S. Hull et Da. Keen, Structural characterization of further high temperature superionic phases of Ag2HgI4 and Cu2HgI4, J PHYS-COND, 13(24), 2001, pp. 5597-5610
The high temperature crystal structure of the superionic compounds Ag2HgI4
and Cu2HgI4 has been investigated using powder neutron diffraction. In addi
tion to the widely studied beta --> alpha superionic transitions observed i
n both compounds just above ambient temperature, we have characterized the
transitions to phases (labelled S) in Ag2HgI4 and Cu2HgI4 which occur at si
milar 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 dis
torted h.c.p. anion sublattice with the two cation species dynamically diso
rdered 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 'unu
sual' behaviour of the alpha <----> delta transition are discussed.