K3[HCO3]2F: a new deep-UV birefringent crystal exhibiting strong optical anisotropy
The development of deep-ultraviolet (DUV) birefringent crystals requires simultaneous satisfaction of a wide bandgap (>6.2 eV) and large birefringence (>0.1 at 1064 nm). Carbonates, despite high polarizability anisotropy, suffer from narrow bandgaps and UV cutoff edges between 200–300 nm, limiting DUV applications. Here we report K3[HCO3]2F, a carbonate fluoride synthesized via synergistic incorporation of highly electronegative fluorine and hydrogen bonding. The coplanar alignment of π-conjugated [HCO3] triangles yields a birefringence of 0.165 at 532 nm. The UV cutoff edge is below 188 nm, confirming DUV transparency. First-principles calculations identify the isolated [HCO3·F·HCO3] block as the dominant contributor to optical anisotropy. This compound demonstrates a viable strategy for balancing bandgap and birefringence in DUV birefringent materials.