NEUTRON POWDER INVESTIGATION OF CUI

CuI is known to undergo two phase transitions at T1=367°C and T2=407°C. The structural changes involve are arrangement of the anions and disordered distributions of the cations. The low temperatureγ-phase has zinc blende structure, space group F43m[a=6.0337(3)Å, Z=4]. The intermediateβ-phase belongs to the hexagonal system with a hcpanion lattice [a=4.289(9)Å), c=7.189(5)Å, Z=2], space group P6m2(co existing with y-ora-phase). The high temperature a-phase is cubicagain, the anions for mafcclattice and the cations are randomly distributed over all tetrahedralsites, space group Fm3m[a=6.148(1)Å, Z=4]. A modified powder profile refinement program, whichincludes an harmonic temperature factors, was used for the refinement of CuI at different temperatures and probability density functions were calculated to elucidate the diffusion paths of this high temperature superionic conductor. Different models were tried with an harmonic coefficients upto the fourth order and split models with an isotropic temperature factors. With increasing temperature the density of Cu cations begins to spread from the center of the tetrahedra towards the vacantoctahedralinterstices. Aproachingtheα-βtransitionfromabovethelobesnearthetetrahedralfacesbecomemo re pronounced indicating a transition from a diffusion to a morestatic (jump) behaviour near the phase transition. © by R. Oldenbourg Verlag