Figure 4.

(A) Raman spectra of MgO + O₂ reaction products collected with 488, 532, and 660 nm excitations. Oxygen Raman spectra collected outside of the laser-heated region is shown for comparison. Dark blue vertical ticks correspond to the computed Raman modes of I4/mcm at 100 GPa (A_1g and B_2g modes may have observable intensities, according to our DFT computations). The 1037 cm⁻¹ peak that can be assigned to the A_1g mode in I4/mcm MgO₂ is marked by an arrow. (B) Pressure dependencies of O-O symmetric stretching vibration (A_1g). Red circles represent positions of the high-frequency mode observed in the A2 sample. Green circles correspond to the positions of the A_1g band in Pa3 MgO₂ measured in this study and in Ref. 19 (green open circle at 1 atm.). Blue triangles are positions of the A_1g mode in BaO₂. Black circles are computed frequencies of the A_1g mode in I4/mcm MgO₂.