A2Se2C2 (A = Na–Cs): crystalline compounds featuring −Se–CC–Se− dianions with short-range structural disorder

Abstract

Crystalline powders of A₂Se₂C₂ (A = Na–Cs) were prepared either by reacting A₂C₂ with two equivalents of elemental (grey) selenium in liquid ammonia or by thermally decomposing ASeC₂H at 200 °C under dynamic vacuum. IR and Raman spectra unambiguously confirm the presence of ⁻Se–CC–Se⁻ anions. X-ray powder diffraction patterns exhibit only a few very broad reflections, consistent with a primitive cubic arrangement of the alkali metal cations. The anions occupy the interstitial voids between these cations and display local orientational disorder. Only through a combined approach employing Rietveld and pair distribution function (PDF) analyses of synchrotron powder diffraction data, supplemented by density functional theory (DFT) calculations, it was possible to establish a consistent structural model for the anion disorder.