Introducing a New d⁰ Sc³⁺ Asymmetric Ion for Functional Materials: Large Birefringence Enhancement by ScO₆ in Ba₃ Sc₂...

Transition metal ions with d⁰ electronic states (Ti⁴⁺ , Zr⁴⁺ , Nb⁵⁺ and Ta⁵⁺ ) are widely investigated as functional materials. This work first illustrates that Sc³⁺ ion, long-time ignored, displays a second-order Jahn-Teller (SOJT) effect similar to asymmetric oxide-coordinated transition metal ions, thus providing a new ground to seek for asymmetric functional materials with enhanced performances. In Ba₃ Sc₂ (BO₃ )₄ , BO₃ groups are parallelly arranged, satisfying the ideal arrangement to produce large birefringence. Importantly, distorted octahedral ScO₆ with Sc³⁺ ion in its d⁰ electronic state enlarges birefringence unexpectedly up to 0.149 @ 550 nm, which is larger than previously reported borates containing solely BO₃ , even to B₃ O₆ units. Subsequently, the SOJT influence of distorted ScO₆ octahedra on birefringence is verified by a comparison between experimental data and theoretical calculations. In addition, Ba₃ Sc₂ (BO₃ )₄ also displays a high transmittance in the range of 230 nm-3.5 μm with a UV cut-off wavelength at 198 nm and a large laser induced damage threshold (2.7 GW/cm² ), comparable to α-BaB₂ O₄ . Above characteristics imply that the title compound may be a promising birefringent material.