Unprecedented Deep Ultraviolet (DUV) Birefringence in Fluorides Constructed from Linear π‐Group Anisotropic Structure Building Units

Advancing laser technologies requires maximizing the anisotropy of crystalline media and overcoming current birefringence limitations. We introduce the strategy of linear π group anisotropic structure building units (ABUCB), leading to an unprecedented large birefringence (Δn), a record-high Δn_obv. of 0.152 in sodium bis(dicyanoborato)fluoride (Na[BF₂(CN)₂]), the highest reported among deep ultraviolet (DUV) materials. This value is 35 % greater than state-of-the-art α-BBO and 1167 % greater than MgF₂. Density functional theory analyses suggest that the linear π group enhances bulk optical anisotropy and ensures a shorter DUV absorption edge. These results establish Na[BF₂(CN)₂] as a groundbreaking DUV birefringent material and open new avenues for research into extending π groups and exploring additional possibilities.