Synergistic dual-modification strategy of π-conjugated cations and tetrahedra: rational design of ultraviolet high-birefringence monofluorophosphates

(CN 4 H 7 ) 2 PO 3 F·H 2 O and (C 4 N 4 H 11 ) 2 PO 3 F·H 2 O exhibit large birefringence values of 0.116 and 0.165 at 546 nm, surpassing those of most previously reported monofluorophosphates/difluorophosphates.