Designing a Heteroleptic Tetrahedral Group with an Ultrahigh Polarizability Anisotropy by Optimizing the Bonding Electrons Activity and Their Distribution

In contrast to regular tetrahedral groups, which struggle to induce a sufficient birefringence to satisfy the phase-matching condition, heteroleptic tetrahedral ones with higher polarizability anisotropy are perfect functional building units for short-wave UV nonlinear optical crystals. However, the intrinsic origin of their properties remains unclear. Here, the high polarizability anisotropy in groups is deconstructed to the large anisotropy of numerical density and confined force of the electron, which corresponds to the optimization of bonding electrons activity and their distribution in the tetrahedral group. It can be perfectly achieved in heteroleptic tetrahedral groups. Then, an elegant hetero-two-site heteroleptic tetrahedral group, [SO₂NH₂Cl], with a high polarizability anisotropy is found, which is inducive to generate a large optical anisotropy as confirmed by the experimental birefringence of 0.085 @ 546 nm and the excellent phase-matching ability in SO₂NH₂Cl crystals. Therefore, this work illuminates a new horizon for the design of heteroleptic tetrahedron-based short-wave UV nonlinear optical materials.