A Sharp Improvement of Sulfate's Birefringence Induced by the Synergistic Effect of Heteroleptic and Dimeric Strategies

As for tetrahedron‐based ultraviolet nonlinear optical crystals, it is so difficult to achieve a sufficient birefringence to satisfy the phase‐matching condition. Thereover, it is necessary to greatly increase the polarizability anisotropy of tetrahedra. Meanwhile, the tetrahedra should be arranged as uniformly as possible. Based on these ideas, a new strategy that dimerizes the heteroleptic tetrahedra with heteroleptic atoms as bridges is proposed and implemented, resulting in a new compound, Na2S3O6, which crystallizes in a centrosymmetric monoclinic phase (phase‐I) and a non‐centrosymmetric orthorhombic one (phase‐II). In the phase‐I, a large birefringence of 0.103 @ 546 nm comparable to those of many triangle‐based crystals is confirmed experimentally, indicating the ability of [S3O6] group in improving the birefringence. Besides, the phase‐II exhibits an improved birefringence of 0.056 @ 546 nm and a moderate second harmonic generation response of 1.4 × KH2PO4. In particular, the phase‐II exhibits a typical phase‐matching behavior under the irradiation of 532 nm laser. Moreover, the excellent optical properties of [S3O6] group are further verified by theoretical calculations. These results completely illustrate the validity of the design strategy. Therefore, this work lights a new route for exploring novel ultraviolet nonlinear optical crystals. A novel strategy, the synergetic dimerization and heterozygosity of tetrahedra, is proposed and verified for enhancing the birefringence. Accordingly, polymorphic Na2S3O6 crystals with [S3O6] groups are obtained with a large birefringence in the monoclinic phase and a moderate second harmonic generation response in the orthorhombic one.