A Giant Optically Anisotropic Phosphate Driven by Mixed Valence Mercury Units

Abstract

Phosphate crystals are known for their low birefringence, a result of the symmetrical tetrahedral structure of PO₄ groups. Attempts to increase their birefringence have traditionally involved the incorporation of highly anisotropic ions and groups. Yet, the majority of the modified phospates still exhibit a birefringence value below 0.1, due to the counteracting effects of anisotropic elements within the tetrahedral coordination. The present study has for the first time discovered a novel birefringence-active Hg^I₂Hg^IIO₉ unit and successfully constructed a highly anisotropic layered structure, which ingeniously prevents the mutual cancellation of anisotropic effects. This synthesis of Hg^I₂Hg^II(Te₂O₄)₂(HPO₄)₂ confirms the effectiveness of this strategy, yielding a birefringence of 0.444 at 546 nm, which outperforms 99% of phosphate crystals and surpasses that of commercial counterparts like YVO₄ (0.209@1064 nm) and CaCO₃ (0.175@533 nm). Structural analysis and PAWED calculations indicate that the significant birefringence is attributed to the synergistic interaction between the Hg^I₂Hg^IIO₉ unit (23.76%), HPO₄ tetrahaedron (24.24%) and TeO₄ group (51.99%). This breakthrough paves the way for phosphates to meet the demands of modern technological advancements, expanding their potential applications in the field of anisotropic optics.