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 pi=bonds is an important functional material for optical applications. Here, we investigated the relationship between the e-ph coupling intensity and the p-conjugated borate groups, including small pi-conjugated (BO3)(3-) and large p-conjugated (B3O6)(3-) motifs. Compared with alpha-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 alpha-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 pi-conjugated functional materials with the e-ph coupling effect, for example, organic laser crystals and light-induced phase transitions.

Automated summary

Electron-phonon (e-ph) coupling, an important topic in physics and chemistry, is found to be dependent on the structural motifs. The e-ph coupling intensity is greatly enhanced in borate crystals containing large pi-conjugated groups, which can be attributed to the shortened bond length and increased electron density.