Ag+diffusion within the rock-salt structured superionic conductor Ag4Sn3S8

The temperature dependence of the crystal structure of the compound Ag4Sn3S8 and its Ag+ ionic conduction mechanisms have been investigated using impedance spectroscopy measurements, powder neutron diffraction studies and computer simulations. Under ambient conditions, Ag4Sn3S8 adopts a cubic structure in space group P4132 with a = 10.808 98(7) Å and Z = 4. The S2− form a slightly distorted fcc structured anion sublattice with the 16 × Ag+ and 12 × Sn4+ arranged in an ordered manner over the 32 available octahedrally coordinated cavities. As a result, the crystal structure of Ag4Sn3S8 can be described as a slightly deformed, cation deficient rock-salt arrangement. Highly anisotropic thermal vibrations of the Ag+ are observed, especially at elevated temperatures. Above K, analysis of the powder neutron diffraction data provides evidence for limited thermally induced cation disorder, associated with an increasing occupancy of the empty octahedrally coordinated positions. This behaviour is successfully reproduced within molecular dynamics simulations, which identify two different Ag+ diffusion mechanisms which occur via a subset of the tetrahedrally coordinated interstices. A brief discussion of the wider issue of superionic behaviour within rock-salt structured compounds is also given.