A single-crystal neutron diffraction study of the distribution and thermal motion of silver ions in alpha- and beta- Ag 3SI

Abstract

Single-crystal neutron diffraction data have been used to examine the distribution of mobile Ag + ions in anion-ordered β-Ag 3SI (space group Pm3m) and anion-disordered α-Ag 3SI (space group Im3m) at 8 temperatures in the overall range 23° to 475°C. The Ag density in the β-phase is confined to a cluster of four tetrahedral sites grouped at the centers of the cell faces, closely spaced as a result of the different distances between Ag + and the two species of anion. The individual sites are not resolved in Fourier syntheses because of large thermal displacements within a flat potential well, but the refinements permit rejection of occupancy of the octahedral site. The Ag + distribution in α-Ag 3SI displays a 2-coordinated site on the body diagonal of the cell and density delocalized in <100 > bands which display fine structure not present in α-AgI or β-Ag 2S. This feature may be qualitatively interpreted as an average over positional disorder of the equilibrium position within the tetrahedral interstice, the location depending on the short-range configuration of anions about the site. Models employing partial Ag + occupancy of the 2-coordinated position plus either octahedral or tetrahedral site occupancy involve the same number of structural parameters and refined to comparable levels. Tetrahedral site occupancy is judged to be somewhat more likely on the basis of agreement for the higher temperature data sets and physical criteria.