Neutron Diffraction Study of NiS Under Pressure

Abstract

Recent neutron diffraction and resistivity measurements on the NiAs form of NiS show the existence of a first‐order metallic to semiconducting, paramagnetic to antiferromagnetic transition at 265°K and and zero pressure, which has a large negative pressure dependence (dT_N/dP)=−6.0°/kbar. Below T_N the nickel moments, of approximately 1.7 μ_B, are coupled ferromagnetically in the basal planes and are aligned along the hexagonal axis. The coupling of adjacent basal planes is antiferromagnetic. The present work has examined the effect of pressures up to 8 kbar at temperatures T>T_N, T=0.9 T_N, T=0.25 T_N using a time‐of‐flight neutron spectrometer. The results show that the compressibility of the lattice is small and essentially independent of temperature. In addition there is a migration of Ni atoms from their octahedral positions to tetrahedral interstitial sites with increasing pressure. In the region of 5 kbars, however, this process is reversed and there is a tendency towards perfect ordering of the NiAs lattice. At the same time there is an increase in the Ni moment towards its fully allowed value.