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 K1−xRbxSb5S8 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 K1−xRbxSb5S8 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.

The KSb5S8 is a good example of a phase-change material with a mixed ionic/covalent bonding. The members of the K1−xRbxSb5S8 series exhibit phase-change properties with greater glass forming ability (GFA) than KSb5S8. The GFA increases with increasing Rb content. In this case, the random alloy disorder in the alkali metal sublattice seems to predominate over the increased degree of ionicity in going from K⋯S to Rb⋯S bonding and works to stabilize the glass forms in K1−xRbxSb5S8.Figure optionsDownload as PowerPoint slide