Crystal structures and ionic conductivities of ternary derivatives of the silver and copper monohalides -: I.: Superionic phases of stoichiometry MA4I5:RbAg4I5, KAg4I5, and KCu4I5

The superionic properties of the compounds RbAg4I5, KAg4I5 and KCu4I5, have been investigated by powder neutron diffraction and complex impedance spectroscopy. RbAg4I5 and KAg4I5 have room-temperature ionic conductivities of sigma = 0.21(6) and 0.08(5) Omega(-1) cm(-1), respectively, which increase gradually on increasing temperature. KCu4I5 is only stable in the temperature range between 515(5) K and its melting point of 605 K, and its ionic conductivity is sigma = 0.61(8) Omega(-1) cm(-1), at T = 540 K. At lower temperatures, KCu4I5 disproportionates into KI + 4CuI and the ionic conductivity falls by over three orders of magnitude. Least-squares refinements of the powder neutron diffraction data for RbAg4I5 at ambient temperature confirm the reported structure (space group P4(1)32, Z = 4, a = 11.23934(3) Angstrom), though with some differences in the preferred locations of the mobile Ag+. KAg4I5 and KCu4I5 are found to adopt the same basic structure as RbAg4I5, with the I- forming a beta-Mn-type sublattice, with the K+ located in a distorted octahedral environment and the Ag+(Cu+) predominantly distributed over two sites which are tetrahedrally co-ordinated to V. The implications for the conduction mechanism within these compounds are discussed, using a novel maximum entropy difference Fourier technique to map the distribution of the Ag+(Cu+) within the unit cell. (C) 2002 Elsevier Science (USA).