Synthesis, crystal structure, chemical bonding, and physical properties of the ternary Na/Mg stannide Na ₂MgSn

A ternary stannide of sodium and magnesium, Na ₂MgSn, was synthesized from the elements, and the crystal structure was determined by single-crystal X-ray diffraction. The compound crystallizes in the Li ₂CuAs structure type (hexagonal, P6 ₃/mmc, Z = 2, a = 5.0486(11) Å, c = 10.095(2) Å), and its structure is built up of two-dimensional honeycomb layers of ² _∞[(MgSn) ^2-] stacked along the c-axis, with Na atoms as "space fillers". First-principles computations at various levels of density functional theory (DFT) verify that the most stable configuration is the one in which Na and Mg atoms occupy the 4f and 2b sites, respectively, and thus DFT provides a necessary complement to X-ray structural elucidation. Our computations also predict that Na ₂MgSn must be a semiconductor with a small band gap. In accord with these predictions, the electrical resistivity measured for a polycrystalline sample of Na ₂MgSn is 9.6-10.4 mω cm in the range of 90-635 K, and the Seebeck coefficient decreases from +390 μV K ^-1 (at 300 K) to +150 μV K ^-1 (at 430 K).