NaSeC 2 H: A Crystalline Compound with Disordered – SeC 2 H Anions

By reaction of NaC 2 H with elemental selenium (grey or red) in liquid ammonia a crystalline brownish powder of NaSeC 2 H was obtained. The existence of the new – SeC 2 H anion – recently discovered in A I SeC 2 H (A I = K, Rb, Cs) – was confirmed by IR spectra showing the expected C≡C, C–H, and C–Se vibrations. A synchrotron powder diffraction pattern (SNBL, ESRF/Grenoble) was indexed in a cubic unit cell with a = 7.044 Å. Analysis of the data led to a pyrite‐type structure ( Pa 3 , Z = 4) with a static orientational disorder of the – SeC 2 H anions over two positions. No phase transition was observed upon cooling to 100 K, where essentially the same structure was obtained with a = 6.989 Å. Upon heating, NaSeC 2 H releases C 2 H 2 as revealed by thermal analysis (DSC/TGA) resulting in potential Na 2 Se 2 C 2 crystallizing in a small cubic unit cell with a ≈ 3.899 Å similar to findings on the decomposition of the other A I SeC 2 H compounds.