Structural characterization of further high temperature superionic phases of Ag2HgI4 and Cu2HgI4

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 beta --> alpha superionic transitions observed in both compounds just above ambient temperature, we have characterized the transitions to phases (labelled S) in Ag2HgI4 and Cu2HgI4 which occur at similar to 410 and similar to 578 K, respectively. Prior to melting, Ag2HgI4 undergoes an additional transition at similar to 445 K to a phase labelled epsilon. The crystal structure of the delta phases comprises a slightly distorted h.c.p. anion sublattice with the two cation species dynamically disordered over the octahedral and tetrahedral interstices. In epsilon -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' alpha -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 alpha <----> delta transition are discussed.