(Meta)stable CaC2

One out of four modifications of CaC2 is the so-called metastable Calcium C arbide, CaC2-III, which was synthesized as pure material. It forms by heati ng monoclinic CaC2-II (C2/c) above 150 degreesC and remains stable after co oling down to room temperature. The structure was refined from X-ray powder patterns (C2/m, Z = 4, a = 722.6(1)pm, b = 385.26(7)pm, c = 737.6(1)pm, be ta = 107.345(2)degrees). After grinding CaC2-III transforms back into CaC2- II. Heating CaC2-III induces a reversible phase transition into the cubic m odification (CaC2-IV) at 460 degreesC. Differences between the three differ ent structures of CaC2 I-III, being stable at ambient conditions are also s hown by C-13-MAS-NMR measurements, especially the presence of two distinct types of carbon atoms in the structure of the title compound.