New σ-phases in the Nb-X-Ga and Nb-X-Al systems (X = Ru, Rh, Pd, Ir, Pt, and Au)

Previously unreported sigma-phases in the ternary niobium-based systems Nb-X-Ga for X = Ru, Rh, Pd, Ir, Pt, Au, and Nb-X-Al for X = Ir, and Pt are presented, prepared by arc-melting followed by annealing at 1000 degrees C for 1 week. The phases are primitive tetragonal (P4(2)/mnm, no. 136) with lattice parameters of a = 9.8336(2)-9.9432(1) angstrom and c = 5.1098(1)-5.1718(1) angstrom. Refinements of the single crystal X-ray diffraction data for the Ru and Ir containing materials gave the formulas Nb20Ru5Ga5 and Nb20Ir4Ga6, showing mixing of X/Ga on the crystallographic 8i(2) and 2a sites of the structure, a type of disorder frequently found in s-phases. We present a graphical classification for ternary sigma-phases that places our materials in context with previous materials. The materials do not show evidence of bulk superconductivity above 2 K. Synthesis of equivalent Ta-based sigma-phases was attempted but they were not found.