CsHgNO3Cl2: A New Nitrate UV Birefringent Material Exhibiting an Optimized Layered Structure

Developing effective strategies to design novel, excellent birefringent materials has become an emerging challenge. Herein, we report a new UV nitrate birefringent material, CsHgNO3Cl2, which is an analogue of the known UV carbonate nonlinear optical crystal KCaCO3F. In CsHgNO3Cl2, a perfect layered structure was produced owing to the alliance of a planar pi-conjugated anion NO3- and highly electropolarized Hg2+ cation. The optimized layered structure that is beneficial in enlarging the optical anisotropy induced a large birefringence (Delta n = 0.145 @ 546 nm) for CsHgNO3Cl2, even superior to that of the well-known commercial birefringent material a-BBO in the UV region. The strategy that the optimization of planar functional group arrangement is able to boost birefringence is expected to guide the development of high-performance birefringent materials in the future, especially in the field of UV.