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Issue 37, 2020
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Chemically driven superstructural ordering leading to giant unit cells in unconventional clathrates Cs8Zn18Sb28 and Cs8Cd18Sb28

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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 Ia[3 with combining macron]d 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, Cs8Zn18−xCdxSb28, 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 Cs8Zn4.5Cd13.5Sb28 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.

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

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Article information


Submitted
14 Jul 2020
Accepted
08 Sep 2020
First published
08 Sep 2020

This article is Open Access
All publication charges for this article have been paid for by the Royal Society of Chemistry

Chem. Sci., 2020,11, 10255-10264
Article type
Edge Article

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

B. Owens-Baird, P. Yox, S. Lee, X. B. Carroll, S. Grass Wang, Y. Chen, O. I. Lebedev and K. Kovnir, Chem. Sci., 2020, 11, 10255
DOI: 10.1039/D0SC03846F

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