A systematic study of the oxygen K edge in the cubic and less common monoclinic phases of the rare earth oxides (Ho, Er, Tm, Yb) by electron energy loss spectroscopy

Transmission electron microscopy and electron energy loss spectroscopy (EELS) were used to distinguish structural details for the cubic and uncommon monoclinic crystal phases of heavy metal rare earth oxide nanoparticles synthesized by a combustion process. Specifically, we systematically examined the EELS oxygen K edge for Ho, Tm, Er, and Yb sesquioxides in both phases. This work presents, to our knowledge, the first EELS study of the monoclinic phase in these materials and the first systematic comparative study between the monoclinic and cubic phases across a range of rare earth sesquioxides. For both phases, we observed the usual double-peak structure of the oxygen K edge typically seen for oxygen atoms tetrahedrally surrounded by metal atoms, but we found the details of the energy loss near-edge structure to differ substantially between the two phases. Our results indicate a greater peak separation for the cubic than for the monoclinic phase. Furthermore, a trend of increasing peak separation with increasing atomic number was also noted for both phases in the series of rare earth oxide nanoparticles. Our results show that the fine structure in EELS can be utilized to determine differences on the nanoscale between the common cubic structure and the less common monoclinic structure.