Tin-magnesium substitution in Ir(3)S(n)7 - structure and chemical bonding in MgxIr3Sn7-x (x=0-1.67)

摘要

Well-shaped single crystals of binary Ir3Sn7 Were obtained from a tin flux (starting composition Ir: Sn = 1:10). The magnesium based starmides MgxIr3Sn7-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 Im (3) over barm, 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 930.6pm (x = 1.67) and the Ir-Ir distances decrease from 294 pin (Ir3Sn7) to 290 pm (Mg1.67Ir3Sn5.33). 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 Sri for Mg. This explains the different site preference of Mg in MgxIr3Sn7-x: Mg occupation of the site 12d retains covalent p-p framework bonding between 16f atoms in the ternary compounds. (C) 2003 Elsevier Inc. All rights reserved. [References: 24]