High-Pressure Crystal Structures of Pb2CO4 and PbC2O5 with Tetrahedral [CO4] and Pyrocarbonate [C2O5] atomic groups

The search of the stable intermediate compounds in the system PbO-CO2 have been performed using crystal structure prediction technique in the pressure range of 0-50 GPa. Stable structures were found for the compositions Pb2CO4 and PbC2O5. The earlier known oxy-carbonate Pb2CO4-P2(1)2(1)2(1) was shown to transform at 10 GPa into another oxy-carbonate Pb2CO4-Pnma and than at 22 GPa - into the sp(3)-hybridized orthocarbonate with the same symmetry. PbC2O5 became thermodynamically stable at almost 10 GPa in the form of the pyrocarbonate structure P2(1)/c. At 35-45 GPa, PbC2O5-P2(1)/c transforms into PbC2O5-Fdd2 with the framework of [CO4] tetrahedra. Another energetically favorable, although dynamically unstable, pyrocarbonate structure PbC2O5-P1? was found in the narrow pressure interval around 10 GPa. We suggest that some analogue of this structure can appear in experiment. One structure, sp(3)-hybridized Pb2CO4-Pnma, is dynamically and thermally stable at ambient pressure, and therefore can be decompressed and extracted from the high-pressure environment.