Chemically driven superstructural ordering leading to giant unit cells in unconventional clathrates Cs8Zn18Sb28 and Cs8Cd18Sb28

Abstract

The unconventional clathrates, Cs₈Zn₁₈Sb₂₈ and Cs₈Cd₁₈Sb₂₈, were synthesized and reinvestigated. These clathrates exhibit unique and extensive superstructural ordering of the clathrate-I structure that was not initially reported. Cs₈Cd₁₈Sb₂₈ orders in the Ia3̄d space group (no. 230) with 8 times larger volume of the unit cell in which most framework atoms segregate into distinct Cd and Sb sites. The structure of Cs₈Zn₁₈Sb₂₈ is much more complicated, with an 18-fold increase of unit cell volume accompanied by significant reduction of symmetry down to P2 (no. 3) monoclinic space group. This structure was revealed by a combination of synchrotron X-ray diffraction and electron microscopy techniques. A full solid solution, Cs₈Zn_18-x Cd _x Sb₂₈, was also synthesized and characterized. These compounds follow Vegard's law in regard to their primitive unit cell sizes and melting points. Variable temperature in situ synchrotron powder X-ray diffraction was used to study the formation and melting of Cs₈Zn₁₈Sb₂₈. Due to the heavy elements comprising clathrate framework and the complex structural ordering, the synthesized clathrates exhibit ultralow thermal conductivities, all under 0.8 W m^-1 K^-1 at room temperature. Cs₈Zn₉Cd₉Sb₂₈ and Cs₈Zn_4.5Cd_13.5Sb₂₈ both have total thermal conductivities of 0.49 W m^-1 K^-1 at room temperature, among the lowest reported for any clathrate. Cs₈Zn₁₈Sb₂₈ has typical p-type semiconducting charge transport properties, while the remaining clathrates show unusual n-p transitions or sharp increases of thermopower at low temperatures. Estimations of the bandgaps as activation energy for resistivity dependences show an anomalous widening and then shrinking of the bandgap with increasing Cd-content.