Two α-SiO₂-Related Deep-Ultraviolet Phase-Matchable Optical Nonlinear Beryllium Silicate Crystals Na₂BeSiO₄ and...

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-SiO₂ 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-SiO₂ as a structural template, Na₂BeSiO₄ and Li₂BeSiO₄, are synthesized successively. By providing flexible coordination BeO₄ tetrahedra for rigid tetrahedral SiO₄ that induces distortion of SiO₄, Na₂BeSiO₄ achieves significant enhancement of phase-matchable SHG effect of 15 × α-SiO₂. Further, by introducing a low coordination number of Li⁺ cations that stimulates uniform orientation of SiO₄, Li₂BeSiO₄ exhibits the strongest phase-matchable SHG effect as large as 40 × α-SiO₂ among the universal SiO₄-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.