Atomic Substitution to Tune ScO 6 Distortion in Ba 2 MSc 2 (BO 3 ) 4 (M = Na, K, Ba) to Acquire a Large Birefringence.

Replacing alkali metals (K, Na atoms) by an alkaline-earth metal (Ba atom), α-Ba 3 Sc 2 (BO 3 ) 4 (high-temperature phase) is successfully obtained by a molten salt method, taking Ba 2 K 1.6 Na 0.4 Sc 2 (BO 3 ) 4 as the parent template. Although both of them exhibit similar layered structures composed of ScO 6 and BO 3 units, α-Ba 3 Sc 2 (BO 3 ) 4 shows largely distorted ScO 6 octahedra (Δ d = 0.56) forced by the uniform tension of a larger space effect from BaO 12 polyhedrons, rather than regular ScO 6 octahedra like in Ba 2 K 1.6 Na 0.4 Sc 2 (BO 3 ) 4 . Experimental measurements and calculated analyses elucidate that distorted ScO 6 octahedra in α-Ba 3 Sc 2 (BO 3 ) 4 , displaying a second-order Jahn-Teller (SOJT) effect, enlarge the experimental birefringence up to 0.14@550 nm, while Ba 2 K 1.6 Na 0.4 Sc 2 (BO 3 ) 4 with regular ScO 6 octahedra only shows Δ n = 0.11 under the same condition. In addition, other optical and thermal properties of the two title compounds were characterized. The experimental results indicate that Ba 2 K 1.6 Na 0.4 Sc 2 (BO 3 ) 4 and α-Ba 3 Sc 2 (BO 3 ) 4 are promising birefringent materials.