Enhanced Electron-Phonon Coupling Effect in Rare-Earth Borate Crystals Containing a "Quasi-Free-Oxygen" Motif

Revealing the interaction between electrons and phonons, e.g., electron-phonon coupling or decoupling, is a great challenge for physics and functional material communities. For rare-earth single crystals, the electron-phonon coupling and fluorescence behaviors strongly depend on the crystal structure and constituent motifs. Here, we proposed a universal "quasi-free O" as an effective structural motif to enhance phonon-assisted electronic transitions and photoluminescence. Using Gd³⁺ ion as a probe, we studied Gd:La₂CaB₁₀O₁₉ (Gd:LCB) and GdMgB₅O₁₀ (GdMB) crystals composed of double B-O layers and dangling "quasi-free O", respectively, which enable strengthened phonon-involved luminescence. Especially, a GdMB crystal features an infinite [O-Gd-O-Gd-O] chain (O represents quasi-free oxygen), thus greatly promoting the energy transfer and electron-phonon coupling effect. As a result, its Huang-Rhys S factor is two times larger than that of a Gd:LCB crystal under room temperature. These results put forward "quasi-free O" to improve the electron-phonon coupling intensity and allow LCB and GdMB crystals to serve as potential hosts for phonon-terminated vibronic lasers.