Comparison of the high-pressure behavior of the cerium oxides Ce sub(2) O sub(3) and CeO sub(2)

The high-pressure behavior of Ce sub(2) O sub(3) was studied using angle-dispersive x-ray diffraction to 70 GPa and compared with that of CeO sub(2). Up to the highest pressure Ce sub(2) O sub(3) remains in the hexagonal phase (space group 164, P 32/m 1) 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 quasihydrostatic case are B sub(0)= 111 + or - 2 GPa, B' sub(0)=4.7 + or - 0.3, and for the case without pressure-transmitting medium B sub(0)= 104 + or - 4 GPa, B' sub(0)= 6.5 + or - 0.4. Starting from ambient-pressure magnetic susceptibility measurements for both oxides in highly purified form, we find that the Ce atom in Ce sub(2) O sub(3) behaves like a trivalent Ce super(3+) ion (2.57 mu sub(B) per Ce atom) in contrast to previously published data. Since x-ray emission spectroscopy of the L gamma (4d sub(3/2) arrow right 2 p sub(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 line shape 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 sub(2) from the high-symmetry, low-pressure fluorite structure to the lower-symmetry orthorhombic phase.