Type‐II Clathrate Na24‐δGe136 from a Redox‐Preparation Route

Abstract The metastable type‐II clathrate Na24‐δGe136 was obtained from Na12Ge17 by applying a two‐step procedure. At first, Na12Ge17 was reacted at 70 °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 °C under an argon atmosphere, the X‐ray amorphous intermediate product was transformed to crystalline Na24‐δGe136 (δ≈2) and α‐Ge in an about 1 : 1 mass ratio. The product was characterized by X‐ray powder diffraction, chemical analysis, and 23Na solid‐state NMR spectroscopy. Metallic properties of Na24‐δGe136 were revealed by a significant Knight shift of the 23Na NMR signals and by a Pauli‐paramagnetic contribution to the magnetic susceptibility. At room temperature, Na24‐δGe136 slowly ages, with a tendency to volume decrease and sodium loss.