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

For the composition-structure-property relationship, material propertiescan often be changed or even adjusted through some specific structural evolutions. Here,we report four antimony phosphates, K3SbP2O9,K2NaSbP2O8,alpha-NaSb3P2O10, and beta-NaSb3P2O10, obtained by using the high-temperature solution method. The fourcompounds were in diverse space groups with isolated PO4groups. The presence ofSb3+cations with lone pairs not only induces the crystal structure symmetry transformationbut also significantly enhances the birefringence.alpha-NaSb3P2O10exhibits a significantlyincreased birefringence which is about 60 times that of K3SbP2O9due to the introductionof lone pairs in metal cations. The useful strategies will have significant meaning for thediscovery of new materials with large birefringence in the future.