Diffuse scattering in nickel arsenide, NiAs

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Abstract
The characteristic diffuse scattering distribution that is observed on electron diffraction photographs of nickel arsenide, NiAs, has been interpreted in terms of a force-constant model of the structure. Thermal diffuse scattering intensities for the system have been obtained, both by direct calculation from a force-constant model and also with the use of Monte Carlo simulation in combination with optical transform techniques. The diffuse scattering can largely be explained in terms of simple nearest-neighbour central-force constants between Ni-Ni (in the c direction and in the basal ab plane), Ni-As and As-As, and does not appear to be closely associated with the formation of the recently reported orthorhombic displacive superlattice, which has been attributed to the condensation of a q=1/3(11-20) Sigma 6 phonon. From the derived force model it is shown that the main driving force required to obtain the superlattice must be a force tending to make the Ni atoms form zigzag chains along c (i.e. alternative Ni atoms are displaced in opposite directions in the basal plane). Additional, albeit small, second-nearest-neighbour forces must also be invoked for the Sigma 6 phonon to condense at 1/3(11-20) rather than at 1/2(11-20), which would correspond to the MnP superlattice distortion.

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