A₃ZnNO₃X₄ (A = Rb, NH₄, X = Cl, I): Regulating Cations and Halides Yields Birefringent Crystals with Significantly Enhanced Optical Anisotropy

Three new isostructural crystals, A₃ZnNO₃X₄ (A = Rb, NH₄; X = Cl, I) were synthesized via a mild aqueous solution method by combining π-conjugated [NO₃] groups with d¹⁰ transition-metal Zn²⁺ ions, crystallizing in the orthorhombic space group <i>Pnma</i> (No. 62). All of the three compounds produce millimeter-sized crystals, with (NH₄)₃ZnNO₃Cl₄ yielding crystal as large as 17 × 14 × 4 mm³. Notably, compared with Rb₃ZnNO₃I₄ (Δ<i>n</i> = 0.051@546 nm), (NH₄)₃ZnNO₃Cl₄ exhibits a short UV cutoff edge (below 243 nm) and significantly enhanced birefringence (Δ<i>n</i> = 0.085@546 nm), while (NH₄)₃ZnNO₃I₄ achieves the highest birefringence value (Δ<i>n</i> = 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 [NO₃] triangles. Detailed analysis further attributes the differences in birefringence to variations in the cation and halide sizes.