| dc.description |
The crystal structures of the four phases of the `anti-perovskite' superionic conductor Ag<sub>3</sub>SI have been investigated by powder neutron diffraction and complex impedance spectroscopy. The high-temperature α-phase is characterized by a random distribution of the two cation species S<sup>2-</sup> and I<sup>-</sup> over the 2(a) positions at 0,0,0 and 1/2,1/2,1/2 within space group Imm. The Ag<sup>+</sup> are found to preferentially occupy the 24(h) trigonal interstices within the anion sublattice, though with significant anisotropy in their thermal vibrations. On quenching from high temperature the disordered cation array is retained, forming the metastable α*-phase, but there is a significant change in the cation distribution. The Ag<sup>+</sup> are predominantly located in 24(g) sites between the octahedral and tetrahedral cavities, though 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 ~0.7 Å in ⟨110⟩ directions to avoid short cation-anion contacts. Slow cooling from the α-phase followed by prolonged annealing at modest temperatures (~473 K) stabilizes the β-phase of Ag<sub>3</sub>SI, in which long-range ordering of the two cation species lowers the symmetry to Pmm. The Ag<sup>+</sup> now occupy 12(h) positions close to half the octahedral sites, such that they do not form close contacts with the larger I<sup>-</sup> species. The displacements are in the four ⟨100⟩ directions and towards the tetrahedral sites, with a single Ag<sup>+</sup> randomly occupying one of these four `split' positions. On cooling, the ionic conductivity is found to drop abruptly by ~4× at T = 156(2) K at the βγ transition. High-resolution powder neutron diffraction studies indicate that γ-Ag<sub>3</sub>SI possesses a very small rhombohedral distortion of the unit cell. The crystal structure of γ-Ag<sub>3</sub>SI (space group R3) can be derived from that of the β-phase by long-range ordering of the Ag<sup>+</sup> onto a subset of the displaced octahedral positions and an associated small displacement of the S<sup>2-</sup> in a ⟨111⟩ direction. |
