Phase Transition and Optical Properties of Solid Oxygen under High Pressure: A Density Functional Theory Study

Crystal structures and optical properties of the δ--O₂ phase and the ε--O₈ phase have been investigated by using the initio pseudopotential plane-wave method. It is found that the phase transition is of the first order with a discontinuous volumetric change from the antiferromagnetic δ--O₂ phase to the nonmagnetic ε--O₈ phase, consistent with the experimental findings. The energy band calculations show that the direct band gap changes into an indirect band gap after the phase transition. The apparent change in the optical properties can be used for identifying the phase transition from δ--O₂ to ε--O₈.