Type‐II Clathrate Na_24‐δGe₁₃₆ from a Redox‐Preparation Route

Abstract

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.

Key properties of inorganic thermoelectric materials—tables (version 1)

“…Besides, band structure engineering through proper substitution of the framework atoms or tuning the vacancy concentration on the framework sites and the guest atom concentrations inside the polyhedral cages with novel synthetic techniques, e.g. low-temperature redox reactions [353,354] melt-centrifugation [44,314] or liquid phase sintering [355,356] may potentially lead to enhanced thermoelectric properties. Leveraging high-throughput calculations and data mining [357][358][359][360], the selection process of inorganic clathrates can be accelerated, and unexplored clathrate phases may be uncovered with high thermoelectric performance.…”

Na2Ga7: A Zintl–Wade Phase Related to “α-Tetragonal Boron”

“…Na 2 Ga 7 was first discovered from the reaction of Na 7 Ga 13 and gaseous NH 3 . The reactions aimed at finding metastable compounds in the sodium–gallium system. − When Na 7 Ga 13 and NH 3 were reacted in a molar ratio of 1:160 at 300 °C, the composition of the reaction product changed with the reaction time according to PXRD, allowing conclusions on the redox process. After 10 min, NaGa 4 was the major product with detectable amounts of Na 2 Ga 7 and remnants of the starting material (Scheme and Figure S2).…”

“…The new equilibrium phase Na 2 Ga 7 extends the series of binary phases with a Ga framework compound built up by [Ga 12 ] 2– Wade and [Ga] − Zintl anions. Na 2 Ga 7 was first discovered by reacting Na 7 Ga 13 with gaseous NH 3 , following previous research on the redox preparation of metastable cage compounds. − Here, preparation methods, phase relations, crystal structures, and physical properties of Na 2 Ga 7 are reported.…”

“…Na 2 Ga 7 was first discovered by reacting Na 7 Ga 13 with gaseous NH 3 , following previous research on the redox preparation of metastable cage compounds. 26 − 28 Here, preparation methods, phase relations, crystal structures, and physical properties of Na 2 Ga 7 are reported.…”

Type II Germanium Clathrates from Zintl Phase Precursor Na4Ge4: Understanding Desodiation Processes and Sodium Migration Using First-Principles Calculations

“…The completely guest-free clathrate (Ge 136 ) has the highest formation energy and therefore should be harder to synthesize than the Na-filled clathrates. Indeed, to our knowledge, most previous attempts at synthesizing guest-free type II germanium clathrates have resulted in the formation of Na-filled clathrates ,, or low-Na/K content clathrates , as one of the major products along with other by-products. Special techniques such as the application of an electric field in an Ar environment, and chemical oxidation, or thermal decomposition in ionic liquid media were needed to obtain nearly guest-free type II Ge clathrates (δ ∼ 0 in Na δ Ge 136 ).…”