Preparation and crystal structure of the ternary uranium rare earth antimonides (U2/3R1/3)Sb2 and (U1/2R1/2)3Sb7 with mixed U/R occupancy of the metal sites and a variety of antimony polyanions

The ternary antimonides (U2/3R1/3)Sb-2 (R = Y, Ce-Nd, Sm, Gd-Tm) and (U1/2R1/2)(3)Sb-7 (R = Y, Gd-Ho) have been prepared by reaction of the uranium antimonide USb2 with the corresponding rare earth antimonides RSb2 and an excess of elemental antimony at high temperatures. The crystal structure of the isotypic series (U2/3R1/3)Sb2 has been determined from single-crystal X-ray data of (U0.675(9)Gd0.325(9))Sb-2. It is isotypic with PbCl2: Pnma, Z = 4, a = 754.6(1), b = 419.6(1), c = 1025.7(3) pm. The compounds of the other series crystallize with a new structure type, which has been determined for (U0.49(1)Ho0.51(1))(3)Sb-7: Immm, Z = 4, a = 410.1(1), b = 1447.7(3), c 1821.2(5) pm. In both structures, the metal positions have mixed occupancy and high coordination numbers. Both structures contain numerous weak Sb-Sb bonds, thus forming a band of antimony atoms in the structure of (U2/3Gd1/3)Sb-2 and a chain and a three-dimensionally infinite network of antimony atoms in the structure of (U1/2Ho1/2)(3)Sb-7. Analyses of the Sb-Sb bonding within the antimony polyanions on the basis of bond-length bond-strength considerations indicate that the uranium atoms have mixed or intermediate +3/+4 valence in these compounds. The structure Of (U1/2Ho1/2)(3)Sb-7 May be considered as a defect variant of the structure of Ce6MnSb15.