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Abstract 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 (Δ n ), a record‐high Δ n obv. of 0.152 in sodium bis(dicyanoborato)fluoride (NaBF 2 (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 MgF 2 . Density functional theory analyses suggest that the linear π group enhances bulk optical anisotropy and ensures a shorter DUV absorption edge. These results establish NaBF 2 (CN) 2 as a groundbreaking DUV birefringent material and open new avenues for research into extending π groups and exploring additional possibilities.
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