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

Abstract

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]_∞ 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 ScO_nF₂ groups when analysed for their polarizability anisotropy-weighted electron density (PAWED).