Mg3Ir3Si8, a new magnesium iridium silicide with Si-4 tetrahedra and Si-12 truncated tetrahedra

摘要

The ternary silicide Mg3Ir3Si8 (cubic, a=1221.4(l) pm, space group F (4) over bar 3m, 8 formula units per unit cell) was prepared by reaction of the elemental components in a sealed molybdenum container (1000 degrees C, I d, cooled with 100 degrees/h). The crystal structure was determined from single crystal data. Short distances in the three-dimensional iridium silicon network indicate strong Ir-Si-bonding (d(Ir-Si) = 240.5(2) and 245.6(l) pin). In addition, homonuclear bonding seems to be important, resulting in the formation Of Si-4-tetrahedra (d(Si-Si) = 257(l) pm), and Mg centered Si-12-polyhedra with the shape of truncated tetrahedra (d(Si-Si) = 241(l) and 261.0(9) pm). Furthermore, Mg-4-tetrahedra with Mg-Mg-distances of 355(2) pm are formed. The structure may be derived from the structure of the isotypic compounds Mg5Pd10Si16 and Mg5Pt10Si16 by adding a Mg siteset and subtracting a platinum metal siteset. It can be described by an expanded cubic "close" packing of MgIr6-octahedra in which Si-4-tetrahedra reside in the octahedral holes while Mg-4-tetrahedra and MgSi12-units occupy one half of the tetrahedral holes each.