Ba ₃ (MoO ₂ F ₃ )PO ₄ F ₂ : Rational Design and Synthesis of an Alkaline-Earth Phosphate Fluoride as a Birefringent Crystal

Phosphate birefringent crystals and nonlinear optical (NLO) crystals are currently in imperative demand for ample applications. However, it is hugely challenging to explore novel phosphate crystals with splendid comprehensive optical and NLO performances due to the drawback of small birefringence. Herein, to address this issue, the strategies of introducing d0 transition metal cations with a second-order Jahn–Teller distortion effect into phosphate and fluorination substitution for polarization optimization and optical anisotropy are coherently adopted. As a result, an alkaline-earth phosphate fluoride, Ba3(MoO2F3)PO4F2, is designed and synthesized. Benefiting from the synergistic effect of tidily aligned [PO4]3– groups and the fluorinated Mo6+ groups of strongly distorted [MoO3F3] polyhedra, the compound exhibits a remarkable birefringence enhancement of 0.062 at 546.1 nm. Theoretical calculation results further indicate that Ba3(MoO2F3)PO4F2 is a promising candidate as a novel birefringent crystal. This work promotes the design and discovery of more phosphate birefringent crystals and NLO materials with large birefringence.