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 crystall ine phases which occur at ambient pressure. The data was modelled usin g the reverse Monte Carlo method. We find that the short range order c hanges relatively continuously with temperature, despite abrupt change s in long range order at the phase transitions. In all phases the Cuions 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 a re not occupied interstitially. Motion in the higher temperature phase s primarily involves tetrahedral positions; in the beta-phase there is about 25% occupation of those sites which should be empty in the wurt zite structure and in the cl-phase all sites are equivalent. The Cu+ d istribution in alpha-CuBr is the same as that of Ag+ in alpha-AgI at t he same temperature.