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

Abstract

Previously unreported σ-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 °C for 1 week. The phases are primitive tetragonal (P4₂/mnm, no. 136) with lattice parameters of a = 9.8336(2)–9.9432(1) Å and c = 5.1098(1)–5.1718(1) Å. 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 8i₂ and 2a sites of the structure, a type of disorder frequently found in σ-phases. We present a graphical classification for ternary σ-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 σ-phases was attempted but they were not found.