Two α‐SiO ₂ ‐Related Deep‐Ultraviolet Phase‐Matchable Optical Nonlinear Beryllium Silicate Crystals Na ₂ BeSiO ₄ and Li ₂ BeSiO ₄ with Enhanced SHG Effect

Abstract Deep‐UV, (wavelength λ < 200 nm) nonlinear‐optical (NLO) silicate crystals have long been troubled by weak second‐harmonic‐generation (SHG) effect and common non‐phase‐matchability, leading to the stagnation of silicate research, despite abundant reserves of silicates on the earth. Representative and simple α‐SiO ₂ is an ideal structural prototype toward improving silicate NLO performance by exploring efficient strategies. Herein, through two‐step structural modulations based on fundamental “distortion and superposition” principles of NLO response, two deep‐UV NLO crystals taking α‐SiO ₂ as a structural template, Na ₂ BeSiO ₄ and Li ₂ BeSiO ₄ , are synthesized successively. By providing flexible coordination BeO ₄ tetrahedra for rigid tetrahedral SiO ₄ that induces distortion of SiO ₄ , Na ₂ BeSiO ₄ achieves significant enhancement of phase‐matchable SHG effect of 15 × α‐SiO ₂ . Further, by introducing a low coordination number of Li ⁺ cations that stimulates uniform orientation of SiO ₄ , Li ₂ BeSiO ₄ exhibits the strongest phase‐matchable SHG effect as large as 40 × α‐SiO ₂ among the universal SiO ₄ ‐governed NLO silicates. Also, computational investigations support the rationality of the structural modulations well. Such efficient structural modulations can greatly promote the exploration and rational design of NLO silicates.