Comparison of the high-pressure behavior of the cerium oxides C e 2 O 3 and Ce O 2

The high-pressure behavior of $\mathrm{C}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$ was studied using angle-dispersive x-ray diffraction to 70 GPa and compared with that of $\mathrm{Ce}{\mathrm{O}}_{2}$. Up to the highest pressure $\mathrm{C}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$ remains in the hexagonal phase (space group 164, $P\overline{3}2/m1)$ 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 quasihydrostatic case are ${B}_{0}=111\ifmmode\pm\else\textpm\fi{}2\phantom{\rule{0.28em}{0ex}}\mathrm{GPa},{B}_{0}^{\ensuremath{'}}=4.7\ifmmode\pm\else\textpm\fi{}0.3,$ and for the case without pressure-transmitting medium ${B}_{0}=104\ifmmode\pm\else\textpm\fi{}4\phantom{\rule{0.28em}{0ex}}\mathrm{GPa},{B}_{0}^{\ensuremath{'}}=6.5\ifmmode\pm\else\textpm\fi{}0.4$. Starting from ambient-pressure magnetic susceptibility measurements for both oxides in highly purified form, we find that the Ce atom in $\mathrm{C}{\mathrm{e}}_{2}{\mathrm{O}}_{3}$ behaves like a trivalent $\mathrm{C}{\mathrm{e}}^{3+}$ ion ($2.57{\ensuremath{\mu}}_{\mathrm{B}}$ per Ce atom) in contrast to previously published data. Since x-ray emission spectroscopy of the $L\ensuremath{\gamma}$ $(4{d}_{3/2}\ensuremath{\rightarrow}2{p}_{1/2})$ 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 $\mathrm{Ce}{\mathrm{O}}_{2}$ from the high-symmetry, low-pressure fluorite structure to the lower-symmetry orthorhombic phase.