Sc2F2(B2O5): a deep ultraviolet scandium borate fluoride exhibiting large birefringence induced by the synergistic effect of B2O5 and ScOnF2 groups

Crystalline borates are a rich source of diverse optical materials. Here, a rare earth borate fluoride, Sc2F2(B2O5), has been obtained by a conventional hydrothermal reaction. It is unlike other rare earth borate fluorides synthesized under harsh conditions. In the layered structure of Sc2F2(B2O5), ScO4F2 octahedra and ScO5F2 pentagonal bipyramids connect to each other to form [Sc4O10F4](infinity) frameworks, which are further linked by B2O54- anion groups. Specifically, Sc2F2(B2O5) does not belong to any types of reported rare earth borate fluoride. More attractively, this compound displays a deep ultraviolet cutoff edge (<200 nm) and very large birefringence (0.19) at 1064 nm based on theoretical calculations. The outstanding birefringence can be seen to originate from the synergistic effect of B2O5 and ScOnF2 groups when analysed for their polarizability anisotropy-weighted electron density (PAWED).