Comparison of partial structures of melts of superionic AgI and CuI and non-superionicAgCl

Abstract

Neutron and high-energy x-ray diffraction analyses of molten AgI have been performed andthe partial structures are discussed in detail with the aid of the structural modellingprocedure of the reverse Monte Carlo (RMC) technique by comparison with those ofmolten CuI and AgCl. It is well known that AgI and CuI have a superionic solid phasebelow the melting point, in which the cations favour a tetrahedral configuration, while solidAgCl has a rock-salt structure with an octahedral environment around both Ag andCl atoms. Even in the molten states, there is a significant difference betweensuperionic and non-superionic melts. The cation is located on the triangular plainformed by three iodine ions in molten AgCl and CuI, while molten AgCl favours a90° Cl–Ag–Cl bond angle, which is understood to maintain a similar local environment to thatin the solid state. The atomic configurations of the RMC model suggest that the cationdistributions in superionic melts of CuI and AgI exhibit large fluctuations, while Agions in the non-superionic melts of AgCl are distributed much more uniformly.