Hg2(HTe2O5)(PO4): A novel phosphate crystal with enhanced birefringence enabled by the synergistic modification of multiple functional groups

Birefringent crystals are crucial for the miniaturization of optical devices. Phosphate crystals, characterized by their highly symmetrical tetrahedral structures, exhibit excellent stability and wide optical bandgaps. However, their intrinsic symmetry typically results in low birefringence, with most phosphate compounds having birefringence values below 0.1. Efforts to enhance birefringence by introducing highly anisotropic ions and groups have been impeded by the tetrahedral coordination of phosphate, which often leads to the cancellation of anisotropic effects. To address this challenge, we propose an approach that leverages the synergistic modification of multiple functional groups to disrupt the anisotropic cancellation in phosphate crystals and significantly enhance their birefringence. Specifically, we incorporate Te(IV), which features stereo-chemically active lone pairs, and Hg(II), known for its high polarizability and deformability, into the phosphate system. We synthesized a novel phosphate compound, Hg2(HTe2O5)(PO4), which exhibits a calculated birefringence of 0.162 at 546 nm and a measured birefringence of 0.168 at 546 nm. This value is comparable to that of the commercial birefringent material CaCO3 (Δn=0.172@546 nm) and surpasses most previously reported phosphate materials. Additionally, Hg2(HTe2O5)(PO4) demonstrates a wide bandgap and excellent stability. Using the PAWED method, we determined that the significant birefringence of Hg2(HTe2O5)(PO4) is primarily due to the combined contributions of the HgO7 polyhedra (19.86%), PO4 tetrahedra (29.17%), and Te2O5 groups (47.40%). Our work demonstrates that the synergistic modification of multiple functional groups is an effective strategy for enhancing the birefringence of tetrahedral compounds, providing a new pathway for the development of high-performance birefringent materials.