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

•The properties of a Pb(II)-based oxyhalide, Pb(OH)I, have been studied by experimental and theoretical methods.•Pb(OH)I has a large birefringence (0.103@1064 nm), which exceeds those of many Pb(II)-based birefringent materials.•Structural and theoretical studies reveal that the distorted [Pb(OH)3I5] polyhedra make the dominant contribution to the birefringence.