Yb14MgSb11 and Ca14MgSb11鈥擭ew Mg-Containing Zintl Compounds and Their Structures, Bonding, and Thermoelectric Properties

详细信息    查看全文

• 作者：Yufei Hu ; Jian Wang ; Airi Kawamura ; Kirill Kovnir ; Susan M. Kauzlarich
• 刊名：Chemistry of Materials
• 出版年：2015
• 出版时间：January 13, 2015
• 年：2015
• 卷：27
• 期：1
• 页码：343-351
• 全文大小：493K
• ISSN：1520-5002

文摘

Magnesium-containing Zintl phase compounds Yb₁₄MgSb₁₁ and Ca₁₄MgSb₁₁ have been prepared by annealing the mixture of the elements at 1075鈥?275 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) 脜, c = 22.24(2) 脜, V = 6145(8) 脜³, and R1/wR2 (0.0194/0.0398)] and Ca₁₄MgSb₁₁ [a = 16.693(2) 脜, c = 22.577(5) 脜, V = 6291(2) 脜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 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 鈭?.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鈥?/sup> and that the bonding of Sb₃7鈥?/sup> can be considered as a hypervalent 3c鈥?e bond.}