Calcium peroxide from ambient to high pressures

Abstract

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.