Neutron powder diffraction and molecular dynamics study of superionic SrBr2

The nature of the dynamic ionic disorder within the high-temperature superionic phase of strontium bromide, β-SrBr2, has been investigated using reverse Monte Carlo (RMC) modelling of neutron powder diffraction data and complementary ab initio molecular dynamics (MD) simulations. The RMC and MD results are in good agreement and indicate the presence of extensive dynamic disorder within the Br(-) sublattice of the cubic fluorite structure. Rapid anion diffusion predominantly occurs as hops between nearest neighbour sites in the 〈100〉 directions, though the trajectories are markedly curved and pass through the peripheries of the octahedral voids in the cation sublattice. In addition, there are extensive correlations between the motions of individual Br(-), often leading to the formation of a short-lived square antiprism co-ordination environment around the Sr(2+). Such polyhedra are observed within the (ambient temperature) ordered tetragonal crystal structure of α-SrBr2. The nature of the ionic disorder in SrBr2 is of particular interest because it is the only known example of a Br(-)-ion superionic. Owing to the large size of this anion, a comparison with the behaviour of other superionic phases gives an insight into the role of ionic size on the conducting properties within these materials.