Local structure and influence of bonding on the phase-change behavior of the chalcogenide compounds K1−xRbxSb5S8

KSb₅S₈ and its solid solution analogs with Rb and Tl were found to exhibit a reversible and tunable glass→crystal→glass phase transition. Selected members of this series were analyzed by differential scanning calorimetry to measure the effect of the substitution on the thermal properties. The solid solutions K_1−xRb_xSb₅S₈ exhibited clear deviations in melting and crystallization behavior and temperatures from the parent structure. The crystallization process of the glassy KSb₅S₈ as a function of temperature could clearly be followed with Raman spectroscopy. The thermal conductivity of both glassy and crystalline KSb₅S₈ at room temperature is ∼0.40 W/m K, among the lowest known values for any dense solid-state material. Electronic band structure calculations carried out on KSb₅S₈ and TlSb₅S₈ show the presence of large indirect band-gaps and confirm the coexistence of covalent Sb-S bonding and predominantly ionic K(Tl)?S bonding. Pair distribution function analyses based on total X-ray scattering data on both crystalline and glassy K_1−xRb_xSb₅S₈ showed that the basic structure-defining unit is the same and it involves a distorted polyhedron of “SbS₇” fragment of ∼7 Å diameter. The similarity of local structure between the glassy and crystalline phases accounts for the facile crystallization rate in this system.