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 (Pa) 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 R2/m (R3m) and lattice parameters a_r = 6.4 Å, α_r = 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 (Rm). 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.