R₆TT‘₂, New Variants of the Fe₂P Structure Type. Sc₆TTe₂ (T = Ru, Os, Rh, Ir), Lu₆MoSb₂, and the Anti-typic Sc₆Te_0.80Bi_1.68

Abstract

The Fe2P structure (P6̄2m) features two 3-fold Fe positions and both 2-fold and 1-fold P sites, and variations in occupancies of the latter pair yield the reported diversity of results. The known Sc6TTe2 examples for T = Fe−Ni are herein extended to four heavier transition metal T derivatives. An attempt to synthesize bismuth analogues led to the novel inverse derivative in which fractional Te (vice T) occupies the smaller tricapped trigonal prismatic (TTP) Sc polyhedron, and Bi rather than Te occurs in the larger TTP of Sc, with parallel reversal of polarity in the bonding. The reported Lu8Te, which is distributed as Lu6TeLu2, is the only example in which a transition metal occupies the normal 2-fold P or Te non-metal position, with corresponding large effects on the bonding. Lutetium otherwise does not form R6TTe2 analogues, but the novel Lu6MoSb2 isotype occurs instead. Extended Hückel calculations are presented for five examples, and the structural and bonding regularities and varieties are discussed further