Determination of the structure of beta -CuI by high-resolution neutron powder diffraction

The structure of the beta form of copper(I) iodide has been investigated. The hexagonal beta -CuI exists as a single phase between 647 K and 655 K and shows marked anisotropy in its thermal expansion within this small temperature range. The high quality of the diffraction data has allowed the corrected structure for beta -CuI to be determined in space group P3m1, with I in 2(d) sites at (1/3,2/3,z) and (2/3,1/3,z) with z=0.242(1), thus forming a slightly distorted hexagonal close-packed array. The Cu+ are situated in the tetrahedral voids, predominantly on 2(d) sites at z=0.6214(8) but with a significant fraction of the cations disordered onto 2(d) sites with z=0.8786(8). The relationship between this and other previously published structures of beta -CuI is discussed.