Pb₂Cl₂(HPO₃)(H₂O) and Pb₃Br₂(HPO₃)₂: Two Phosphite Halides with 3D Structural Networks and Enlarged Birefringence

Compounds constructed by mixed anions are widely favorable for their diverse crystal structures and physical performances. Here, two lead phosphite halides, Pb₂Cl₂(HPO₃)(H₂O) (1) and Pb₃Br₂(HPO₃)₂ (2), were obtained by facile hydrothermal reactions. Both compounds crystallize in the space group Pnma. 1 exhibits a 3D structure composed of HPO₃ units, PbCl₄O₃ pentagonal bipyramids, and PbO₃ pyramids. 2 also shows a 3D framework built by HPO₃ units, PbO₂Br₄ octahedron, PbO₅ and PbO₃Br₂ square pyramids. By introducing halogen anions to the lead phosphite system, 1 and 2 show enhanced birefringences of 0.083 and 0.072 at 1064 nm, respectively, compared with 0.023@1064 nm for Pb₂(HPO₃)₂. In this work, syntheses, crystal structures, optical properties, and theoretical calculation results have been studied in detail.