Short-UV Metal Halide Nitrates: Moderate SHG in K3Sb(NO3)3F3 and Enhanced Optical Anisotropy in K3Zn(NO3)X4 (X = Cl, Br)

The development of nonlinear optical (NLO) and birefringent crystals remains crucial for advancing laser science and optical technologies. The [NO3]- anionic group, characterized by a planar π-conjugated geometry, pronounced polarizability anisotropy, and a substantial hyperpolarizability tensor, is an attractive functional unit for designing NLO and birefringent materials. Guided by this structural motif, four ultraviolet (UV) metal halide nitrate crystals were synthesized: K3Sb(NO3)3F3, KSb(NO3)F3, K3Zn(NO3)Cl4, and K3Zn(NO3)Br4. K3Sb(NO3)3F3 shows a favorable second-harmonic generation (SHG) response (∼1.2 × KH2PO4, KDP) and an appropriate experimental birefringence (Δnexp = 0.086 at 546 nm) for phase-matching behavior. KSb(NO3)F3 and K3Zn(NO3)X4 (X = Cl, Br) display modest calculated birefringence (Δncal) values within the range of 0.052-0.079 at 546 nm, among which K3Zn(NO3)Br4 shows the largest Δnexp (0.096 at 546 nm). Notably, all four nitrate compounds exhibit short UV cutoff wavelengths (208-212 nm) and can be cultivated as centimeter-sized crystals, achieving dimensions of up to 1.0 × 0.7 × 0.4 cm3. Collectively, the combination of short UV cutoffs, moderate SHG responses, and large birefringence establishes nitrate-based compounds as promising candidates for UV optical applications.