Crystal structures and P-T phase diagrams of SrC O and BaC O

In this study, we present the results of a search for new stable structures of SrC $$ O $$ and BaC $$ O $$ in the pressure range of 0-100 GPa based on the density functional theory and crystal structure prediction approaches. We have shown that the recently synthesized pyrocarbonate structure SrC $$ O $$ - $$ is thermodynamically stable for both SrC $$ O $$ and BaC $$ O $$ . Thus, SrC $$ O $$ - $$ is stable relative to decomposition reaction above 10 GPa, while the lower-pressure stability limit for BaC $$ O $$ - $$ is 5 GPa, which is the lowest value for the formation of pyrocarbonates. For SrC $$ O $$ , the following polymorphic transitions were found with increasing pressure: $$ at 40 GPa and 1000 K, $$ at 90 GPa and 1000 K. SrC $$ O $$ - $$ and SrC $$ O $$ - $$ are characterized by the framework and layered structures of [CO $$ ] $$ tetrahedra, respectively. For BaC $$ O $$ , with increasing pressure, decomposition of BaC $$ O $$ - $$ into BaCO $$ and CO $$ is observed at 34 GPa without any polymorphic transitions.