Screening Large Birefringent Materials via Halogen Regulation in Ternary d⁰‑Transition Metal Oxyhalides

Birefringent crystals assume a pivotal role in the realm of optical materials, because of their capacity to manipulate the polarization state of light waves. Herein, the first-principles high-throughput screening methodology was implemented to search for materials with large birefringence (Δn > 0.1), and 15 compounds were highlighted from 102 d⁰-transition metal oxyhalides AO_mX_n (A = V, Nb, Mo, W; X = F, Cl, Br, I; m and n = integers). Notably, 12 compounds exhibit large birefringences (Δn > 0.14 at 1064 nm), four of them (MoO₂Br₂, WOBr₄ and NbOX₂ (X = Br, I)) even larger than 0.6 at 1064 nm, surpassing those of YVO₄ (Δn = 0.21 at 1064 nm) and TiO₂ (Δn = 0.256 at 1530 nm), the well-known commercial birefringent materials. Additionally, the nonlinear optical coefficients of 8 noncentrosymmetric compounds in the 15 compounds were also calculated. However, most of them have small nonlinear optical coefficients, except WOBr₄ (d₁₅ = −15.01 pm/V), which is 1.1 times that of the infrared benchmark AgGaS₂ (d₃₆ = 13.4 pm/V). The exploration of the d⁰-transition metal oxyhalides AO_mX_n with exceptionally large birefringence sets a direction in future searches for birefringent crystals.