Studies of the behaviour of solids at ultra-high pressures, those beyond 200 GPa, contribute to our fundamental understanding of materials properties and allow an insight into the processes happening at such extreme conditions relevant for terrestrial and extra-terrestrial bodies. The behaviour of magnesium oxide, MgO, is of a particular importance, as it is believed to be a major phase in the Earth lower mantle and the interior of super-Earth planets. Here we report the results of studies of MgO at ultra-high static pressures up to ca. 660 GPa using the double-stage diamond anvil cell technique with synchrotron X-ray diffraction. We observed the B1-B2 phase transition in the pressure interval from 429(10) GPa to 562(10) GPa setting an unambiguous reference mark for the B1-B2 transition in MgO at room temperature. Our observations allow constraining theoretical predictions and results of available so far dynamic compression experiments.
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