Tin-magnesium substitution in Ir3Sn7—structure and chemical bonding in MgxIr3Sn7-x (x=0-1.67)

Well-shaped single crystals of binary Ir₃Sn₇ were obtained from a tin flux (starting composition Ir:Sn=1:10). The magnesium based stannides Mg_xIr₃Sn_7-x (x=0.61-1.67) were synthesized from the elements in glassy carbon crucibles in a water-cooled sample chamber of a high-frequency furnace. The samples were characterized by X-ray diffraction on powders and single crystals. All compounds crystallize with the cubic Ir₃Ge₇ type structure (space group Imm, Z=4). In this structure type the p-block atoms occupy the Wyckoff positions 12d and 16f and form two interpenetrating frameworks consisting of cubes and square antiprisms. The transition metal atoms center the square antiprisms and are arranged in pairs. With increasing magnesium substitution the lattice parameter of Ir₃Sn₇ (935.3 pm) decreases from 934.7 pm (x=0.61) to 930.6 pm (x=1.67) and the Ir-Ir distances decrease from 294 pm (Ir₃Sn₇) to 290 pm (Mg_1.67Ir₃Sn_5.33). In the ternary compounds Mg substitutes Sn on both framework sites. However, the 12d site shows a substantially larger preference for Mg occupation. By performing first-principles calculations we investigated the bonding situation in Ir₃Sn₇ and its alteration upon Mg incorporation. For binary Ir₃Sn₇ there are considerable bonding interactions between Ir and Sn atoms (d-p bonding) and between neighboring Sn atoms on the site 16f (p-p bonding). Both types of interactions diminish when substituting Sn for Mg. This explains the different site preference of Mg in Mg_xIr₃Sn_7−x: Mg occupation of the site 12d retains covalent p-p framework bonding between 16f atoms in the ternary compounds.