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Powder neutron diffraction and molecular dynamics (MD) simulations have been used to investigate the structural behaviour of silver sulfide, Ag2S, at elevated temperatures. Above ~450 K Ag2S adopts the β phase in which the S2- possess a body-centred cubic arrangement. Analysis of the neutron diffraction is in good agreement with the previously proposed structural model in which the Ag+ predominantly reside within the tetrahedral interstices. At ~865 K Ag2S transforms to the α phase in which the anion sublattice adopts a face-centred cubic arrangement. Structural refinements of this phase indicate that the cations are distributed predominantly in the tetrahedral cavities but with a significant fraction in the octahedral holes. MD simulations, using established potentials for this compound, confirm the stability of the two high-temperature superionic phases and show good agreement with the measured Ag+ distribution within the unit cell.

The nature of the superionic transition in Ag2 HgI4 and Cu2 HgI4 has been investigated using temper dependent powder neutron diffraction and impedance spectroscopy techniques. In the case of Ag2 HgI4 , the superionic transition occurs at Tc = 326(2) K and is accompanied by a 50-fold increase in the ionic conductivity. In the Cu+ analogue, which has a lower conductivity for a given temperature, the corresponding values are Tc = 338(4) K and / ~ 6. The ambient temperature crystal structures of the two compounds are different (space group I for -Ag2 HgI4 and I 2m for -Cu2 HgI4 ) but, in contrast to the most recent study, the high temperature polymorphs are found to be isostructural (space group F 3m ). Possible explanations for the different behaviour of the ionic conductivity of the two compounds are given.

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The effects of temperature on the crystal structure and ionic conductivity of the compounds Ag 2 CdI 4 , Ag 2 ZnI 4 and Ag 3 SnI 5 have been investigated by powder diffraction and impedance spectroscopy techniques. ε-Ag 2 CdI 4 adopts a tetragonal crystal structure under ambient conditions and abrupt increases in the ionic conductivity are observed at 407(2), 447(3) and 532(4) K, consistent with the sequence of transitions ε-AgThe ambient-temperature β phase of Ag 2 ZnI 4 is orthorhombic and the structures of β-Ag 2 CdI 4 and β-Ag 2 ZnI 4 can, respectively, be considered as ordered derivatives of the wurtzite (β) and zincblende (γ ) phases of AgI. On heating Ag 2 ZnI 4 , there is a 12-fold increase in ionic conductivity at 481(1) K and a further eightfold increase at 542(3) K. These changes result from decomposition of β-Ag 2 ZnI 4 into α-AgI + ZnI 2 , followed by the appearance of superionic α-Ag 2 ZnI 4 at the higher temperature. The hexagonal crystal structure of α-Ag 2 ZnI 4 is a dynamically disordered counterpart to the β modification. Ag 3 SnI 5 is only stable at temperatures in excess of 370(3) K and possesses a relatively high ionic conductivity (σ ≈ 0.19 −1 cm −1 at 420 K) due to dynamic disorder of the Ag + and Sn 2+ within a cubic close packed I − sublattice. The implications of these findings for the wider issue of high ionic conductivity in AgI-MI 2 compounds is discussed, with reference to recently published studies of Ag 4 PbI 6 and Ag 2 HgI 4 and new data for the temperature dependence of the ionic conductivity of the latter compound.

The high temperature crystal structure of the superionic compounds Ag2HgI4 and Cu2HgI4 has been investigated using powder neutron diffraction. In addition to the widely studied β→α superionic transitions observed in both compounds just above ambient temperature, we have characterized the transitions to phases (labelled δ) in Ag2HgI4 and Cu2HgI4 which occur at ~410 and ~578 K, respectively. Prior to melting, Ag2HgI4 undergoes an additional transition at ~445 K to a phase labelled ε. The crystal structure of the δ phases comprises a slightly distorted h.c.p. anion sublattice with the two cation species dynamically disordered over the octahedral and tetrahedral interstices. In ε-Ag2HgI4 the cations are distributed over the tetrahedral and trigonal interstices formed by a b.c.c. anion sublattice and can, therefore, be considered to be a `cation-deficient' α-AgI-type superionic phase. The implications of these experimental findings in the wider context of the family of copper- and silver-based superionic conductors and for previous suggestions of `unusual' behaviour of the α↔δ transition are discussed.

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