Atomic Substitution to Tune ScO₆ Distortion in Ba₂MSc₂(BO₃)₄ (M = Na, K, Ba) to Acquire a Large Birefringence

Replacing alkali metals (K, Na atoms) by an alkaline-earth metal (Ba atom), α-Ba₃Sc₂(BO₃)₄ (high-temperature phase) is successfully obtained by a molten salt method, taking Ba₂K_1.6Na_0.4Sc₂(BO₃)₄ as the parent template. Although both of them exhibit similar layered structures composed of ScO₆ and BO₃ units, α-Ba₃Sc₂(BO₃)₄ shows largely distorted ScO₆ octahedra (Δd = 0.56) forced by the uniform tension of a larger space effect from BaO₁₂ polyhedrons, rather than regular ScO₆ octahedra like in Ba₂K_1.6Na_0.4Sc₂(BO₃)₄. Experimental measurements and calculated analyses elucidate that distorted ScO₆ octahedra in α-Ba₃Sc₂(BO₃)₄, displaying a second-order Jahn-Teller (SOJT) effect, enlarge the experimental birefringence up to 0.14@550 nm, while Ba₂K_1.6Na_0.4Sc₂(BO₃)₄ with regular ScO₆ octahedra only shows Δn = 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 Ba₂K_1.6Na_0.4Sc₂(BO₃)₄ and α-Ba₃Sc₂(BO₃)₄ are promising birefringent materials.