The NbB2-phase revisited: Homogeneity range, defect structure, superconductivity

The discovery of superconductivity below 40 K in MgB2 has motivated new investigations on similar compounds, especially on binary diborides. The great majority of these compounds represent the AlB2-type structure (P6/mmm space group, number 191) and comprise line compounds. However, among those, NbB2 and TaB2 are reported to present a significant homogeneity region, a value of 12 at.% being reported for the case of NbB2. In this work we have evaluated the homogeneity range of the NbB2-phase through 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 = 66.7 at.% B). The results clearly showed that the width of the homogeneity range of this phase is nearly 5 at.%, extending from 65 at.% B (NbB1.86) up to 70 at.% 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 supercell was evaluated; however, a simple AlB2-type is 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 (almost equal) 3.5 K.