A Combined Metal–Halide/Metal Flux Synthetic Route towards Type‐I Clathrates: Crystal Structures and Thermoelectric Properties of A₈Al₈Si₃₈ (A=K, Rb, and Cs)

Abstract

Single‐phase samples of the compounds K8Al8Si38 (1), Rb8Al8Si38 (2), and Cs7.9Al7.9Si38.1 (3) were obtained with high crystallinity and in good quantities by using a novel flux method with two different flux materials, such as Al and the respective alkali‐metal halide salt (KBr, RbCl, and CsCl). This approach facilitates the removal of the product mixture from the container and also allows convenient extraction of the flux media due to the good solubility of the halide salts in water. The products were analyzed by means of single‐crystal X‐ray structure determination, powder X‐ray and neutron diffraction experiments, 27Al‐MAS NMR spectroscopy measurements, quantum chemical calculations, as well as magnetic and transport measurements (thermal conductivity, electrical resistivity, and Seebeck coefficient). Due to the excellent quality of the neutron diffraction data, the difference between the nuclear scattering factors of silicon and aluminum atoms was sufficient to refine their mixed occupancy at specific sites. The role of variable‐range hopping for the interpretation of the resistivity and the Seebeck coefficient is discussed.

Distribution of Al atoms in the clathrate-I phase Ba8AlxSi46−x at x = 6.9

“…The assignment of the 27 Al signals in this work is consistent with that recently reported for M 8 Al 8 Si 38 (M = K, Rb, Cs) where the signals of Al at site 6c had a distinctly smaller shift than those of Al on 16i and 24k. 12 The 27 Al NMR experiments thus agree with the model that site 6c is occupied mainly by Al atoms. An occupation of site 16i and/or 24k cannot be decided from these experiments alone, and was studied with the 29 Si NMR experiments.…”

Ab initiocomputational study on the lattice thermal conductivity of Zintl clathrates[Si19P4]Cl4andHärkönen1, Karttunen2 2016Phys. Rev. BSelf Cite11190View full textAdd to dashboardCite

“…This facilitates the comparative analysis of these materials because some quantities for them are identical due to symmetry reasons. Type-I Zintl clathrates A 8 [Al 8 Si 38 ] (A = Na, K, Rb, Cs) and A 8 [Ga 8 Si 38 ] (A = K, Rb, Cs) have been recently synthesized [39][40][41]. The thermoelectric properties of the synthesized silicon clathrates have also been investigated and they show rather low thermal conductivity (about 2 W/(m K) for Na 8 [Al 8 Si 38 ] and below 0.5 W/(m K) for K 8 [Ga 8 Si 38 ] at T = 300 K, polycrystalline samples).…”

Cu and Zn Substituted Silicon Clathrates with the Cubic Type‐II Structure: Synthesis and Characterization of Cs8Na16Cu3.8Si132.2 and Cs8Na16Zn6.9Si129.1

“…Two recent reports on the preparation of otherwise hardly accessible germanium clathrates using the metastable Ge (cF136) as a precursor, [27] and on the successful use of metal halide/metal flux synthesis to obtain alkali metal-Al-Si clathrates [28] might be good strategies toward more new clathrates with the type-II structure.…”