Phonon-Assisted Photoluminescence-Excited (PLE) Spectrum in Eu³⁺-Doped Single Crystals

The europium (Eu3+) ion is an important lanthanide cation for rare-earth phosphors, with red emission around 620 nm. It exhibits two typical hypersensitive transitions, namely, absorbed 7F0 → 5D2 and emitted 5D0 → 7F2, where the transition electric dipole intensities among electron levels are strongly correlated to ligands and lattices. This case indicates that Eu3+ should be an excellent fluorescence probe to investigate the weak and sensitive electron–phonon coupling effect in 4f–4f transitions. Here, we established a qualitative relation between the hypersensitive transitions and electron–phonon coupling by two parameters, bond length and average polarizability of ligand, that short length and large polarizability are beneficial for strengthened electron–phonon coupling. In addition, we grew three Eu3+-doped crystals: Eu:La2CaB10O19, Eu:Lu2O3, and Eu:Y2O(SiO4), and measured their photoluminescence-excited (PLE) and photoluminescence (PL) spectra, respectively. We found clear phonon sidebands in their PLE spectra, with some satellites at shorter wavelengths associated with phonon-assisted excitation. Based on the calculated Huang–Rhys factors, we demonstrated that the special “free-oxygen” motif in Eu:Y2O(SiO4) is beneficial for strengthening phonon-assisted excitations. This work provides a helpful guideline for searching for strong electron–phonon coupling crystals in rare-earth materials.