Anion-Ordered [B3O5(OH)2] Induced by Crystal Water with Deep Ultraviolet Transparency and Large Birefringence in Boron-Rich Borophosphates

The utilization of deep ultraviolet (deep-UV) birefringent materials is crucial in the modulation of polarized light and poses various challenges. In this work, the synthesis and verification of three borophosphate compounds with the functional building block (FBB) [B6PO12(OH)4], namely, NaB6PO10(OH)4, RbB6PO10(OH)4, and RbB6PO10(OH)4·H2O, were accomplished via structural, spectral, and thermal analysis. All three compounds have wide bandgaps (HSE06 result, 7.32–7.65 eV) and large birefringence (0.070–0.094 at 532 nm). The birefringence discrepancy is analyzed quantitatively based on the arrangement of anions, and the theoretical calculations shed light on the origin of optical properties. The flexible intramolecular hydrogen bonds of crystal water play a pivotal role in facilitating the uniform arrangement of structures. This work not only enriches the chemical structure of borophosphate but also provides a horizon for the exploration of deep-UV birefringent materials.