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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 ͑QNS͒ 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 Ni 50 Ga 50 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.

Quasielastic neutron scattering at the backscattering spectrometer IN16 at the ILL has been used to determine the elementary jump vector of diffusing Co atoms in the B2 phase of CoGa. Measurements have been performed on Co single crystals with 54 at. % Co and 64 at. % Co. For both compositions a maximum of the quasielastic broadening has been observed near a reciprocal lattice point corresponding to a B2 superlattice reflection. This gives unambiguous evidence of Co diffusion via nearest neighbor jumps. From the residence times on the antistructure sites high defect concentrations are deduced. The correlation of Co diffusion has been found to be surprisingly weak. A diffusion mechanism is suggested which takes advantage of the high degree of disorder and, therefore, cannot be described in terms of defined cycles.

The structure of the intermetallic alloy Ni 3 Sb (the high-temperature phase) has been studied by means of neutron diffraction. Alloys with Ni contents between 71 and 75 at.% were examined at temperatures ranging from 600 to 1000 • C. The results confirm DO 3 structure as well as the existence of vacancy concentrations of up to 20% of the Ni sites. The high number of vacancies certainly plays a crucial role in stabilizing the DO 3 phase; it is also held responsible for the extraordinarily high Ni diffusivity values found in tracer diffusion studies.

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