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.