Nickel I 7300Rare Earth Metal Rich Magnesium Compounds Ln4NiMg (Ln: Y, Pr-Nd, Sm, Gd-Tm, Lu) -Synthesis, Structure, and Hydrogenation Behavior. -The title compounds are prepared from the elements (sealed tantalum tubes, 1300 K, 2 min, 920 K, 2 h) and their structures are determined by powder and single crystal XRD. The Ln4NiMg compounds are isotypic with Gd4RhIn and crystallize in the cubic space group F43m with Z = 16. The structure consists of a three-dimensional network of edge-sharing NiLn 6 trigonal prisms. Mg 4 tetrahedra and additional Ln atoms are located within voids of this framework. The hydrogenation behavior of Y4NiMg and Gd4NiMg is studied. These compounds absorb up to 11 H atoms per formula unit under a hydrogen pressure of 1 MPa at room temperature. At temperatures >363 K decomposition to LnH2/LnH3 hydrides occurs. Gd4NiMg exhibits antiferromagnetic behavior below T N = 92 K, whereas its hydride Gd 4 NiMgH 11 is paramagnetic down to 1.8 K. -(TUNCEL, S.; ROQUEFERE, J. G.; STAN, C.; BOBET, J.-L.; CHEVALIER, B.; GAUDIN, E.; HOFFMANN, R.-D.; RODEWALD, U. C.; POETTGEN*, R.; J. Solid