Structural behaviour at the gamma-beta phase transition of Ag3SI

Temperature-dependent powder neutron diffraction has been used to investiga te the structure of the superionic compound Ag3SI in the region of the gamm a-beta phase transition at Tgamma-beta = 157 K. On cooling through Tgamma-b eta the structure changes from cubic to rhombohedral as the three silver io ns per unit cell preferentially occupy six of the twelve sites which are ra ndomly occupied in the cubic phase. This is accompanied by a slight distort ion of the anion sublattice along one of the threefold axes. Cooling below Tgamma-beta produces a further gradual ordering of the silver ions with eac h silver ion on a single site and an increased distortion of the anion subl attice. This behaviour is used to interpret the observed ionic conductivity of Ag3SI and to resolve the ambiguity over the nature of the phase transit ion.