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 Δnobv. 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.