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

• Published in: Chemistry : a European journal 2014-11, Vol.20 (46), p.15077-15088
• Main Authors: Baran, Volodymyr, Senyshyn, Anatoliy, Karttunen, Antti J., Fischer, Andreas, Scherer, Wolfgang, Raudaschl-Sieber, Gabriele, Fässler, Thomas F.
• Format: Article
• Language: English
• Subjects: Chemistry; clathrates; neutron diffraction; Neutrons; thermochemistry; transport properties; zintl phases
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201403416

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. It takes two: A large‐scale synthesis of clathrate compounds is described by the reaction of an arc‐melted aluminum–silicon alloy with alkali metals in alkali‐metal halide flux media. The Si/Al ratios are determined by using X‐ray and neutron diffraction as well as 27Al‐MAS NMR spectroscopy methods. The three materials A8Al8Si38 (A=K, Rb, and Cs) are further characterized by magnetic and transport measurements.