Quasielastic neutron scattering study of the Ni diffusion mechanism in the intermetallic alloy NiSb

Ni diffusion in the hexagonal NiSb intermetallic phase of approximately equiatomic composition is surprisingly fast, more than two orders of magnitude faster than Sb diffusion. In order to determine the atomic jump mechanism of the Ni atoms, quasielastic neutron scattering was carried out at two single crystals kept at 1040 degrees C, 1080 degrees C and 1100 degrees C. Jumps of the Ni atoms between regular sites (octahedral interstices of the Sb lattice) and double tetrahedral interstices (DTI) and back yield very satisfactory fits on the measured data. Such jumps had been proposed by Frohberg in order to explain the difference in tracer diffusivities parallel and vertical to the hexagonal c axis. Even with a 12% surplus of Ni atoms over Sb atoms in the alloy Ni₅₃Sb₄₇, about 4% vacancies remain on the regular Ni sites. The authors measurements permit the exclusion of jumps just between the regular sites or just between the DTI sites.