3D nitride frameworks with variable channel sizes; synthesis and powder neutron diffraction study of the nitride carbodiimdes, Ca₄N ₂(CN₂) and Ca11N6(CN₂)₂

The synthesis of powders of the calcium nitride carbodiimides, Ca ₄N₂(CN₂) (1) and Ca₁₁N ₆(CN₂)₂ (2) has been studied and the products characterised initially by powder X-ray diffraction. The latter phase (2) is unobtainable in pure bulk form and is always accompanied by the former compound. Nevertheless, definitive structures of the compounds were obtained from Rietveld refinements against time-of-flight powder neutron diffraction data. Each of the nitride carbodiimides are composed of Ca-N three dimensional networks built from edge- and vertexsharing NCa6 octahedra arranged to create one-dimensional channels. The size of the channels is dependent on the Ca:C (host:guest) ratio and the larger channels in (1) are occupied by twice as many guest species as those in (2). Importantly, neutron diffraction data confirm that the identity of these guest species is carbodiimide (as opposed to isoelectronic azide), that neither the framework nor the channels therefore contain excess electrons and that the compounds are hence not "nitride electrides" as per the parent subnitride, Ca₂N.