The Distribution of Silver Ions in α-AgI

Abstract

Different distributions have been proposed for the mobile Ag ions among the bcc array of I in α-AgI. Unambiguous specification of the distribution has been difficult because (a) single crystals shatter during the β→α transition at 150°C; all previous work has been powder diffraction with fewer intensities than adjustable structural parameters, and (b) the difficulty in evaluating structure factors for a “smeared” distribution of mobile ions. We have been able to reproducibly nucleate a single-crystal β→α transformation on a precession camera or four circle diffractometer through a “reverse Bridgman” technique. The silver atom distribution was completely specified, as a function of temperature and with no adjustable parameters, through use of a probability function derived from the potential calculations of Flygare and Huggins. The bulk of the metal ions are concentrated near the tetrahedral interstice. Published intensity data obtained at 150°C provide R values of 21–23% for previously proposed distributions, but 13.0% for the present model. With diffractometer data obtained at 212°C, all models provide R of 17–20% but electron density and difference maps support the proposed distribution.