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.