(N 2 H 6 )[HPO 3 F] 2 : maximizing the optical anisotropy of deep-ultraviolet fluorophosphates.

Although phosphates are a rich source of deep-ultraviolet optical materials, the realization of large optical anisotropy in them still remains a challenge because of the small polarizability anisotropy of [PO 4 ] units. Inspired by the fluoridation strategy and hydrogen bond interaction, a new metal-free monofluorophosphate, (N 2 H 6 )[HPO 3 F] 2 , was synthesized, which exhibits a large birefringence (cal. 0.077) and wide band gap (∼6.51 eV). Such a large birefringence in (N 2 H 6 )[HPO 3 F] 2 sets a new record among available fluorophosphates, and the [HPO 3 F] unit is theoretically confirmed to be a new birefringence-active unit.