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Synthesis, Crystal Structure, and Properties of Three La–Zn–P Compounds with Different Dimensionalities of the Zn–P Framework

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journal contribution
posted on 25.05.2018, 00:00 authored by Jian Wang, Philip Yox, Jackson Voyles, Kirill Kovnir
Two novel ternary compounds in a La–Zn–P system, La2Zn11P9 and La7Zn2P11, were synthesized via high-temperature transport reactions. The crystal structures for both compounds were established by means of single crystal X-ray diffraction. The complex three-dimensional (3D) crystal structure of metal-rich La3Zn2P4 is composed of a Zn–P framework with large channels accommodating four atomic columns of La atoms. The isolated columns of La atoms alternating with Zn–P tetrahedral chains and disordered P3 chains, resembling polyacene fragments, build up the crystal structure of phosphorus-rich La7Zn2P11. The previously reported La3Zn2–xP4 compound with intermediate phosphorus content has a two-dimensional (2D) structural motif composed of Zn2P2 and La3P2 layers. A structural dimensionality reduction from 3D La2Zn11P9 to 2D La3Zn2‑xP4 to 1D La7Zn2P11 is due to both the flexibility of the Zn–P framework with ZnP4 tetrahedra and ZnP3 planar building units and the ability of phosphorus to form homonuclear bonds and polyatomic phosphorus chains. A polycrystalline sample La3Zn1.75P4 was purified by a high-temperature solid-state method. The electron counting rules and computations predict the n-type metallic nature of La3Zn1.75P4. The transport properties tests performed on a sintered pellet of La3Zn1.75P4 confirm its metallic behavior with negative thermopower indicating that the major carriers are electrons. La3Zn1.75P4 exhibits moderate thermal conductivity, 4.5 W m–1 K–1 at 300 K, where lattice thermal conductivity has the dominating contributions.