Stable magnesium peroxide at high pressure

TL;DR

This study discovers a stable high-pressure magnesium peroxide phase (MgO2) formed from MgO and oxygen, which could alter our understanding of planetary compositions in high-oxidation environments.

Contribution

The paper reports the first experimental synthesis and identification of stable MgO2 at high pressures and temperatures, expanding knowledge of planetary mineral phases.

Findings

MgO and oxygen react above 94 GPa and 2150 K to form MgO2.

Raman spectroscopy confirms peroxide ion presence in MgO2.

Recovered samples show higher oxygen content than pure MgO.

Abstract

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.