Abstract

In this study, we present the results of a search for new stable structures of SrC2$$ {}_2 $$O5$$ {}_5 $$ and BaC2$$ {}_2 $$O5$$ {}_5 $$ 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 SrC2$$ {}_2 $$O5$$ {}_5 $$-P21/c$$ P{2}_1/c $$ is thermodynamically stable for both SrC2$$ {}_2 $$O5$$ {}_5 $$ and BaC2$$ {}_2 $$O5$$ {}_5 $$. Thus, SrC2$$ {}_2 $$O5$$ {}_5 $$-P21/c$$ P{2}_1/c $$ is stable relative to decomposition reaction above 10 GPa, while the lower-pressure stability limit for BaC2$$ {}_2 $$O5$$ {}_5 $$-P21/c$$ P{2}_1/c $$ is 5 GPa, which is the lowest value for the formation of pyrocarbonates. For SrC2$$ {}_2 $$O5$$ {}_5 $$, the following polymorphic transitions were found with increasing pressure: P21/c→Fdd2$$ P{2}_1/c\to Fdd2 $$ at 40 GPa and 1000 K, Fdd2→C2$$ Fdd2\to C2 $$ at 90 GPa and 1000 K. SrC2$$ {}_2 $$O5$$ {}_5 $$-Fdd2$$ Fdd2 $$ and SrC2$$ {}_2 $$O5$$ {}_5 $$-C2$$ C2 $$ are characterized by the framework and layered structures of [CO4$$ {}_4 $$]4−$$ {}^{4-} $$ tetrahedra, respectively. For BaC2$$ {}_2 $$O5$$ {}_5 $$, with increasing pressure, decomposition of BaC2$$ {}_2 $$O5$$ {}_5 $$-P21/c$$ P{2}_1/c $$ into BaCO3$$ {}_3 $$ and CO2$$ {}_2 $$ is observed at 34 GPa without any polymorphic transitions.