Synthesis and characterization of metal silicides including volatile metals

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New ternary sodium-magnesium stannides and plumbides, Na2MgX (X=Sn, Pb), Na2Mg3X2, (X=Sn, Pb), and Na4Mg4Sn3, were synthesized by heating the constituent elements. Their crystal structures were determined by single-crystal X-ray diffraction and the thermoelectric power factors were estimated from the measured Seebeck coefficients and electrical conductivities. Na2MgSn, which crystallizes in the Li2CuAs-type structure (hexagonal, P63/mmc, Z = 2, a = 0.50486(11) nm, c = 1.0095(2) nm), showed highest power factor of 1.5x10-3 Wm-1K-1 at 300 K among the novel compounds.Polycrystalline Si powder consisting of micro-porous grains was prepared by heating Mg2Si powder at 873 K for 40 h under a pressure of 500 Pa. The porous Si grains were composed of submicrometer- to micrometer-sized Si crystals with open spaces. The formation of the porous Si grains was attributed to Mg evaporation from Mg2Si grains, followed by crystallization of Si.