Structural phase transitions and electronic properties of LI2o2 and Na2o2 under high pressure for Co2 capture application

Abstract

Since pressure induces exotic chemical and physical properties of materials, investigation of the high-pressure effect on structural phase transition and the related properties is paramount. Structural phase transitions and electronic properties of Li2O2 and Na2O2 under high pressure were investigated using first-principles calculations based on the density functional theory. Structural phase transitions of Li2O2 up to 500 GPa were predicted at ~75 GPa from the P63/mmc to the P21 structures and at ~136 GPa from the P21 to the P21/c structures. The calculated band gaps of all phases increase with elevated pressure. At 11 and 40 GPa, the band gaps decrease, while the O-O bond lengths and the electron localization function (ELF) values increase, resulting in the O-O stretching mode softening. Structural phase transitions of Na2O2 up to 300 GPa were investigated with elevated temperature up to 600 K based on the quasi-harmonic approximation. Two new phases were predicted consisting of the Amm2 and the P21/c structures, which are stable at low temperatures in the 0-22 and 22-28 GPa pressure ranges, respectively. At the elevated temperature, the P-62m and the Pbam structures more favor temperature than the Amm2 and the P21/c structures, respectively. In the 2-3 and 9-10 GPa pressure ranges, some phonon modes softening and some elastic stiffnesses decreasing result in the phonon free energies decreasing and the ELF values increasing. Their band gaps also increase with increasing pressure. High-pressure Raman measurements and Raman calculations for Na2O2 provide the evidence for the presence of an unidentified structure at 8.6-21.5 GPa and the Pbam structure at 24.8 GPa. The calculated Gibbs free energy changes of the CO2 capture reactions by Li2O2 and Na2O2 suggest that the reactions are spontaneous under high pressure and temperature studied.