Tetrahedra Modification of Phosphates for Optical Anisotropy Enhancement

Abstract As an exceptional optical gene, the [PO₄] tetrahedron is indispensable in the field of optical crystals due to its gain in the band gap of solid materials. However, the high symmetry of [PO₄] tetrahedron hinders the achievement of large optical anisotropy in the lattice for phosphate crystal forms. In this work, the heteroleptic tetrahedra strategy, which involves replacing some of the oxygen atoms on regular oxy‐tetrahedra, is proven to be a feasible approach for preserving the wide transmission of the original [PO₄] tetrahedra and improving the polarizability anisotropy. Based on this, eight methylphosphates are designed and synthesized for the phosphate system by substituting a [CH₃] group for the O atom on the [PO₄] tetrahedra. Theoretically, as compared to [PO₄] units, [CH₃PO₃] and [CH₃PO₃H] units can improve the polarizability anisotropy, particularly [CH₃PO₃H] units, which have the potential to be birefringence‐active. The strong birefringence (exp. 0.108@546.1 nm) and short deep‐UV cutoff edge (195 nm) of nonmetallic methylphosphates [C(NH₂)₃][CH₃PO₃H] in the series suggest that it may be a possible short‐wave UV birefringent crystal. In addition to providing new birefringence‐active units for optical material design, this study validates the viability of modifying tetrahedra to improve inherent short board of nearly rigid [PO₄] tetrahedra in phosphate family.