Breaking Symmetry Cancellation: Heterovalent Substitution Unlocks Ultrastrong, Phase-Matchable Nonlinearity in Deep-Ultraviolet Tetrahedral Crystals

Abstract

Deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals demand a challenging integration of strong second-harmonic generation (SHG), sufficient birefringence for phase matching (PM), and a wide bandgap (λ < 200 nm). In tetrahedron-based materials, the hyperpolarizabilities of constituent tetrahedra usually nearly cancel due to structural symmetry, resulting in weak SHG and small birefringence─a classic example being α-SiO2. Breaking this symmetry-driven cancellation while retaining deep-UV transparency thus represents a key chemical design challenge. Here, we report a deep-UV transparent NLO crystal BeSO4 derived from α-SiO2 via stepwise heterovalent tetrahedral substitution. This crystal crystallizes in the noncentrosymmetric tetragonal space group, with alternating BeO4 and SO4 tetrahedra forming [Be3S3O18] six-membered rings. By effectively breaking the symmetry cancellation via alternating low-valence (Be2+) and high-valence (S6+) tetrahedra, BeSO4 achieves a deep-UV cutoff at 176 nm, an exceptional SHG response of 5.1 × KDP at 1064 nm─the strongest among all deep-UV sulfates and Be-containing deep-UV crystals, and a birefringence of 0.038 at 590 nm, almost seven times that of the isostructural BPO4. Unlike BPO4 and α-SiO2, BeSO4 exhibits SHG behavior consistent with phase-matchability, and under 532 nm excitation, it produces a detectable second-harmonic signal at 266 nm. First-principles calculations reveal that the enhanced performance originates from shorter S–O bonds, stronger orbital hybridization, a narrower bandgap, and highly anisotropic electron density, all stemming from the successful cancellation-breaking strategy. It should be noted that the main novelty of this work lies in single-crystal growth, linear/nonlinear optical properties, and mechanistic analysis, since the crystal structure of BeSO4 has been previously reported.