Preparation and crystal structure of the intermetallics La4Mo7Al51 and La4W7Al51

The title compounds have been prepared by reaction of the elemental components in alumina crucibles. Their crystal structure was determined from single-crystal X-ray data of La4Mo7Al51: P bar 3m1, a = 1042.4(3) pm, c = 1174.3(3) pm, Z = 1, R = 0.023 for 892 structure factors and 62 variables. The lattice constants of the isotypic compound La4W7Al51 are a = 1041.3(4) pm, c = 1173.3(4) pm. The structure is closely related to that reported for Ca4Cr7Al48. The atoms occupy 12 atomic positions, of which four show significant deviations from the full occupancy, resulting in the formula La3.662(4)Mo7Al50.26(8). All atoms have high coordination numbers. The two different lanthanum atoms are coordinated by 1 lanthanum and 15 aluminum atoms, the two molybdenum sites by 12 and 11 aluminum atoms, respectively, in the form of bi and monocapped pentagonal antiprisms. The eight different aluminum atoms have between 11 and 14 neighbors with between 2 and 4 lanthanum and/or molybdenum atoms. All coordination polyhedra of the aluminum atoms have at least some rectangular or nearly rectangular faces