  Structures of calcium peroxide (CaO2) are investigated in the pressure range
0-200 GPa using the ab initio random structure searching (AIRSS) method and
density functional theory (DFT) calculations. At 0 GPa, there are several CaO2
structures very close in enthalpy, with the ground-state structure dependent on
the choice of exchange-correlation functional. Further stable structures for
CaO2 with C2/c, I4/mcm and P21/c symmetries emerge at pressures below 40 GPa.
These phases are thermodynamically stable against decomposition into CaO and
O2. The stability of CaO2 with respect to decomposition increases with
pressure, with peak stability occurring at the CaO B1-B2 phase transition at 65
GPa. Phonon calculations using the quasiharmonic approximation show that CaO2
is a stable oxide of calcium at mantle temperatures and pressures, highlighting
a possible role for CaO2 in planetary geochemistry. We sketch the phase diagram
for CaO2, and find at least five new stable phases in the pressure/temperature
ranges 0<P<60 GPa, 0<T<600 K, including two new candidates for the
zero-pressure ground state structure.
