Comparative analysis for meaningful interpretation of rare-earth oxide M$_{4,5}$ energy loss edges

Abstract

The magnetic, electronic, and optical properties of rare earth-oxides are directly influenced by the valency of the metallic cation. With the development of next generation electron energy-loss spectrometers, high-energy lanthanide fine structure can be studied with improved signal-to-noise for quantitative analysis. Unfortunately, the behavior of rare-earth $4f$ orbital electrons is not well understood. To establish best practices for analysis of energy-loss spectra from lanthanide oxides, we have performed a comparative study of the four traditional white line analysis methods extended to lanthanide $M_{4,5}$ edges resulting from $3d \rightarrow 4f$ orbital transitions using data from Gatan's EELS Atlas. The ${M_4}/{M_5}$ spectral feature ratios were examined as a function of $4f$ occupancy. The ${M_4}/{M_5}$ spectral feature ratio decreases exponentially as $4f$ occupancy increases, except for a plateau between S$\text{m}^{3+}$ and D$\text{y}^{3+}$. The full-width at half the maximum intensity of the $M_4$ edges shows increased broadening for S$\text{m}^{3+}$ through D$\text{y}^{3+}$. We suggest that the plateau results from $4f$ orbital half-filling and is explained through the relationship between electron transition probability and transition lifetime as expressed through Fermi's Golden Rule. Of the four spectral analysis methods described, only the integrated area method can be ascribed a quantitative physical interpretation.