Type‐II Clathrate Na(24‐δ )Ge(136) from a Redox‐Preparation Route

The metastable type‐II clathrate Na(24‐δ )Ge(136) was obtained from Na(12)Ge(17) by applying a two‐step procedure. At first, Na(12)Ge(17) 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 Na(12)Ge(17), 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 Na(24‐δ )Ge(136) (δ≈2) and α‐Ge in an about 1 : 1 mass ratio. The product was characterized by X‐ray powder diffraction, chemical analysis, and (23)Na solid‐state NMR spectroscopy. Metallic properties of Na(24‐δ )Ge(136) were revealed by a significant Knight shift of the (23)Na NMR signals and by a Pauli‐paramagnetic contribution to the magnetic susceptibility. At room temperature, Na(24‐δ )Ge(136) slowly ages, with a tendency to volume decrease and sodium loss.