Revealing Electron–Phonon Coupling Dependence on the π-Conjugated Groups in Rare-Earth Borates

Electron–phonon (e–ph) coupling, representing an interaction term connecting the electronic states and lattice vibrations, is an important hot topic in physical and chemistry frontiers. However, the e–ph coupling intensity dependence on the structural motifs is not well understood at present. Borate crystal containing conjugated π-bonds is an important functional material for optical applications. Here, we investigated the relationship between the e–ph coupling intensity and the π-conjugated borate groups, including small π-conjugated (BO3)3– and large π-conjugated (B3O6)3– motifs. Compared with α-Ba3Gd(BO3)3, the e-ph coupling intensity of Ba3Gd(B3O6)3 was greatly enhanced. The fitted Huang–Rhys factor of Ba3Gd(B3O6)3 was 3 times larger than that of α-Ba3Gd(BO3)3, thus manifesting the strengthened phonon-assisted photon luminescence. Such substantial improvement could be attributed to the shortened B–O bond length and increased electron density on the conjugated (B3O3) six-member ring. Our work not only provides a helpful guideline to search strong coupling in rare-earth materials but also paves new routes for π-conjugated functional materials with the e–ph coupling effect, for example, organic laser crystals and light-induced phase transitions.