A₂Se₂C₂ (A = Na-Cs): crystalline compounds featuring ^-Se-C[triple bond, length as m-dash]C-Se^- dianions with...

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-C[triple bond, length as m-dash]C-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.