Ionic Diffusion Within Superionic Ag₃SI

S. Hull¹, D. A. Keen^1,2

¹The ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, United Kingdom.

²Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford, OX1 3PU, United Kingdom.

The structural properties of silver iodide (AgI) and silver sulphide (Ag₂S) have been extensively studied for many years because both compounds undergo superionic transitions at relatively modest temperatures (420K and 452K, respectively). At temperatures immediately above the superionic transition their high conductivity phases are both characterised by rapid jump diffusion of the Ag⁺, predominantly between the tetrahedrally co-ordinated interstices formed by an immobile body centred cubic (b.c.c.) sublattice of anions (I^- or S^2-). The AgI-Ag₂S system contains only a single ternary compound of composition Ag₃SI. By varying the sample preparation procedure it is possible to synthesise this material in two forms, characterised by a disordered (a* phase) or ordered (b phase) arrangement of the two cation species over the 0,0,0 and ¹/₂,¹/₂,¹/₂ positions which generate the b.c.c. array. Interestingly, the a* phase possesses an ionic conductivity at ambient temperature over two orders of magnitude higher than the b form. To understand the factors underlying this observation, powder neutron diffraction experiments (including analysis of the diffuse scattering) and molecular dynamics simulations have been used to probe the relationship between the long-range order within the anion sublattice and the dynamic cation disorder responsible for the macroscopic ionic conductivity. The results of these studies will be presented, together with a brief discussion of their implications for the wider question of ionic mobility within perovskite structured compounds.