Soft-phonon anomalies and crystal electric field–phonon coupling in cubic lanthanide sesquioxides Eu2O3 and Yb2

Phonon softening is a common identifier of strong coupling between electronic and vibrational degrees of freedom in materials, which is responsible for many of the exotic phenomena observed in scientifically interesting and technologically relevant materials. Two cubic rare-earth sesquioxides (RESO), ${\mathrm{Eu}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}$, display anomalously soft phonons compared to trends in phonon energy exhibited by other cubic RESOs. In an effort to elucidate the mechanisms responsible for the anomalous mode softening in ${\mathrm{Eu}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}$, we performed temperature- and magnetic field-dependent Raman scattering studies of polycrystalline samples of these materials. Our data suggest that two different underlying phenomena are responsible for these anomalies. While we observe crystal electric field (CEF) electronic transitions in both materials, only in ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}$ does the energetic proximity of CEF excitations to phonons appear to lead to significant electron-phonon coupling in the form of CEF-phonon coupling. We find evidence that defects---rather than interactions between vibrational and electronic excitations---are responsible for the soft phonons in ${\mathrm{Eu}}_{2}{\mathrm{O}}_{3}$. These results place constraints on proposed phonon mode softening mechanisms in rare-earth sesquioxides and illustrate the varied effects of electron-phonon coupling in these materials.