  Calcium and magnesium carbonates are believed to be the host compounds for
most of the oxidized carbon in the Earth's mantle. Here, using evolutionary
crystal structure prediction method USPEX, we systematically explore the
MgO-CO2 and CaO-CO2 systems at pressures ranging from 0 to 160 GPa to search
for thermodynamically stable magnesium and calcium carbonates. While MgCO3 is
the only stable magnesium carbonate, three calcium carbonates are stable under
pressure: well-known CaCO3, and newly predicted Ca3CO5 and CaC2O5. Ca3CO5
polymorphs are found to contain isolated orthocarbonate (CO4)4- tetrahedra, and
are stable at relatively low pressures (>11 GPa), whereas CaC2O5 is stable
above 33 GPa and its polymorphs feature polymeric motifs made of CO4
tetrahedra. Detailed analysis of chemical stability of CaCO3, Ca3CO5 and CaC2O5
in the environment typical of the Earth's lower mantle reveals that none of
these compounds can exist in the Earth's lower mantle. We conclude that MgCO3
is the main host of oxidized carbon throughout the lower mantle.
