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

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 E_g 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.