K₂C₂O₅─The First Finding of Pyrocarbonate Stable at Ambient Pressure

A potassium pyrocarbonate thermodynamically stable at atmospheric pressure has been revealed for the first time using first-principles calculations and modern crystal structure prediction technique. Our calculations indicate that at ambient pressure K2C2O5 is stable in the C2 structure, which contains paired [CO3] triangles forming [C2O5] pyrogroups. According to the calculation even at ambient pressure, it will not decompose into the mixture K2CO3 + CO2 and thus can be obtained in the experiment. With increasing pressure to 4 GPa, a polymorphic transition to the K2C2O5–P21 structure was observed, which is also characterized by the presence of [C2O5] pyrogroups. Thus, these predicted K2C2O5 structures complement the recently discovered family of pyrocarbonates. K2C2O5–P21 retains its stability up to a pressure of 37 GPa, where its decomposition into K2CO3 and CO2 was observed. At a pressure of 87 GPa, K2C2O5 is again formed as a result of the reaction K2CO3 + CO2 and it is stable in the C2-hp structure. In the high-pressure modification C2-hp, the carbon atoms are in the sp3-hybridized state, and the paired triangles [CO3] are replaced by polymerized [CO4] tetrahedra in layers. This type of polymerization well established for the alkaline-earth carbonates, is observed for the first time for alkali metal carbonates. In addition, our calculations indicate the dynamic and thermal stability of the predicted structures in the corresponding pressure ranges.