The tau-borides τ-(Fe0.54Ir0.46)20Fe3B6 and τ-(Co0.64Ir0.36)21Co0.16B4B6

Two tau-borides, τ-(Fe0.54Ir0.46)20Fe3B6 and τ-(Co0.64Ir0.36)21Co0.16B4B6 were studied by X-ray powder and single crystal diffraction with respect to precise atom site distribution and metal atom ordering. X-ray structure determination for a single crystal of τ-(Fe0.54Ir0.46)20Fe3B6 revealed isotypism with a partially ordered Cr23C6-type (space group Fm 3 ¯ m, a = 1.11020 nm) but with a high degree of random replacement of Fe/Ir atoms in sites 48h and 32f. X-ray single crystal data for τ-(Co0.64Ir0.36)21Co0.16B4B6 showed a novel low-symmetry structure variant of tau-borides characterized by a noncentrosymmetric distribution of boron tetrahedra and a defect Co-site (space group F 4 ¯ 3m as a translationengleiche, nonisomorphic maximal subgroup of index 2 of Fm 3 ¯ m, a = 1.09359 nm). The crystal structures of both τ-phases are described with nested polyhedra geometrically related to the α-Mn structure. Stability ranges and various structural arrangements of (MMʹ)23B6 phases are discussed. ystal structures of (Fe1−xIrx)3B (x = 0.24; X-ray single crystal data; Fe3C-type, space group Pnma, a = 0.5430(2), b = 0.6958(2) nm, c = 0.4654(2) nm), and of isotypic (Fe1−xRhx)3B (XPD Rietveld refinement, x = 0.25, space group Pnma, a = 0.54560(1) nm, b = 0.69552(1) nm, c = 0.46169(1) nm) show both a high degree of randomness in the metal sites.