Yb₁₄MgSb₁₁ and Ca₁₄MgSb₁₁-New Mg-containing zintl compounds and their structures, bonding, and thermoelectric properties

Magnesium-containing Zintl phase compounds Yb₁₄MgSb₁₁ and Ca₁₄MgSb₁₁ have been prepared by annealing the mixture of the elements at 1075-1275 K. These compounds are isostructural with the Zintl compound Ca₁₄AlSb₁₁ and crystallize in the tetragonal space group I4₁/acd (Z = 8). Single-crystal X-ray data (90 K) were refined for Yb₁₄MgSb₁₁ [a = 16.625(9) Å, c = 22.24(2) Å, V = 6145(8) ų, and R1/wR2 (0.0194/0.0398)] and Ca₁₄MgSb₁₁ [a = 16.693(2) Å, c = 22.577(5) Å, V = 6291(2) ų, R1/wR2 (0.0394/0.0907)]. This structure type has been shown to be highly versatile with a large number of phases with the general formula A₁₄MPn₁₁ (A = Ca, Sr, Ba, Yb, Eu; M = Mn, Zn, Nb, Cd, Group 13 elements; Pn = Group 15 elements). The two compounds reported in this paper are the first Mg-containing analogs. Replacing M with Mg, which is divalent with no d-orbitals, probes electronic structure and properties of this structure type. Mg²⁺ is well-known to prefer tetrahedral geometry and allows for integration of the properties of a main group analog isoelectronic to the transition metal (Mn²⁺) containing compounds of this structure type. Thermoelectric properties of both compounds were measured from room temperature to 1075 K on fully dense pellets. Yb₁₄MgSb₁₁ shows metallic behavior, whereas Ca₁₄MgSb₁₁ is a semiconductor with a much larger electrical resistivity. The figure of merit reaches 0.32 for Ca₁₄MgSb₁₁ at 1075 K, and 1.02 at 1075 K for Yb₁₄MgSb₁₁, which is a 45% improvement over the reported zT_1075K of Yb₁₄MnSb₁₁ prepared from Sn-flux. Density functional band structure calculations on Ca₁₄MgSb₁₁ yield p-type materials with a band gap of ∼0.6 eV and shows Ca and Sb 5p orbitals contributes to the top of valence band. The electron localization function calculations show that Mg and Sb are covalently bonded in MgSb₄ ^9- and that the bonding of Sb₃ ^7- can be considered as a hypervalent 3c-4e bond.