LiB5O5F2(OH)4: A new deep-ultraviolet birefringent crystal with [B5O5F2(OH)4] anionic group

A new deep-ultraviolet (DUV) birefringent crystal, LiB 5 O 5 F 2 (OH) 4 (LBOFH), was designed and synthesized by a high-temperature solution method. Its structure features a [B 5 O 5 F 2 (OH) 4 ] anionic group that is constructed by four π-conjugated units, i.e., two [BO 3 ] units and two [BO(OH) 2 ] units connected by the [BO 2 F 2 ] unit, which forms a near-coplanar configuration. The group exhibits large anisotropic polarizability and highest unoccupied molecular orbital-lowest unoccupied molecular orbital (HUMO-LUMO) gap, thus providing a good balance between the large birefringence (0.124 at 546 nm) and band gap (7.99 eV) in LBOFH. This work suggests new possibilities for the design and fabrication of advanced DUV birefringent crystals using the novel anionic group and the expansion of their alternative system with new hydroxyfluorooxoborates.