Application of backscatter Kikuchi diffraction in the scanning electron microscope to the study of NiS2

Backscatter Kikuchi diffraction has been applied to an investigation of a selection of nickel sulfide crystals. In the first instance the technique itself was investigated to see whether it could be used successfully to distinguish between the space groups P21/a{ 3} (Pa{ 3}) of pyrite and Pnnm of marcasite. Subsequently, it was used to investigate the structure of a thin epilayer which was observed to have grown on selected facets of a certain synthetic nickel pyrite crystal. The epilayer was shown to be rhombohedral with possible space groups R{ 3}2/m (R3m) and lattice parameters ar = 6.4 Å, αr = 36°. In an attempt to positively identify this structure, the technique was applied to a number of related compounds including the mineral millerite (NiS, R3m), α-NiS – a high temperature polymorph of millerite (P63/mmc and P63mc), and the layer compound NiI2 (R{ 3}m). It was concluded that the epilayer was most probably NiCl2 formed by reaction of the NiS2 crystal with chlorine contained in the carrier gas used during the chemical vapour deposition growth process.