HIGH-PRESSURE POLYMORPHISM OF THE COPPER(I) HALIDES - A NEUTRON-DIFFRACTION STUDY TO SIMILAR-TO-10 GPA

The structural changes within the copper(I) halides induced by hydrostatic pressure have been investigated using the powder neutron-diffraction technique. The expected transition from the ambient zinc-blende structure with tetrahedral coordination to the octahedrally coordinated rocksalt structure is observed in both CuCl and CuBr. No rocksalt phase is observed in CuI, presumably due to the limited maximum pressure of our apparatus (approximately 10 GPa). The lower symmetry phases which occur as intermediate structures between the zinc-blende and rocksalt phases have been studied in detail. CuCl and Cul undergo abrupt transitions whilst CuBr has three sluggish transitions, involving extensive coexistence of neighboring phases. There is one intermediate phase in CuCl (CuCl-IV), two in CuBr (CuBr-IV and CuBr-V) and at least two in CuI (CuI-IV and CuI-V). Three different structure types have been identified, in space groups Pa3BAR (CuCl-IV and CuBr-V), P4/nmm (CuBr-IV and CuI-V) and R3mBAR (CuI-IV). The intermediate structures all retain the tetrahedral cation coordination of the Cu+, though the environment is rather more distorted in CuCl and CuBr than in Cul. The evolution of the structures of these I-VII compounds is very different from that observed in the more covalent II-VI and III-V systems.