Crystal structures and optical properties of two layered lead halide phosphites Pb3(HPO3)Cl4·H2O and Pb3(HPO3)2Cl2·H2O.

Two new all-inorganic lead halide phosphites, Pb3(HPO3)Cl4·H2O and Pb3(HPO3)2Cl2·H2O, have been synthesized and structurally characterized as layered frameworks constructed from distorted Pb-O(H2O) polyhedra linked by pseudo-tetrahedral HPO3 groups, with interlayer Cl- ions providing charge balance and imparting intrinsic structural anisotropy. UV-vis diffuse reflectance spectra analyzed using Tauc's fitting show that both compounds are direct wide-band-gap materials, with optical band gaps of 4.20 eV for Pb3(HPO3)Cl4·H2O and 4.14 eV for Pb3(HPO3)2Cl2·H2O, indicating good transparency in the UV region. Birefringence measurements at 546 nm using a Berek compensator yield Δn ≈ 0.033 and 0.057 for Pb3(HPO3)Cl4·H2O and Pb3(HPO3)2Cl2·H2O, respectively, demonstrating useful optical anisotropy for polarization-dependent applications. Density functional theory calculations reproduce the wide-gap insulating character of both phases and predict birefringence values in good agreement with experiment, supporting their identification as phosphite-based candidates for UV/visible birefringent optical crystals.