Record‐Short Phase Matchability of Silicate: Charge‐Driven Lattice Distortion Enables Deep‐UV Phase Matching and Strong Optical Nonlinearity

ABSTRACT The development of deep‐ultraviolet (deep‐UV, λ < 200 nm) nonlinear‐optical (NLO) crystals based on tetrahedral groups faces a fundamental limitation: insufficient birefringence to achieve phase matching. Herein, we address this critical challenge through strategic charge modulation of tetrahedral groups ([PO 4 ] → [SiO 4 ]) to control Y(III) cation polyhedral stacking configurations, enabling the successful synthesis of two deep‐UV transparent NLO crystals YBeP 5 O 15 and Y 2 Be 2 SiO 7 . Structural analysis uncovers a progressive evolution from isolated YO 6 to 2D [Y 2 O 7 ] ∞ layers composed of highly distorted YO 8 polyhedra, directly driving a great birefringence enhancement to 0.083 in Y 2 Be 2 SiO 7 . Remarkably, Y 2 Be 2 SiO 7 manifests the shortest phase‐matching second‐harmonic‐generation wavelength in all reported silicates, marking the first entry of this material class into the deep‐UV spectral region. More importantly, it also retains large second‐order optical nonlinearity (1.7 × KDP) and short deep‐UV cut‐off edge (178 nm), suggesting its great potential as UV and deep‐UV NLO crystals. First‐principles calculations confirm superior optical properties dominantly from the YO 8 groups. This work highlights the transformative potential of engineering cationic lattice sites, particularly through rare‐earth polyhedral distortion, to break long‐standing trade‐off between deep‐UV transparency and strong optical nonlinearity in tetrahedron‐based systems. This paradigm opens new avenues for developing high‐performance deep‐UV NLO crystals beyond conventional materials.