Lone Pair-Driven Enhancement of Birefringence in Polar Alkali Metal Antimony Phosphates

For the composition–structure–property relationship, material properties can often be changed or even adjusted through some specific structural evolutions. Here, we report four antimony phosphates, K3SbP2O9, K2NaSbP2O8, α-NaSb3P2O10, and β-NaSb3P2O10, obtained by using the high-temperature solution method. The four compounds were in diverse space groups with isolated PO4 groups. The presence of Sb3+ cations with lone pairs not only induces the crystal structure symmetry transformation but also significantly enhances the birefringence. α-NaSb3P2O10 exhibits a significantly increased birefringence which is about 60 times that of K3SbP2O9 due to the introduction of lone pairs in metal cations. The useful strategies will have significant meaning for the discovery of new materials with large birefringence in the future.