Hierarchical van der Waals Assemblies to Boost Deep-Ultraviolet Nonlinear-Optical Capabilities in Boron Phosphate

Cristobalite-type boron phosphate BPO4, an important nonlinear-optical (NLO) crystal, exhibits nearly the largest bandgap (E-g approximate to 9.3 eV) and a significant second-harmonic-generation (SHG) effect (d(36) approximate to 0.7 pm V-1) in the deep-ultraviolet (DUV, < 200 nm) spectral region. However, its conventional tetrahedral framework structure tends to have low structural anisotropy, resulting in a severe deficiency of birefringence (Delta n < 0.01), which fails to achieve DUV birefringent phase matching. In this work, the local anisotropy of tetrahedral structures are proposed to enlarge by constructing van der Waals (vdW) anisotropic motifs under synthetic chemical conditions, which can effectively convert unattainable extreme external forces into internal stresses in chemical bond structures. Depending on unique hierarchical vdW assemblies, enormous tetrahedral distortions can be achieved by stable in-plane or in-line bond matching between rigid tetrahedral primitives. First-principles calculations demonstrate that the birefringence of 2D layered and 1D chained BPO4 achieved by vdW engineering are scaled from Delta n < 0.01 to 0.07-0.13, with the shortest birefringent phase-matched wavelengths down to DUV 189-154 nm. The finding is of seminal importance for the design of vdW NLO crystals in structural chemistry and would provide opportunities to explore unique vdW DUV NLO materials under extreme synthetic conditions.