Crystal Structure of the Silicide Phosphide Gd4Co13(Si,P)9, the isotypic Lanthanoid Cobalt Silicide Phosphides Ln4Co13(Si,P)9 und the Uranium Iron Phosphide U4Fe13P9

The title compounds crystallize with a new hexagonal structure type, which was determined from single-crystal diffractometer data of Gd4Co13(Si, P)9: space group Pequation imagem2, a = 1041.9(4) pm, c = 372.7(1) pm, Pearson symbol hP26. The structure was refined to a residual of R = 0.033 for 18 variable parameters and 190 structure factors. The silicon and phosphorus atoms (X) are located at three crystallographic positions with eight or nine metal neighbors forming capped trigonal prisms most likely with random occupancy. The gadolinium atoms occupy two sites, each with 6 X neighbors forming trigonal prisms. This coordination is augmented by 14 metal atoms (mostly cobalt atoms), thus increasing the coordination numbers of both gadolinium positions to 20. The four different cobalt atoms have 3, 4, or 5 X neighbors with 6 or 8 additional metal neighbors. The relative strengths of the chemical bonds correlate very well with the coordination numbers. The structure belongs to a large structural family with a metal to metalloid ratio of approximately 2:1, where all atoms are situated on two alternating mirror planes and most of the metalloid atoms (M, mainly silicon, phosphorus, and their homologues) with the coordination number 9 are situated in tricapped prisms of the metal atoms (R = large electropositive metals, mainly rare earth elements and actinoids; T = transition metals). It can be classified as the third member (n = 3) of a topological series Requation imageTn(n+2)–2Mequation image, where the structures of WC and α-UCr6P4 (LiCo6P4) are the members with n = 1 and n = 2, respectively.