Electron−phonon coupling emerges as a growing frontier in the heart of condensed matter from physical symmetry to the electronic quantum state, but its quantitative strength dependence on the chemical structure has not been assessed. Here, we originally proposed the anion-centered polyhedron (ACP) strategy for elaborating the electron−phonon coupling interaction in rare-earth (RE) materials comprising three chemical factors, RE−O bond length, the effective charge of the coordinated atom, and structural dimensionality. Using Gd 3+ cation with 4f 7 configuration as a fluorescence probe, we found that the "free-O"-centered polyhedron is the most crucial motif in strengthening the phonon-assisted energy transfer and photon emission. The temperature-dependent Huang−Rhys S factors were calculated to identify the electron−phonon coupling intensity based on the fluorescence spectrum quantitatively. Finally, beyond conventional wisdom, a series of structural criteria were presented, serving as useful guidelines for discovering strongly coupled rare-earth optical materials. Our study breaks the long-time "blind"-searching diagram and provides reliable principles for many functional materials associated with electron−phonon coupling, such as superconductors, multiferroics, and phosphors.

Phase-matchable (PM) capacity is an indispensable factor in evaluating the applicable prospect of a new nonlinear optical crystal. Obtaining an accurate and credible PM curve by the Kurtz−Perry method using a small single crystal is indeed crucial. Here, we reported a new borate NLO crystal Rb 7 SrY 2 (B 5 O 10 ) 3 (I) and studied the influence of the arrangement of micro-[B 5 O 10 ] groups on its birefringence. Interestingly, the synthesized polycrystalline powder and grinded single crystal display divergent PM and non-PM behavior, respectively. Combining theoretical calculations and analysis, I is demonstrated to be non-phase matchable for the secondharmonic generation (SHG) process of a 1064 nm laser.