Unlocking Hidden Potential via Reprogrammed Mineral Structures: How Anionic Transformation Achieves Ultraviolet...

Mineral crystals, with their diverse structural motifs and highly flexible coordination configurations, afford a virtually limitless structural library for developing nonlinear optical materials. However, intrinsic structural limitations frequently result in inadequate optical performance, most notably insufficient birefringence that hinders phase-matching at shorter wavelengths. Isomorphic substitution is a straightforward optimization method, but it typically yields only limited improvements. To overcome this, we developed a strategy involving the monovalent anionic substitution (F-, Cl-, Br-, I-, [BO2]-, [NO3]-, [BF4]-, [B(OH)4]-) within apatite and zeolite-like borate frameworks. By leveraging the correlation between channel dimensions and anionic van der Waals volume, together with theoretical modeling, this approach enabled the synthesis of new compounds and the prediction of additional isostructural crystals. Notably, the introduction of [NO3]- groups in both families drives a dramatic 7.5- and 21.5-fold enhancement in birefringence while maintaining strong second-harmonic generation responses of 1.2-4.6 × KDP. This synergistic improvement shifts the shortest phase-matching wavelength toward the ultraviolet from the near-infrared in the zeolite-like crystals. Consequently, this work provides an effective strategy to achieve significant performance enhancement for mineral-type nonlinear optical crystals and expands the functional wavelength range of mineral crystals.