Ternary magnesium compounds RE23Ni7Mg4 (RE = La, Ce, Pr, Nd, Sm) with Pr23Ir7Mg4 type structure

New rare earth-rich compounds RE23Ni7Mg4 (RE = La, Ce, Pr, Nd, Sm) were synthesized from the elements in sealed tantalum tubes in a water-cooled sample chamber of an induction furnace. The samples were characterized via X-ray powder diffraction. The RE23Ni7Mg4 compounds crystallize with the hexagonal Pr23Ir7Mg4 type structure, space group P63mc. The structures of La23Ni7Mg4 (a = 1013.7(3), c = 2295.2(8) pm, wR2 = 0.0371, 2283 F2 values, 75 variables) and Pr23Ni7Mg4 (a = 992.8(3), c = 2243.0(5) pm, wR2 = 0.0634, 2462 F2 values, 74 variables) were refined from single crystal X-ray diffractometer data. Striking structural motifs are nickel centered trigonal prisms formed by the rare earth elements and isolated Mg4 tetrahedra (319 pm as Mg-Mg distance in La23Ni7Mg4). The trigonal RE6Ni prisms exhibit short RE-Ni distances (287-306 pm in La23Ni7Mg4). The prisms are condensed via common edges, leading to a rigid three-dimensional network in which the Mg4 tetrahedra are embedded. Temperature dependent magnetic susceptibility data of Ce23Ni7Mg4 indicate Curie-Weiss behavior with an experimental magnetic moment of 2.54 B/Ce atom, indicative for stable trivalent cerium. No magnetic ordering is evident down to 3 K