The high-temperature structural behaviour of copper(I) iodide

The structural behaviour of copper(I) iodide, CuI, has been investigated between room temperature and its melting point (T M =878 K) using neutron powder diffraction. Detailed measurements have been made in the vicinity of the two known structural phase transitions gamma to beta and beta to alpha , which are observed at 643+or-2 K and 673+or-8 K. Within the zinc-blende-structured gamma -phase (space group F43 m) increasing disorder of the Cu + ion sublattice is observed as temperature approaches the gamma to beta transition, in addition to a non-linear thermal expansion. The hexagonal beta -phase (space group P3m1) is observed as a single phase in the temperature range 645-668 K, but on first heating it is found to coexist with a rhombohedral phase. This transient phase was observed in isolation for only a short time but this was sufficient to show that its structure was that of CuI-IV (space group R3m), which had only been observed previously at elevated pressures. The high-temperature phase alpha -CuI has Fm3m symmetry with Cu + ions distributed randomly over all the tetrahedral sites within the cubic close-packed I $ sublattice.