Ta11Pt4 − a new coloring variant of the σ-phase.

Well-shaped single crystals of Ta11Pt4 formed through crucible attack during phase analytical studies in the Ca–Pt–Zn system. Phase-pure Ta11Pt4 was subsequently obtained from the pure elements by arc-melting. The crystal structure of Ta11Pt4 was refined from single crystal X-ray diffractometer data: <italic>P</italic>42/<italic>mnm</italic>, <italic>a</italic> = 992.94(8), <italic>c</italic> = 516.59(4) pm, <italic>wR</italic>2 = 0.0672, 576 <italic>F</italic>2 values and 27 variables. Ta11Pt4 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@Ta11Pt icosahedra are condensed by interpenetration forming a Pt2 pair with 275 pm Pt–Pt distance. These double-icosahedra are the monomeric building unit of the Ta11Pt4 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 <italic>bcc</italic> tantalum. Temperature dependent magnetic susceptibility studies indicate Pauli paramagnetism. [ABSTRACT FROM AUTHOR]