THE HIGH-TEMPERATURE STRUCTURAL BEHAVIOR 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 --> beta and beta --> alpha, which are observed at 643 +/- 2 K and 673 +/- 8 K. Within the zinc-blende-structured gamma-phase (space group F (4) over bar 3m) increasing disorder of the Cu+ ion sublattice is observed as temperature approaches the gamma --> beta transition, in addition to a non-linear thermal expansion. The hexagonal beta-phase (space group P (3) over bar m1) 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 R (3) over bar m), which had only been observed previously at elevated pressures. The high-temperature phase alpha-CuI has Fm (3) over bar m symmetry with Cu+ ions distributed randomly over all the tetrahedral sites within the cubic close-packed I- sublattice.