The homogeneity range of the NbB2-phase was evaluated by means of the detailed microstructural characterization of as-cast, as-cast + heat-treated and solid state sintered Nb–B alloys. Neutron diffraction experiments were performed to assess the defect mechanism responsible for accommodating the non-ideal NbB2 stoichiometries (ideal = 66.7at%B). The results clearly showed that the width of the homogeneity range of this phase was nearly 5at%, extending from 65at%B (NbB1.86) up to 70at%B (NbB2.34). Rietveld refinement of the neutron intensity diffraction data indicated a random distribution of vacancies in the Nb-subnet for hyperstoichiometric NbB2. The occurrence of a possible Nb-vacancy ordered super-cell was evaluated; however, a simple AlB2-type was observed throughout the entire homogeneity range. The superconducting properties of selected alloys were checked via magnetic measurements. The Nb-deficient samples were found to contain traces of a superconducting phase with Tc ≈ 3.5K.

The NbB2-Phase Revisited - Homogeneity Range, Defect Structure, Superconductivity. C.A.Nunes, D.Kaczorowski, P.Rogl, M.R.Baldissera, P.A.Suzuki, G.C.Coelho, A.Grytsiv, G.André, F.Boureé, S.Okada: Acta Materialia, 2005, 53[13], 3679-87