Rare Earth (RE) Gallides with Closely Related Compositions: REIrGa and RE6Ir5Ga7

The ternary rare-earth iridium gallides RE6Ir5Ga7 (RE = Sc, Y, Nd, Sm, Gd - Tm, and Lu) were synthesized from the elements by arc-melting or by induction melting in sealed niobium ampoules. The samples were structurally characterized by X-ray powder diffraction. The structure of Sc6Ir5Ga7 was refined from single-crystal X-ray diffractometer data: Yb6Ir5Ga7 (Nb6.4Ir4Al7.6) type, P6(3)/mcm, a = 912.84(7), c = 828.07(6) pm, wR(2) = 0.0362, 504 F-2 values and 22 variables. The RE6Ir5Ga7 gallides crystallize with a fully ordered superstructure of the hexagonal Laves phase MgZn2. The zinc substructure consists of ordered and condensed Ir3Ga and IrGa3 tetrahedra. Further phase analytical studies in the RE-Ir-Ga systems led to the new equiatomic gallides SmIrGa and LuIrGa with the orthorhombic, TiNiSi type structure, space group Pnma. The samarium compound shows a pronounced solid solution SmIr1-xGa1+x, which was confirmed by single crystal structure refinements for SmIr0.95Ga1.05, SmIr0.69Ga1.31 and SmIr0.55Ga1.45. The RE6Ir5Ga7 and REIrGa gallides show pronounced covalent Ir-Ga bonding.