Design Principles and Identification of Birefringent Materials

Birefringence ($\Delta n$) is the dependence of the refractive index of a material on the polarization of light travelling through it. Birefringent materials are used as polarizers, waveplates, and for novel light-matter coupling. While several birefringent materials exist, only a handful of them show large $\Delta n$ > 0.3, and are primarily limited to the infrared region. The variation of $\Delta n$ across diverse materials classes and strategies to achieve highly birefringent materials with transparency covering different regions of the electromagnetic spectrum are missing. We have calculated the $\Delta n$ of 967 non-cubic, formable crystals having vastly different structures, polyhedral connectivity and chemical compositions. From this set of compounds, we have screened highly birefringent crystals ($\Delta n$ greater than 0.3) having transparency in different regions of the electromagnetic spectrum. The screened compounds belong to several families such as A3'MN3, AMO2, AN3, and A'N6 (A = Li, Na, K; A'= Ca, Sr, Ba; M = V, Nb, Ta). By analyzing the electronic structures of these compounds, we have distilled rules to enable the design of crystals with large $\Delta n$.