An Amorphous Phase of Zinc and Silicon at Composition Zn2Si5(:H, OH)

Deciphering the Polymorphism of CaSi2: The Influence of Heat and Composition

The Zintl phase CaSi2 is a layered compound with stacking variants known as 1P, 3R, and 6R. We extend the series by the 21R polytype formed by rapid cooling of the melt. The crystal structure of 21R-CaSi2 (space group R3̅m) was derived from HRTEM images, and the atomic positions were optimized by using the FPLO code (a = 3.868 Å, c = 107.276 Å). We explore polytype transformations by powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron backscattering diffraction (EBSD), and thermal analysis. While 6R-CaSi2 is thermodynamically stable at ambient conditions, nanosized impurities of silicon stabilize 3R-CaSi2 as a bulk phase.

Reactivity and Controlled Redox Reactions of Salt-like Intermetallic Compounds in Imidazolium-Based Ionic Liquids

Substituted imidazolium ionic liquids (ILs) were investigated for their reactivity towards Na₁₂Ge₁₇ as a model system containing redox‐sensitive Zintl cluster anions. The ILs proved widely inert for imidazolium cations with a 1,2,3‐trisubstitution at least by alkyl groups, and for the anion bis(trifluoromethylsulfonyl)azanide (TFSI). A minute conversion of Na₁₂Ge₁₇ observed on long‐time contact with such ILs was not caused by dissolution of the salt‐like compound, and did thus not provide dissolved Ge clusters. Rather, a cation exchange led to the transfer of Na⁺ ions into solution. In contrast, by using benzophenone as an oxidizer, heterogeneous redox reactions of Na₁₂Ge₁₇ were initiated, transferring a considerable part of Na⁺ into solution. At optimized conditions, an X‐ray amorphous product NaGe_6.25 was obtained, which was thermally convertible to the crystalline type‐II clathrate Na_24–δGe₁₃₆ with almost completely Na‐filled polyhedral cages, and α‐Ge. The presented method thus provides unexpected access to Na_24–δGe₁₃₆ in bulk quantities.