KRb 2 (NO 3 ) 2 Cl: a new birefringent crystal exhibiting a perovskite-related framework and a short cutoff edge.

The combination of π-conjugated groups [NO 3 ] and Cl-centered polyhedra generates a new birefringent crystal with a perovskite-related framework, KRb 2 (NO 3 ) 2 Cl, which is the first alkali metal nitrate chloride synthesized by a mild hydrothermal method. It crystallizes in the orthorhombic space group pbam (no. 55). In addition, KRb 2 (NO 3 ) 2 Cl crystals with dimensions up to 7 × 1.5 × 1 mm 3 were grown. Notably, KRb 2 (NO 3 ) 2 Cl has a short UV cut-off edge (below 228 nm) and a significantly enhanced birefringence (Δ n = 0.084 at 1064 nm). Theoretical calculations indicate that the birefringence enhancement mainly derives from π-conjugated [NO 3 ] plane triangles.