Pb(OH)I: A lead(II)-based oxyhalide with a large birefringence

Birefringent crystals are very important optical functional materials in optical communication and the laser industry. By introducing Pb(II) into oxyiodide, a Pb–oxyhalide birefringent crystal of Pb(OH)I was solvothermally synthesized. It crystallizes in Pnma space group. Pb2+ cations are coordinated by OH– and I- to form distorted Pb–oxyhalide polyhedra, [Pb(OH)3I5], which are arranged in a 3D dense structure. Importantly, according to the first-principles calculations, Pb(OH)I has a large birefringence of 0.103 at 1064 nm. Structural and theoretical studies reveal that the distorted [Pb(OH)3I5] polyhedra with large polarizability anisotropy make the dominant contribution to the birefringence. The work indicates that the metal–oxyhalide polyhedra with mixed anions have advantages in producing large optical anisotropies, and are preferable structural units for designing birefringent materials.