Multinuclear NMR and Powder X-ray Diffraction Studies of Si and Sn Clathrates of Alkali Metals: Vacancies, Disorder and
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Multinuclear NMR and Powder X-ray Diffraction Studies of Si and Sn Clathrates of Alkali Metals: Vacancies, Disorder and Knight Shifts. Michael J. Ferguson, Igor L. Moudrakovski, Christopher I. Ratcliffe and John S. Tse. Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario K1A 0R6, Canada. ABSTRACT The Structure I type binary metal clathrates of K/Si, Rb/Si and Cs/Sn have been synthesised and studied by powder X-ray diffraction and solid state NMR. Rietveld analysis shows that in all three materials some of the cages are empty, and that in the Cs/Sn clathrate there are vacancies in the Sn framework. The NMR results yield Knight shifts for 29Si and 39K and confirm that the Cs/Sn clathrate is not conducting. Many of the features of the NMR spectra can be understood in terms of the distributions of atom vacancies. INTRODUCTION Two structural types of clathrate (Str.I and II) consisting of 4-coordinate tetrel atom frameworks forming cages which trap single metal atoms were first discovered in the 1960’s [15]. They are now well known as potential thermoelectric materials [6] and numerous techniques have been applied to study their thermal and electrical properties. One of their attractive features is the possibility of property tuning by varying the components and also the stoichiometry. Most NMR studies of the binary Str.I clathrates to date have focussed on the Knight shifts as a source of information about the conduction electrons [7-11]. NMR, however, can also be used as a probe of local structure, and is more sensitive in some aspects than X-ray diffraction. Here we present preliminary results of studies of the synthesis, PXRD and NMR ( 39K, 87Rb, 133Cs, 29Si and 119Sn) of Str.I clathrates with frameworks of Si or Sn, with alkali metals K, Rb or Cs in the cages. Emphasis is placed on the complementary use of the two techniques for structural information regarding vacancies, in addition to the determination of NMR Knight shifts. EXPERIMENTAL DETAILS Synthesis. Str.I K/Si clathrate was prepared by vacuum decomposition of the binary silicide, KSi, at elevated temperatures. Manipulations were performed in an Ar-filled glovebox. KSi was produced from a 1:1 mixture of potassium (Aldrich, 99.5%) and powdered silicon (Aldrich, 99.999+%) (1g total mass) loaded into a steel bomb under Ar, and heated in a tube furnace at 600 ºC for 3 hours. KSi was loaded into an alumina boat, and placed inside a quartz tube which was inserted into a tube furnace, and attached to a vacuum line. The tube was then heated at 1 ºC/min to 410 ºC, and held there for 48 hours under a dynamic vacuum of ~2 × 10-4 mbar. Str.I Rb/Si clathrate was prepared in an analogous manner, except (a) the preparation of RbSi required heating of the stoichiometric mixture of Rb (United Mineral & Chemical Corp., 99.8%) and Si at 600 ºC for 3 hours, and (b) the vacuum decomposition of RbSi was performed under a static vacuum. Str.I Cs/Sn clathrate was prepared by direct reaction of Cesium (United Mineral & Chem
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