Figure 1. X-ray diffraction (XRD) pattern of the A1 sample before laser heating (black line), at high temperature (red line) and quenched to 300 K (blue line).

Arrows mark new peaks that appear at high temperature. A thermal shift of the MgO peaks is seen at T = 2150 K indicating a uniform heating of the sample. Miller indices correspond to the indexed tetragonal unit cell. Expected positions of I4/mcm MgO₂ Bragg reflections¹² are shown by red ticks. Blue bars correspond to MgO. Oxygen peaks are not resolved. The wavelength is 0.3344 Å. The inset shows the experimental assemblage of type-A runs.