Strategically designed anisotropic organic-inorganic metal halides (OIMHs) with superior balance between bandgap and birefringence

This study reports two novel organic-inorganic metal halides (OIMHs) with outstanding ultraviolet birefringences, namely Cd(C6H4NO2)I·H2O (1) and Cd(C6H5NO2)2I2 (2). Both of them feature 1D neutral chain, in which the 1D [Cd(C6H4NO2)I·H2O]∞ is built from Cd2I2N2O6 dimers and (C6H4NO2)- anions via the connection of Cd-N bonds, whereas each pair of antiparallel (C6H5NO2) groups hang at 1D [CdI2]∞ forming 1D [Cd(C6H5NO2)2I2]∞ chain. Importantly, birefringence measurements exhibit that both 1 and 2 features exceptional birefringent properties with values of 0.243 @ 550 nm and 0.388 @ 550 nm, respectively, which are not only greater than commercial materials (α-BaB2O4, 0.122 @ 546 nm), but also 8.1 and 12.8 times that of inorganic CdI2. Additionally, the UV-vis spectra show that both 1 and 2 have short adsorption edges of 280 nm and 284 nm, relating to wide bandgaps of 4.23 eV and 3.94 eV. Therefore, they, especially 2, are strong candidates for potential ultraviolet birefringent materials. This study innovatively proposes a design strategy for the NA-Cd-I system, which means construct organic-inorganic metal halide systems (OIMHs), providing new insights into the exploration of new high-performance birefringent crystals.