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Issue 11, 2018
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Phonon glass behavior beyond traditional cage structures: synthesis, crystal and electronic structure, and properties of KMg4Sb3

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Abstract

Three new ternary antimonides, KMg4Sb3 and A2Mg5Sb4 (A = Rb, Cs), were synthesized via high-temperature solid-state reactions and their crystal structures were determined by single crystal X-ray diffraction. All three compounds feature three-dimensional anionic frameworks composed of edge-shared MgSb4 tetrahedra with channels trapping the alkali metal cations. KMg4Sb3 crystallizes in β-BaCu4S3 structure type (Pearson symbol oS32) while Rb2Mg5Sb4 and Cs2Mg5Sb4 are isostructural and crystallize in K2Zn5As4 structure type (Pearson symbol oC44). Band structure calculations predict KMg4Sb3 to be a direct bandgap semiconductor with Eg of ∼1 eV. Characterizations of the transport properties indicate that KMg4Sb3 is a semiconductor with impurity levels. KMg4Sb3 exhibits ultralow total thermal conductivity of 0.9 W m−1 K−1 at 300 K. Potassium cations in the structure of KMg4Sb3 exhibit abnormally large anisotropic displacement parameters at 90 K, a behavior typical for rattling cations. Calculations of the phonon dispersions and density of states support the K rattling as an important contributor to overall thermal conductivity reduction. A Phonon-Glass thermal behavior with K atoms rattling in open channels of Mg4Sb3 framework shines new light on designing low thermal conductivity materials.

Graphical abstract: Phonon glass behavior beyond traditional cage structures: synthesis, crystal and electronic structure, and properties of KMg4Sb3

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Supplementary files

Article information


Submitted
18 Jan 2018
Accepted
19 Feb 2018
First published
19 Feb 2018

J. Mater. Chem. A, 2018,6, 4759-4767
Article type
Paper

Phonon glass behavior beyond traditional cage structures: synthesis, crystal and electronic structure, and properties of KMg4Sb3

J. Wang, L. Wang and K. Kovnir, J. Mater. Chem. A, 2018, 6, 4759
DOI: 10.1039/C8TA00553B

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