Te3O3(PO4)2: a phosphate crystal with large birefringence activated by the highly distorted [TeO5] group and antiparallel [PO4] pseudo-layer

Abstract

Phosphate crystals typically display small birefringence, due to the very small polarizability anisotropy of the highly symmetric [PO₄] tetrahedral unit. Here, we discovered a novel phosphate crystal, Te₃O₃(PO₄)₂, which exhibits an unexpectedly enhanced birefringence of 0.148 at 1064 nm, which hits a new high in inorganic phosphate materials. Theoretical calculations and structural analysis reveal that the large birefringence can be attributed to the combined effects of the highly distorted [TeO₅] group and antiparallel [PO₄] pseudo-layer. Notably, bulk Te₃O₃(PO₄)₂ single crystals can be grown by the Czochralski method, and a centimeter-size crystal has been successfully obtained. Moreover, the Te₃O₃(PO₄)₂ crystal exhibits a wide transmission range (0.29–4.76 μm) and a high laser damage threshold (544 MW cm⁻²). These excellent properties suggest that Te₃O₃(PO₄)₂ is a potential phosphate birefringent material. More importantly, this work confirmed that the distorted [TeO₅] group and antiparallel [PO₄] pseudo-layer are two excellent functional units for increasing birefringence, which provides a novel strategy for the further design of new phosphate compounds with large birefringence.