The crystal structures of superionic Ag3SI

The crystal structures of the four phases of the 'anti-perovskite' superion ic conductor Ag3SI have been investigated by powder neutron diffraction and complex impedance spectroscopy. The high-temperature cu-phase is character ized by a random distribution of the two cation species S2- and I- over the 2(a) positions at 0, 0, 0 and 1/2, 1/2, 1/2 within space group Im (3) over barm. The Ag+ are found to preferentially occupy the 24(h) trigonal inters tices within the anion sublattice, though with significant anisotropy in th eir thermal vibrations. On quenching from high temperature the disordered c ation array is retained, forming the metastable alpha*-phase, but there is a significant change in the cation distribution. The Ag+ are predominantly located in 24(g) sites between the octahedral and tetrahedral cavities, tho ugh a significant proportion are found in 48(i) sites close to the 1/4, 1/4 , 1/4 position midway between two anions. The latter are displaced by simil ar to0.7 Angstrom in (110) directions to avoid shea cation-anion contacts. Slow cooling from the alpha -phase followed by prolonged annealing at modes t temperatures (similar to 473 K) stabilizes the beta -phase of Ag3SI, in w hich long-range ordering of the two cation species lowers the symmetry to P m (3) over barm. The Ag+ now occupy 12(h) positions close to half the octah edral sites, such that they do not form close contacts with the larger I- s pecies. The displacements are in the four (100) directions and towards the tetrahedral sites, with a single Ag+ randomly occupying one of these four ' split' positions. On cooling, the ionic conductivity is found to drop abrup tly by similar to 4x at T = 156(2) K at the B --> gamma transition. High-re solution powder neutron diffraction studies indicate that gamma -Ag3SI poss esses a very small rhombohedral distortion of the unit cell. The crystal st ructure of gamma -Ag3SI (space group R3) can be derived from that of the be ta -phase by long-range ordering of the Ag+ onto a subset of the displaced octahedral positions and an associated small displacement of the S2- in a ( 111) direction.