  Rocky planets are thought to comprise compounds of Mg and O as these are
among the most abundant elements, but knowledge of their stable phases may be
incomplete. MgO is known to be remarkably stable to very high pressure and
chemically inert under reduced condition of the Earth's lower mantle. However,
in 'icy' gas giants as well as in exoplanets oxygen may be a more abundant
constituent (Ref. 1,2). Here, using synchrotron x-ray diffraction in
laser-heated diamond anvil cells, we show that MgO and oxygen react at
pressures above 94 GPa and T = 2150 K with the formation of the theoretically
predicted I4/mcm MgO2 (Ref.3). Raman spectroscopy detects the presence of a
peroxide ion (O22-) in the synthesized material as well as in the recovered
specimen. Likewise, energy-dispersive x-ray spectroscopy confirms that the
recovered sample has higher oxygen content than pure MgO. Our finding suggests
that MgO2 may substitute MgO in rocky mantles and rocky planetary cores under
highly oxidizing conditions.
