Sc3Co, a new structure type related to Fe3C and Re3B by periodic unit-cell twinning

Sc3Co, orthorhombic, Pnma, a = 13.102 (9), b = 8.624 (6), c = 5.829 (3) A, Z = 8 Dsub(x) = 3.913g cm-3, μ(Mo Kα) = 107 cm-1. Least-squares refinement gave R = 0.048 for 535 independent diffractometer data which were corrected for absorption. The structure is characterized by trigonal prisms of Sc atoms which are centred by Co atoms. Centred trigonal prisms are also found in the Fe3C and Re3B structure types. Using the concept of periodic unit-cell twinning, the point positions of all three structure types can be derived. Fe3C is based on a hexagonal, and Re3B on a cubic close-packed base structure, while Sc3Co derives from a close-packed base structure with mixed stackings corresponding to the atom arrangement in the Nd structure type. (Auth.)