Abstract
The ternary silicides Mg5Pd10Si16 and Mg5Pt10Si16 have been prepared by reaction of magnesium with the binary platinum-metal silicides in sealed tantalum containers (Pt-compound: 1200°C, 3 d, up 20 °/h, down 5 °/h; Pd-compound: 1000°C, 2 d, up and down 50 °/h). In the case of the Pd-compound contact with the tantalum had to be avoided by using a boron nitride crucible. The isotypic compounds crystallize in the cubic space group F4̄3m with 4 formula units per unit cell. The crystal structures were determined from single crystal data, lattice constants from Guinier patterns. The following data were obtained: a = 1258.81(8) pm for Mg5Pd10Si16 and a = 1256.94(9) pm for Mg5Pt10Si16. Short distances in the three-dimensional platinum-metal silicon network indicate strong, covalent Pd(Pt)-Si-bonding (d(Pd-Si) = 240.2 to 256.1 pm; d(Pt-Si) = 237.1 to 258.5 pm). In addition, homonuclear bonding seems to be important, resulting in the formation of Si4-tetrahedra (d(Si-Si) = 250.4 pm (Mg5Pd10Si16) and 255 pm (Mg5Pt10Si16)), empty Si12-polyhedra with the shape of truncated tetrahedra (d(Si- Si): 234.5 and 248.2 pm (Mg5Pd10Si16); 236 and 248.2 pm (Mg5Pt10Si16)), and Mg-centered Pd(Pt)10-clusters with the shape of adamantane (d(Pd-Pd) = 282.3 pm; d(Pt-Pt) = 284.5 pm). Furthermore, Mg4-tetrahedra with Mg-Mg-distances of 360 pm are formed. The structure may be described by an expanded cubic “close” packing of MgPd(Pt)10-units in which the Si4-tetrahedra occupy the octahedral holes while the Si12-polyhedra and the Mg4-tetrahedra reside in one half of the tetrahedral holes each.
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