Ta₁₁Pt₄ − a new coloring variant of the σ-phase

Well-shaped single crystals of Ta 11 Pt 4 formed through crucible attack during phase analytical studies in the Ca–Pt–Zn system. Phase-pure Ta 11 Pt 4 was subsequently obtained from the pure elements by arc-melting. The crystal structure of Ta 11 Pt 4 was refined from single crystal X-ray diffractometer data: P 4 2 / mnm , a = 992.94(8), c = 516.59(4) pm, wR 2 = 0.0672, 576 F 2 values and 27 variables. Ta 11 Pt 4 is a new coloring variant in the large family of sigma phase compounds. Of the five crystallographically independent sites, four are occupied by tantalum and the fifth one by platinum in a fully ordered manner. The platinum atoms have distorted icosahedral coordination by 11 tantalum and 1 platinum atom. Two of these Ta@Ta 11 Pt icosahedra are condensed by interpenetration forming a Pt 2 pair with 275 pm Pt–Pt distance. These double-icosahedra are the monomeric building unit of the Ta 11 Pt 4 structure and further condense via common edges to a three-dimensional dense sphere packing. The Ta–Ta distances cover a broader range from 256 to 329 pm, comparable to the structure of bcc tantalum. Temperature dependent magnetic susceptibility studies indicate Pauli paramagnetism.