Structural and superionic properties of Ag+-rich ternary phases within the AgI-MI2 systems

The effects of temperature on the crystal structure and ionic conductivity of the compounds Ag2CdI4, Ag2ZnI4 and Ag3SnI5 have been investigated by powder diffraction and impedance spectroscopy techniques. epsilon-Ag2CdI4 adopts a tetragonal crystal structure under ambient conditions and abrupt increases in the ionic conductivity are observed at 407(2),447(3) and 532(4) K, consistent with the sequence of transitions epsilon-Ag2CdI4 --> beta-Ag2CdI4 + beta-AgI + CdI2 --> alpha-AgI + CdI2 --> alpha-Ag2CdI4. Hexagonal beta-Ag2CdI4 is metastable at ambient temperature. The ambient-temperature beta phase of Ag2ZnI(4) is orthorhombic and the structures of beta-Ag2CdI4 and beta-Ag2ZnI4 can, respectively, be considered as ordered derivatives of the wurtzite (beta) and zincblende (gamma) phases of AgI. On heating Ag2ZnI4, there is a 12-fold increase in ionic conductivity at 481(l) K and a further eightfold increase at 542(3) K. These changes result from decomposition of beta-Ag2ZnI4 into alpha-AgI + ZnI2, followed by the appearance of superionic alpha-Ag2ZnI4 at the higher temperature. The hexagonal crystal structure of alpha-Ag2ZnI4 is a dynamically disordered counterpart to the beta modification. Ag3SnI5 is only stable at temperatures in excess of 370(3) K and possesses a relatively high ionic conductivity (sigma approximate to 0.19 Omega(-1) cm(-1) at 420 K) due to dynamic disorder of the Ag+ and Sn2+ within a cubic close packed I- sublattice. The implications of these findings for the wider issue of high ionic conductivity in AgI-MI2 compounds is discussed, with reference to recently published studies of Ag4PbI6 and Ag2HgI4 and new data for the temperature dependence of the ionic conductivity of the latter compound.