KRb2(NO3)2Cl: a new birefringent crystal exhibiting a perovskite-related framework and a short cutoff edge

Abstract

The combination of π-conjugated groups [NO₃] and Cl-centered polyhedra generates a new birefringent crystal with a perovskite-related framework, KRb₂(NO₃)₂Cl, 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, KRb₂(NO₃)₂Cl crystals with dimensions up to 7 × 1.5 × 1 mm³ were grown. Notably, KRb₂(NO₃)₂Cl has a short UV cut-off edge (below 228 nm) and a significantly enhanced birefringence (Δn = 0.084 at 1064 nm). Theoretical calculations indicate that the birefringence enhancement mainly derives from π-conjugated [NO₃] plane triangles.