Ames Laboratory; Chemistry
Chemistry, Ames Laboratory
Journal of Materials Chemistry A
Three new ternary antimonides, KMg4Sb3 and A(2)Mg(5)Sb(4) (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 beta-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 E-g of similar to 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.
Department of Energy Subject Categories
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
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