Synthesis and characterization of Na₅M_2+xSn _10-x (x ≈ 0.5, M ) = Zn, Hg - A doped tetrahedral framework structure

Two homologous and isostructural compounds Na₅M _2+xSn_10-x (M ) Zn, Hg) were obtained by direct reaction of the elements at high temperature. The crystal structures of these novel phases were determined from single-crystal X-ray diffraction data and represent a new structure type in tin chemistry. They crystallize in the space group Pbcn (No. 60, Z = 4) with a = 12.772(1), b = 10.804(1), and c = 12.777(1) Å, V = 1763.1(2) ų for Na₅Zn_2.28Sn _{9.72(2) (I)} and a = 12.958(1), b = 10.984(1), and c = 12.960(1) Å, V = 1844.5(2) ų for Na₅Hg _2.39Sn_{9.61(1) (II)}. The structures consist of an anionic 3D open framework of tetrahedrally coordinated Sn and M atoms interwoven with a cationic 2D array of interconnected {NaNa₄} tetrahedra. The framework can be partitioned into fragments of realgar-like units {Sn_8-xM _x}^2x- and twice as many {Sn-M}^2- dimers. Formally, the compounds are charge-balanced Zintl phases for x = 0.5. As the structure refinements lead to x = 0.28 and 0.39 for I and II, respectively, both structures are electron-rich and expected to be metallic. Theoretical investigations at the density functional theory level reveal a deep minimum at the Fermi level for x = 0.5. According to rigid band analyses, the electronic structure of the phases with the experimentally observed compositions corresponds to heavily doped semiconductors, thereby meeting an important requirement of thermoelectric materials.