Stability and crystal structure of MgSiO₃ perovskite to the core-mantle boundary

We report in situ angle-dispersive X-ray diffraction measurements for MgSiO₃ at 90-118 and 135-144 GPa and 300-2900 K. At 90-144 GPa, we observed a new diffraction peak at 2.57-2.62 Å as reported in Shim et al. [2001]. The pressure-induced position shift of the new peak is consistent with those of MgSiO₃ perovskite (Mg-Pv), supporting that the new peak is associated with a modification in crystal structure of Mg-Pv at 88 GPa. During laser heating above 2500 K at 144 ± 10 GPa, in addition to peaks of Mg-Pv, we observed new peaks as well as peak splittings and intensity changes of some peaks. The major features in the diffraction patterns can be explained by the modified Mg-Pv proposed by Shim et al. [2001] together with the post-perovskite phase proposed by Murakami et al. [2004]. This supports the existence of a major phase transition in MgSiO₃ under conditions close to those at the base of Earth's mantle.