From borophosphate to fluoroborophosphate: a rational design of fluorine-induced birefringence enhancement

The introduction of oxofluoride anion into anionic group assists to tune optical properties owing to the change of coordination, electronegativity, and according anionic framework. Here, we proposed a rational design of new compounds by fluorine-driven structure and optical property evolution. A new borophosphate Ba2BP3O11 with the monoclinic space group P21/c has been synthesized in the sealed system. Ba2BP3O11 exhibits a rare P–O–P bridge formation, which is the first example in alkaline-earth metal borophosphates. By further substituting [BO4]3− with [BO3F]4−, the first alkaline-earth metal/lead fluoroborophosphates M2BP2O8F (M = Ba and Pb) with the same space group were designed. Since the scissors effect of fluorine, in M2BP2O8F (M = Ba and Pb), a BO3F tetrahedron corner-sharing with three PO4 tetrahedra forms 1D chains along the b-axial direction, which are filled by MOn (M = Ba/Pb, n = 5, 6, 8) distorted polyhedra. The first principles calculation shows that the borophosphate Ba2BP3O11 has a birefringence about 0.013 @1,064 nm, while the fluoroborophosphates M2BP2O8F (M = Ba and Pb) have the values of 0.035 and 0.043 @1,064 nm, respectively. Such an apparent enhancement in birefringence is derived from synergies of the oxyfluoride and cation. The introduction of fluorine-containing heteroanionic groups provides a feasible strategy to design novel promising optical materials.