Yb14MgSb11andCa14MgSb11-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 I4(1)/acd (Z = 8). Single-crystal X-ray data (90 K) were refined for Yb14MgSb11 [a = 16.625(9) angstrom, c = 22.24(2) angstrom, V = 6145(8) angstrom(3), and R1/wR2 (0.0194/0.0398)] and Ca14MgSb11 [a = 16.693(2) angstrom, c = 22.577(5) angstrom, V = 6291(2) angstrom(3), 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 A(14)MPn(11) (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 zT1075K of Yb14MnSb11 prepared from Sn-flux. Density functional band structure calculations on Ca14MgSb11 yield p-type materials with a band gap of similar to 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 MgSb49- and that the bonding of Sb-3(7-) can be considered as a hypervalent 3c-4e bond.