High‐Throughput Discovery of Large Birefringence Materials in Mixed‐N Hf‐O Systems

Birefringence is a key property of optical functional crystals, underpinning their vital applications in angle phase‐matching, polarization control, and various advanced photonic technologies. Currently, TiO 2 exhibits a large birefringence in the visible region, serving as a benchmark material. With the advancement of science and industry, the exploration of new birefringent materials in this wavelength band has become increasingly important. In this work, we proposed a mixed anion strategy to enhance the optical anisotropy, using the Hf‐O‐N system as a case study. Structures with Hf‐O‐N were screened from the NOEMD database for further first‐principles calculations. Six structures with ( E hull ≤ 0.05 eV/atom) possess large bandgaps (1.67–5.02 eV) and significant birefringence (0.17–0.403 @1064 nm). Four structures exhibit potential birefringence in the visible region, I 4/ mmm ‐Hf 2 N 2 O (0.37 @1064 nm), Cm ‐Hf 5 N 6 O (0.219 @1064 nm), C 2/ m ‐Hf 7 N 8 O 2 (0.333 @1064 nm), and Cm ‐Hf 7 N 8 O 2 (0.403 @1064 nm). The birefringence of three of these structures exceeds that of TiO 2 (0.256 @1064 nm). Through structural analysis, we identified that the [Hf 4 NO 2 ], [Hf 4 NO], and [Hf 2 NO 4 ] polyhedra are outstanding optical functional units, which tend to offer significant optical anisotropy. This result offers novel potentialities for the application of birefringent materials and guideline for novel uses of Hafnium oxynitride materials.