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Abstract:
The metastable type-II clathrate Na24-delta Ge136 was obtained from Na12Ge17 by applying a two-step procedure. At first, Na12Ge17 was reacted at 70 degrees C with a solution of benzophenone in the ionic liquid (IL) 1,3-dibutyl-2-methylimidazolium-bis(trifluoromethylsulfonyl) azanide. The IL was inert towards Na12Ge17, but capable of dissolving the sodium salts formed in the redox reaction. By annealing at 340 degrees C under an argon atmosphere, the X-ray amorphous intermediate product was transformed to crystalline Na24-delta Ge136 (delta approximate to 2) and alpha-Ge in an about 1 : 1 mass ratio. The product was characterized by X-ray powder diffraction, chemical analysis, and Na-23 solid-state NMR spectroscopy. Metallic properties of Na24-delta Ge136 were revealed by a significant Knight shift of the Na-23 NMR signals and by a Pauli-paramagnetic contribution to the magnetic susceptibility. At room temperature, Na24-delta Ge136 slowly ages, with a tendency to volume decrease and sodium loss.
