Enclathration of X@La4 Tetrahedra in Channels of Zn–P Frameworks in La3Zn4P6X (X = Cl, Br)

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• 作者：Jian Wang ; Derrick Kaseman ; Kathleen Lee ; Sabyasachi Sen ; Kirill Kovnir
• 刊名：Chemistry of Materials
• 出版年：2016
• 出版时间：July 12, 2016
• 年：2016
• 卷：28
• 期：13
• 页码：4741-4750
• 全文大小：643K
• 年卷期：0
• ISSN：1520-5002

文摘

Two new quaternary lanthanum zinc phosphide-halides were synthesized via high-temperature solid-state reactions. Their complex crystal structures were determined by a combination of X-ray diffraction and advanced solid-state ³¹P NMR spectroscopy. La₃Zn₄P₆Cl and La₃Zn₄P_6.6Br_0.8 share a common structural feature: a polyanionic Zn–P framework with large channels hosting complex one-dimensional cations. The cations are built from X@La₄ tetrahedral chains with X = Cl (La₃Zn₄P₆Cl) or Br_0.8P_0.2 (La₃Zn₄P_6.6Br_0.8). The X@La₄ tetrahedra share two vertices forming one-dimensional chains. To accommodate larger bromine-containing cations the Zn–P framework is rearranged by breaking and forming several Zn–P and P–P bonds. This results in the formation of a unique [P₃]^3– cycle, which is isoelectronic to cyclopropane. Analysis of the electron localization and orbital overlaps confirmed the presence of different chemical bonding in the Zn–P networks in the Cl- and Br-containing compounds. La₃Zn₄P₆Cl was predicted to be a narrow bandgap semiconductor, while the formation of the [P₃]^3– units in the structure of La₃Zn₄P_6.6Br_0.8 was shown to lead to a narrowing of the bandgap. Characterization of the transport properties confirmed both La₃Zn₄P₆Cl and La₃Zn₄P_6.6Br_0.8 to be narrow bandgap semiconductors with electrons as dominating charge carriers at low temperatures. La₃Zn₄P₆Cl exhibits a n-p transition around 250 K. Due to the complex crystal structure and segregation of the areas of different chemical bonding, both title compounds exhibit ultralow thermal conductivities of 0.7 Wm^–1 K^–1 and 1.5 Wm^–1 K^–1 at 400 K for La₃Zn₄P₆Cl and La₃Zn₄P_6.6Br_0.8, respectively.