LAUSR

Abstract Bulk calcium peroxide (CaO2) was synthesized in 1941, where experiments assigned a tetragonal ‘calcium carbide’ phase. However our first-principles calculations of thermodynamic and dynamical properties evidence that the tetragonal calcium carbide phase of CaO is unstable. Therefore we revisit here the stability of the Ca–O system at ambient pressure by using evolutionary methodology for crystal structure prediction. Besides the well-known rocksalt phase of CaO, unexpected new hexagonal nearly thermodynamically stable, and unstable tetragonal calcium carbide phases have been discovered at ambient conditions. Computed elastic constants and phonon reveal that all polytype phases of CaO are mechanically stable; whereas rocksalt and hexagonal (tetragonal) are dynamically stable (unstable). Furthermore, the electronic, mechanical and dynamical properties of rocksalt, hexagonal and tetragonal structures of Ca–O system are examined in relation with calcium oxide polytypes.