Neutron powder investigation of CuI*

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Abstract
CuI is known to undergo two phase transitions at T1 = 367°C and T2 = 407°C. The structural changes involve a rearrangement of the anions and disordered distributions of the cations. The low temperature γ-phase has zincblende structure, space group F[unk]3m [a = 6.0337(3) Å, Z = 4]. The intermediate β-phase belongs to the hexagonal system with a hcp anion lattice [a = 4.289(9) Å), c = 7.189(5) Å, Z = 2], space group P[unk]m2 (coexisting with γ- or α-phase). The high temperature α-phase is cubic again, the anions form a fcc lattice and the cations are randomly distributed over all tetrahedral sites, space group Fm[unk]m [a = 6.148(1) Å, Z = 4]. A modified powder profile refinement program, which includes anharmonic 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 anharmonic coefficients upto the fourth order and split models with anisotropic temperature factors. With increasing temperature the density of Cu cations begins to spread from the center of the tetrahedra towards the vacant octahedral interstices. Aproaching the α-β transition from above the lobes near the tetrahedral faces become more pronounced indicating a transition from a diffusion to a more static (jump) behaviour near the phase transition.