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• 作者：Gnu Nam ; Woongjin Choi ; Hongil Jo ; Kang Min OkKyunghan Ahn ; Tae-Soo You
• 刊名：Chemistry of Materials
• 出版年：2017
• 出版时间：February 14, 2017
• 年：2017
• 卷：29
• 期：3
• 页码：1384-1395
• 全文大小：721K
• ISSN：1520-5002

文摘

The solid-solution Zintl compounds with the mixed cations of Ca²⁺and Yb²⁺ in the Ca_5–xYb_xAl₂Sb₆ (1.0 ≤ x ≤ 5.0) system have been synthesized by high-temperature solid-state reactions. Two slightly different crystal structures of the Ba₅Al₂Bi₆-type and Ca₅Ga₂Sb₆-type phases have been characterized for seven compounds with 2.5 ≤ x ≤ 5.0 and three compounds with 1.0 ≤ x ≤ 2.0, respectively, by both powder and single-crystal X-ray diffraction analyses. The two title phases adopt the orthorhombic space group Pbam (Z = 2, oP26) with seven independent asymmetric atomic sites and share certain structural similarities, including infinite one-dimensional [Al₂Sb₈] double chains and isolated space-filling Ca²⁺/Yb²⁺ cations. Interestingly, we reveal the crystal-to-crystal solid-state structural transformation of the Yb-rich compound Ca_1.5Yb_3.5Al₂Sb₆ from the Ba₅Al₂Bi₆-type to the Ca₅Ga₂Sb₆-type phase through the postannealing process, which can be rationalized as the phase transition from the kinetically more stable structure to the thermodynamically more stable crystal structure on the basis of theoretical calculations. Discrepancies of the local coordination geometries of the anionic [Al₂Sb₈] units and the geometrical arrangements of structural building moieties in the two distinct phases provoke the different electrical properties of metallic and semiconducting conduction, respectively, for the Ba₅Al₂Bi₆-type and Ca₅Ga₂Sb₆-type phases. Density of states and crystal orbital Hamilton population analyses based on tight-binding linear muffin-tin orbital calculations prove that the band-gap opening in the Ca₅Ga₂Sb₆-type phase should mainly be attributed to an extended bond distance of the bridging Sb–Sb in the [Al₂Sb₈] unit. A series of thermoelectric (TE) property measurements indicates that the phase transition via the postannealing process eventually results in an enhancement of the TE performance of Yb-rich Ca_1.5Yb_3.5Al₂Sb₆.