Experimental evidence for a phase transition in magnesium oxide at exoplanet pressures

Magnesium oxide is an important component of the Earth's mantle and has been extensively studied at pressures and temperatures relevant to Earth. However, much less is known about the behaviour of this oxide under conditions likely to occur in extrasolar planets with masses up to 10 times that of Earth, termed super-Earths, where pressures can exceed 1,000 GPa (10 million atmospheres). Magnesium oxide is expected to change from a rocksalt crystal structure (B1) to a caesium chloride (B2) structure at pressures of about 400-600 GPa (refs,). Whereas no structural transformation was observed in static compression experiments up to 250 GPa (ref.), evidence for a solid-solid phase transition was obtained in shockwave experiments above 400 GPa and 9,000 K (ref.), albeit no structural measurements were made. As a result, the properties and the structure of MgO under conditions relevant to super-Earths and large planets are unknown. Here we present dynamic X-ray diffraction measurements of ramp-compressed magnesium oxide. We show that a solid-solid phase transition, consistent with a transformation to the B2 structure, occurs near 600 GPa. On further compression, this structure remains stable to 900 GPa. Our results provide an experimental benchmark to the equations of state and transition pressure of magnesium oxide, and may help constrain mantle viscosity and convection in the deep mantle of extrasolar super-Earths.