Nickel diffusion in B2-NiGa studied with quasielastic neutron scattering

The weak concentration dependence of Ni diffusivity in NiGa has formerly been interpreted as evidence for Ni diffusion via next-nearest-neighbor jumps, which seems reasonable in the light of the comparably high enthalpy of formation found in this alloy. Quasielastic neutron scattering (QNS) at the backscattering spectrometer IN16 at ILL has been used to study the elementary diffusion jump of Ni in NiGa single crystals near the stoichiometric composition Ni50Ga50 as well as in polycrystals with 57 at. % Ni and 62 at. % Ni. While the weak concentration dependence of the Ni diffusion coefficient has been confirmed over a wide concentration range on the Ni-rich site of the NiGa phase diagram, the diffusive jump of Ni atoms unequivocally turned out to be a jump via nearest-neighbor sites, i.e., antistructure sites. For the near-stoichiometric compositions it was possible to determine the residence time of the Ni atoms on the antistructure sites directly from the QNS measurements. These results indicate very high defect concentrations near the melting temperature.