Femtosecond X-ray Diffraction of Laser-shocked Forsterite (Mg2SiO4) to 122 GPa

The behavior of forsterite, Mg2SiO4, under dynamic compression is of
fundamental importance for understanding its phase transformations and
high-pressure behavior. Here, we have carried out an in situ X-ray
diffraction study of laser-shocked poly- and single-crystal forsterite
(a-, b-, and c- orientations) from 19 to 122 GPa using the Matter in
Extreme Conditions end-station of the Linac Coherent Light Source. Under
laser-based shock loading, forsterite does not transform to the
high-pressure equilibrium assemblage of MgSiO3 bridgmanite and MgO
periclase, as was suggested previously. Instead, we observe forsterite and
forsterite III, a metastable polymorph of Mg2SiO4, coexisting in a
mixed-phase region from 33 to 75 GPa for both polycrystalline and
single-crystal samples. Densities inferred from X-ray diffraction data are
consistent with earlier gas-gun shock data. At higher stress, the behavior
observed is sample-dependent. Polycrystalline samples undergo
amorphization above 79 GPa. For [010]- and [001]-oriented crystals, a
mixture of crystalline and amorphous material is observed to 108 GPa,
whereas the [100]-oriented crystal adopts an unknown crystal structure at
122 GPa. The Q values of the first two sharp diffraction peaks of
amorphous Mg2SiO4 show a similar trend with compression as those observed
for MgSiO3 glass in both recent static and laser-compression experiments.
Upon release to ambient pressure, all samples retain or revert to
forsterite with evidence for amorphous material also present in some
cases. This study demonstrates the utility of femtosecond free-electron
laser X-ray sources for probing the time evolution of high-pressure
silicates through the nanosecond-scale events of shock compression and
release.