  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.
