A Rare-Earth Borophosphate Ultraviolet Nonlinear Optical Crystal Featuring P-O-P Linkages.

Noncentrosymmetric compounds have long attracted considerable attention in functional solid-state chemistry due to their intrinsic symmetry-breaking characteristics and the resulting nonlinear optical, piezoelectric, and ferroelectric properties. In this work, two novel anhydrous rare-earth borophosphate crystals, centrosymmetric CsScBP3O11 and noncentrosymmetric K1.41Rb0.59Sc2B2P6O22, featuring rare P-O-P linkages, were synthesized via spontaneous crystallization, representing the first observation of such structural motifs in rare-earth borophosphate systems. Notably, K1.41Rb0.59Sc2B2P6O22 exhibits a short UV cutoff edge at 218 nm, a wide band gap of 4.26 eV, robust thermal stability, and moderate SHG response, highlighting its promise as a UV NLO optical material and overturning the conventional centrosymmetric nature of P-O-P-containing borophosphates. Structural analysis combined with first-principles calculations reveals that variation of A-site cations induces a flexible contraction of the Cairo pentagonal anionic network, which effectively breaks inversion symmetry and drives the centrosymmetric-to-noncentrosymmetric transformation, while also elucidating the origin of the observed optical properties. Overall, these findings expand the structural landscape of rare-earth borophosphates and provide an effective strategy for the rational design of new UV NLO materials through unconventional framework engineering.