AZnNOX (A = Rb, NH, X = Cl, I): Regulating Cations and Halides Yields Birefringent Crystals with Significantly Enhanced Optical Anisotropy.

Three new isostructural crystals, AZnNOX (A = Rb, NH; X = Cl, I) were synthesized via a mild aqueous solution method by combining π-conjugated [NO] groups with d transition-metal Zn ions, crystallizing in the orthorhombic space group (No. 62). All of the three compounds produce millimeter-sized crystals, with (NH)ZnNOCl yielding crystal as large as 17 × 14 × 4 mm. Notably, compared with RbZnNOI (Δ = 0.051@546 nm), (NH)ZnNOCl exhibits a short UV cutoff edge (below 243 nm) and significantly enhanced birefringence (Δ = 0.085@546 nm), while (NH)ZnNOI achieves the highest birefringence value (Δ = 0.091@546 nm) among the three crystals, demonstrating the potential as excellent UV birefringent crystals. Theoretical calculations reveal that the large birefringence of the three crystals primarily originates from the planar [NO] triangles. Detailed analysis further attributes the differences in birefringence to variations in the cation and halide sizes.