Self diffusion in the B2 ordered alloy NiGa

Complete text of publication follows. The CsCl structure, called B2 structure for intermetallic alloys, is the simplest structure of ordered solids. Even though B2 ordered intermetallic alloys are of great fundamental and technological interest, their diffusion process is still not completely understood. This is due to the fact that jumps to nearest neighbour sites in these structures are associated with a disturbance of local ordering. Quasielastic neutron scattering investigations on NiGa are presented, carried out at various concentrations near the melting point. The quasielastic line found in the experiments consists of two Lorentzians. This implies that the elementary diffusion jump of Ni in NiGa is as a jump between sites of a non-Bravais lattice, i.e. a nearest neighbour jump to the 'foreign' sublattice. From the ratio of the intensities of the two Lorentziaus it is even possible to determine the concentration of the thermal vacancies in stoichiometric NiGa. A comparison with the only other B2 ordered alloy where an elementary diffusion jump has been determined, i.e. FeAl (1), shows differences in the diffusion mechanisms. The differences are suggested to be due to the very different vacancy concentrations. (author)