We present evidence for an isostructural, first-order Mott transition in MnO at 105+/-5 GPa, based on high-resolution x-ray emission spectroscopy and angle-resolved x-ray diffraction data. The pressure-induced structural and spectral changes provide a coherent picture of MnO phase transitions from paramagnetic B1 to antiferromagnetic distorted B1 at 30 GPa, to paramagnetic B8 at 90 GPa, and to diamagnetic B8 at 105+/-5 GPa. The last is the Mott transition, accompanied by a significant loss of magnetic moment, an approximately 6.6% volume collapse and the insulator-metal transition as demonstrated by recent resistance measurements.

Vanadium has been reported to undergo phase transition upon compression from bcc to rhombohedral structure around 62 GPa. In this paper we confirm the bcc to rhombohedral phase transition at 61.5 GPa under quasi hydrostatic compression in Ne pressure medium. Under nonhydrostatic condition we find the phase transition occurring at 30 GPa at ambient temperature, and 37 GPa at 425 K. We find the transition under hydrostatic condition is hindered and it can occur at much lower pressure under non-hydrostatic condition.

The high pressure behavior of Ce 2 O 3 was studied using angle dispersive X-ray diffraction to 70 GPa and compared with that of CeO 2. Up to the highest pressure Ce 2 O 3 remains in the hexagonal phase (space group 164, P-32/m1) typical for the lanthanide sesquioxides. A theoretically predicted phase instability for 30 GPa is not observed. The isothermal bulk modulus and its pressure derivative for the quasi-hydrostatic case are B 0 = 111 ± 2 GPa, B 0 ' = 4.7 ± 0.3 and for the case without pressure transmitting medium B 0 = 104 ± 4 GPa, B 0 ' = 6.5 ± 0.4. Starting from ambient pressure magnetic susceptibility measurements for both oxides in highly purified form we find that the Ce atom in Ce 2 O 3 behaves like a trivalent Ce 3+ ion (2.57 μ B per Ce atom) in contrast to previously published data. Since X-ray emission spectroscopy of the Lγ (4d 3/2 → 2p 1/2) transition is sensitive to the 4f-electron occupancy, we also followed the high-pressure dependence of this line for both oxides up to 50 GPa. No change of the respective lineshape was observed, indicating that the 4f-electron configuration is stable for both materials. We posit from this data that the 4f-electrons do not drive the volume collapse of CeO 2 from the high-symmetry, low-pressure fluorite structure to the lower-symmetry orthorhombic phase. .