STRUCTURAL DISORDER IN CUBR

The total structure factor of CuBr, both Bragg and diffuse scattering, has been measured by powder neutron diffraction in the three crystalline phases which occur at ambient pressure. The data was modelled using the reverse Monte Carlo method. We find that the short range order changes relatively continuously with temperature, despite abrupt changes in long range order at the phase transitions. In all phases the Cu+ ions are in tetrahedrally coordinated sites, with conduction involving motion towards vacant positions. In the gamma-phase the octahedrally and tetrahedrally coordinated vacant positions serve as pathways and are not occupied interstitially. Motion in the higher temperature phases primarily involves tetrahedral positions; in the beta-phase there is about 25% occupation of those sites which should be empty in the wurtzite structure and in the cl-phase all sites are equivalent. The Cu+ distribution in alpha-CuBr is the same as that of Ag+ in alpha-AgI at the same temperature.