Local boron environment in Ni100-xBx metallic glasses: An NMR study

From the analysis of the B11 NMR spectra in a-Ni100-xBx (18.5x40) the electric-field-gradient (EFG) components on the B sites and their distributions are deduced, giving an insight into the local atomic arrangements. For x=18.5 and 40, one finds relatively narrow distributions of the EFG components, indicating weak fluctuations of the bonding angles, distances, and nature of the atoms in the B coordination shell. The comparison with the compositionally closest nickel borides (Ni3B and Ni4B3) immediately suggests that the local structure around B is similar in these glasses and in the related crystals, i.e., nickel trigonal prisms (6 Ni) whose rectangular faces are capped by 3 Ni (Ni3B) or 3 Ni and/or B (Ni4B3). For intermediate concentrations the EFG component distributions are found significantly broader and comparable with those expected from random packing of spheres. However, the continuous evolution of the NMR spectra shape with increasing B content rather suggests an admixture of c-Ni3Blike and c-Ni4B3like B local environment; i.e., when B concentration increases, more prisms share rectangular faces allowing boron-boron contact as in c-Ni4B3. No sign of a B coordination shell similar to that in c-Ni2B (8 Ni anticubes+2 B) is found in the glasses. Measurements of transverse relaxation times T2, which reflect essentially the BB bondings, qualitatively support these conclusions.