Of the more than 6,000 confirmed and candidate extrasolar planets discovered to date those that are 1-4 times the radius of the Earth are found to be the most abundant. The silicate compounds that dominate the Earth’s mantle likely dissociate into component oxides at the extreme pressures (200-2,000 GPa) and temperatures (5,000-10,000 K) corresponding to conditions of super-Earth mantles. MgO (periclase), an endmember of the (Mg,Fe)O solid solution, is expected to be a major component of the deep mantles of terrestrial planets and exoplanets. Its high-pressure transformation from a rocksalt (B1-type) structure to the B2 (CsCl-type) structure is expected to be observable in rocky exoplanets greater than about 5 Earth masses in size. Empirical systematics and theoretical studies have suggested that the MgO phase transformation may be accompanied by a strong change in rheological properties with the high-pressure B2-phase exhibiting a reduction in viscosity. However, theoretical studies also show significant differences in predictions for the B1-B2 onset pressure and P-T Clapeyron slope.

In this study, the structure of MgO upon shock compression over the 200-700 GPa pressure range was examined at the Omega-EP Laser facility at the Laboratory for Laser Energetics, University of Rochester. Laser drives of up to 2 kJ over 10 ns focused onto a polyimide ablator were used to shock compress 70-μm thick polycrystalline or single-crystal MgO. At peak compression, the sample was probed with He-α X-rays from a laser-plasma source. Diffracted X-rays were collected using the PXRDiP diagnostic which consists of image plates lining the inner walls of a box attached to the target package. For each pressure we measure pressure (velocity interferometry), density (x-ray diffraction) and shock temperature (pyrometry).

Our study provides the first direct measurement of lattice-level changes in MgO under shock compression. We constrain the onset of rocksalt (B1) to cesium chloride (B2) phase transformation over the 397-425 GPa and 9,610-9,730 K pressure and temperature range. Above these pressures we measure a pressure-dependent timescale for the B1→B2 transition from 850 ps (425 GPa) to 5 ps (493 GPa). Our data is consistent with B2-liquid co-existence in shocked MgO above 470 GPa and total melt within the 545-634 GPa range (11,450-14,150 K). A textural analysis of the diffraction data reveals that uniaxially compression of MgO [100] single crystals into the B2 phase produce a well-orientated multiple fiber textured microstructure.