CsSn₂(HPO₃)₂I: The first inorganic metal phosphite iodide as a promising birefringent material exhibits a large birefringence with ideally layered framework

The first inorganic metal phosphite iodide, CsSn2(HPO3)(2)I, was successfully synthesized. It represents the first phosphite tin halide and features a [Sn(HPO3)](infinity) layer framework structure built by the SnO3 pyramid and HPO32- groups. Attractively, it shows the most giant experimental birefringence (Delta n = 0.200@0.546 mu m) among reported inorganic metal phosphite birefringent crystals. This work proves that the reasonable utilization of metal cations with lone pair electrons is an effective strategy for developing excellent birefringent materials.