Crystal structures and P-T phase diagrams of SrC 2 $$ {}_2 $$ O 5 $$ {}_5 $$ and BaC 2 $$ {}_2 $$ O 5 $$ {}_5 $$ .

In this study, we present the results of a search for new stable structures of SrC 2 $$ {}_2 $$ O 5 $$ {}_5 $$ and BaC 2 $$ {}_2 $$ O 5 $$ {}_5 $$ 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 2 $$ {}_2 $$ O 5 $$ {}_5 $$ - P 2 1 / c $$ P{2}_1/c $$ is thermodynamically stable for both SrC 2 $$ {}_2 $$ O 5 $$ {}_5 $$ and BaC 2 $$ {}_2 $$ O 5 $$ {}_5 $$ . Thus, SrC 2 $$ {}_2 $$ O 5 $$ {}_5 $$ - P 2 1 / c $$ P{2}_1/c $$ is stable relative to decomposition reaction above 10 GPa, while the lower-pressure stability limit for BaC 2 $$ {}_2 $$ O 5 $$ {}_5 $$ - P 2 1 / c $$ P{2}_1/c $$ is 5 GPa, which is the lowest value for the formation of pyrocarbonates. For SrC 2 $$ {}_2 $$ O 5 $$ {}_5 $$ , the following polymorphic transitions were found with increasing pressure: P 2 1 / c → F d d 2 $$ P{2}_1/c\to Fdd2 $$ at 40 GPa and 1000 K, F d d 2 → C 2 $$ Fdd2\to C2 $$ at 90 GPa and 1000 K. SrC 2 $$ {}_2 $$ O 5 $$ {}_5 $$ - F d d 2 $$ Fdd2 $$ and SrC 2 $$ {}_2 $$ O 5 $$ {}_5 $$ - C 2 $$ C2 $$ are characterized by the framework and layered structures of [CO 4 $$ {}_4 $$ ] 4 - $$ {}^{4-} $$ tetrahedra, respectively. For BaC 2 $$ {}_2 $$ O 5 $$ {}_5 $$ , with increasing pressure, decomposition of BaC 2 $$ {}_2 $$ O 5 $$ {}_5 $$ - P 2 1 / c $$ P{2}_1/c $$ into BaCO 3 $$ {}_3 $$ and CO 2 $$ {}_2 $$ is observed at 34 GPa without any polymorphic transitions.