phases, apparently comprising orthorhombic and a further phase of probable composition . However, the ionic conductivity of these samples show an abrupt increase at T = 520(5) K, as the two minority phases react to form a single, superionic phase. Within this high-temperature modification the anions are dynamically disordered over the tetrahedral and, to a lesser extent, the octahedral interstices created by a body centred cubic (bcc) cation sublattice formed by and . This phase is, therefore, an example of an anion conducting bcc superionic and its structure is `anti-' to that adopted by the archetypal superionic phase . A plausible model describing the structural relationship between these two anti-types, including the short-range order between mobile ions within the bcc- phase, is given.

The structural properties of copper (I) iodide have been investigated at elevated pressures and temperatures using the neutron powder diffraction technique, to probe the effects of pressure on the superionic properties of this compound. On increasing temperature at a pressure of p = 1.30(8) GPa, three structural phase transitions are observed. The first is from the ambient temperature zincblende structured phase CuI-III to rhombohedral CuI-IV at T = 444(6) K. There is only limited cation disorder in CuI-IV which increases gradually with temperature. The preferred locations of the interstitial cations are sites between the tetrahedral and octahedral interstices within the slightly distorted face-centred cubic (f.c.c.) anion sublattice. A subsequent transition to the disordered f.c.c. structured phase CuI-I occurs at T = 694(5) K. This phase shows complete cation disorder at all measured pressures and temperatures. Finally, CuI undergoes a further phase transition at a temperature of T = 920(15) K. The first diffraction studies of this high pressure phase (labelled CuI-VII) are presented, which indicate that this phase is a body-centred cubic (b.c.c.) superionic with complete disorder of the cation sublattice. The cations are found to preferentially occupy the tetrahedral sites, in a manner similar to that in isostructural (ambient pressure) superionic phases such as and . The structural systematics of the superionic binary halide compounds and their thermally induced disorder are briefly summarized.

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