Introducing a New d 0 Sc 3+ Asymmetric Ion for Functional Materials: Large Birefringence Enhancement by ScO 6 in Ba 3 Sc 2 (BO 3 ) 4 .

Transition metal ions with d 0 electronic states (Ti 4+ , Zr 4+ , Nb 5+ and Ta 5+ ) are widely investigated as functional materials. This work first illustrates that Sc 3+ 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 Ba 3 Sc 2 (BO 3 ) 4 , BO 3 groups are parallelly arranged, satisfying the ideal arrangement to produce large birefringence. Importantly, distorted octahedral ScO 6 with Sc 3+ ion in its d 0 electronic state enlarges birefringence unexpectedly up to 0.149 @ 550 nm, which is larger than previously reported borates containing solely BO 3 , even to B 3 O 6 units. Subsequently, the SOJT influence of distorted ScO 6 octahedra on birefringence is verified by a comparison between experimental data and theoretical calculations. In addition, Ba 3 Sc 2 (BO 3 ) 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/cm 2 ), comparable to α-BaB 2 O 4 . Above characteristics imply that the title compound may be a promising birefringent material.