Structural characterization of further high temperature superionic phases of Ag2HgI4and Cu2HgI4

The high temperature crystal structure of the superionic compounds Ag 2 HgI 4 and Cu 2 HgI 4 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 Ag 2 HgI 4 and Cu 2 HgI 4 which occur at ~410 and ~578 K, respectively. Prior to melting, Ag 2 HgI 4 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-Ag 2 HgI 4 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.