A bidentate-anionic-group strategy for enhancing electron–phonon coupling and vibronic fluorescence in rare-earth crystals†
Abstract
Electron–phonon coupling, representing the energy transfer between electron states and lattice vibrations, is a fundamental physical cornerstone for many interesting phenomena. Phonon-assisted electronic transition manifests as fluorescence emission, providing a good platform for studying electron–phonon coupling. Herein, we obtained K3Gd(BO3)2 and Li3K9Gd3(BO3)7 crystals and studied the relations of the coordination type between the central Gd3+ ions and the [BO3]3− ligand. Using Gd3+ ions as a fluorescence probe, we found that the bidentate-BO3 group, better than monodentate-BO3, can greatly enhance the electron–phonon coupling and phonon-assisted fluorescence at longer wavelengths. Compared with K3Gd(BO3)2, the Huang–Rhys S factor of Li3K9Gd3(BO3)7 realized about triple improvement (from 0.057 to 0.163), which is attributed to the increased bidentate–BO3 ratio (from 50% in K3Gd(BO3)2 to 100% in Li3K9Gd3(BO3)7). More impressively, this bidentate-anionic-group strategy can be extended to rare-earth phosphates, in which KGd(PO3)4 (without bidentate-PO4) and K3Gd(PO4)2 (with 50% bidentate-PO4) exhibit S factors of 0.08 and 0.133, respectively. These discoveries provide some potential fluorescence matrices and a helpful guideline for searching new rare-earth hosts with strong electron–phonon coupling for tunable broadband vibronic lasers.
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