Structural and superionic properties of Ag -rich ternary phases within the AgI MI2systems

The effects of temperature on the crystal structure and ionic conductivity of the compounds Ag 2 CdI 4 , Ag 2 ZnI 4 and Ag 3 SnI 5 have been investigated by powder diffraction and impedance spectroscopy techniques. ε-Ag 2 CdI 4 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 ε-Ag 2 CdI 4 → β-Ag 2 CdI 4 + β-AgI + CdI 2 → α-AgI + CdI 2 → α-Ag 2 CdI 4 . Hexagonal β-Ag 2 CdI 4 is metastable at ambient temperature. The ambient-temperature β phase of Ag 2 ZnI 4 is orthorhombic and the structures of β-Ag 2 CdI 4 and β-Ag 2 ZnI 4 can, respectively, be considered as ordered derivatives of the wurtzite (β) and zincblende (γ) phases of AgI. On heating Ag 2 ZnI 4 , there is a 12-fold increase in ionic conductivity at 481(1) K and a further eightfold increase at 542(3) K. These changes result from decomposition of β-Ag 2 ZnI 4 into α-AgI + ZnI 2 , followed by the appearance of superionic α-Ag 2 ZnI 4 at the higher temperature. The hexagonal crystal structure of α-Ag 2 ZnI 4 is a dynamically disordered counterpart to the β modification. Ag 3 SnI 5 is only stable at temperatures in excess of 370(3) K and possesses a relatively high ionic conductivity (σ ≈ 0.19Ω −1 cm −1 at 420 K) due to dynamic disorder of the Ag + and Sn 2+ within a cubic close packed I − sublattice. The implications of these findings for the wider issue of high ionic conductivity in AgI–MI 2 compounds is discussed, with reference to recently published studies of Ag 4 PbI 6 and Ag 2 HgI 4 and new data for the temperature dependence of the ionic conductivity of the latter compound.