Pb2[C2O6]-P3̄m1: new insights into the high-pressure behavior of carbonates.

In the present study, based on density functional theory and crystal structure prediction approaches, we found a new high-pressure structure of lead carbonate, named Pb2[C2O6]-P3̄m1. This structure differs significantly from previously known modifications of lead carbonate. The Pb2[C2O6]-P3̄m1 structure is characterized by the presence of ethane-like [C2O6] groups, which can also be classified as orthooxalate groups. This structure is most energetically favorable at pressures above 92 GPa at low temperatures, while Pmmn (post-aragonite structure) is most favorable below this pressure. As temperature increases to 2000 K, the pressure required for the Pmmn → P3̄m1 phase transition increases to 100 GPa. The high-pressure modification Pb2[C2O6]-P3̄m1 retains its stability at least up to 200 GPa. In addition, the Raman spectrum of the newly discovered modification was calculated, which may be useful for subsequent identification of this phase in high-pressure experiments. At 100 GPa, the most intense band located at 1148 cm-1 corresponds to the symmetric stretching mode of the C-C bond in the [C2O6] orthooxalate groups. The second and third most intense modes appear at 1021 and 726 cm-1, correspondingly.