Ternary Anion‐Engineered Rare Earth Crystals for Broadband <scp>UV</scp> Transparency and Enhanced Second‐Harmonic Generation

Comprehensive Summary Rare earth (RE) crystals have garnered considerable attention as second‐order nonlinear optical (NLO) materials, owing to highly electropositive nature and large ionic radii of RE cations, which facilitate strong ionic bonding, structural versatility, and enhanced NLO susceptibilities. Despite these intrinsic advantages, achieving simultaneously strong NLO activity and deep‐ultraviolet (deep‐UV) transparency in RE crystals remains a formidable challenge. Herein, we report the rational design and synthesis of a new family of RE crystals enabled by a ternary anion‐engineering strategy, in which π‐conjugated carbonate, non‐π‐conjugated sulfate, and lone‐pair‐electron‐containing hydroxyl group are synergistically integrated into a single crystal lattice. The cooperative interplay among these chemically and electronically distinct anions induces a new noncentrosymmetric structural motif, giving rise to exceptional deep‐UV transparency, with an absorption edge below 190 nm, surpassing that of many reported RE NLO crystals constructed via complementary anion engineering. Furthermore, such crystals exhibit a strong second‐harmonic generation response, with an intensity 2.5 times that of Y‐cut quartz at 860 nm, along with a suitable birefringence of 0.023@1064 nm. This work provides a versatile design paradigm for the development of high‐performance RE‐based deep‐UV NLO materials, and underscores the critical role of multi‐anions in tuning structural distortion and optical functionality.