Medium-range correlation of Ag ions in superionic melts ofAg2Se and AgI by reverse Monte Carlo structural modelling—connectivity and voiddistribution

Abstract

High-energy x-ray diffraction measurements on moltenAg2Se were performed. Partial structure factors and radial distribution functionswere deduced by reverse Monte Carlo (RMC) structural modelling on thebasis of our new x-ray and earlier published neutron diffraction data. Thesepartial functions were compared with those of molten AgI. Both AgI andAg2Se have a superionic solid phase prior to melting. New RMC structural modellingfor molten AgI was performed to revise our previous model with a bond-anglerestriction to reduce the number of unphysical Ag triangles. The refined model ofmolten AgI revealed that isolated unbranched chains formed by Ag ions are thecause of the medium-range order of Ag. In contrast with molten AgI, moltenAg2Se has ‘cage-like’ structures with approximately seven Ag ions surrounding aSe ion. Connectivity analysis revealed that most of the Ag ions in moltenAg2Se are located within 2.9 Å of each other and only small voids are found, whichis in contrast to the wide distribution of Ag-void radii in molten AgI. Itis conjectured that the collective motion of Ag ions through small voidsis required to realize the well-known fast diffusion of Ag ions in moltenAg2Se, which is comparable to that in molten AgI.