Two new ammonium/alkali-rare earth metal difluorophosphates ALa(PO2F2)4 (A = NH4 and K) with moderate birefringence and short cutoff edges

Herein, two new ammonium/alkali-rare-earth metal fluorophosphates ALa(PO2F2)4 (A = NH4 and K) have been successfully obtained via a facile route. The introduction of F- anions with high electronegativity and non-π-conjugated species [PO2F2]- was found in the title compounds. Different from the [PO4]3- unit in phosphates, the (PO2F2)- group retains the merit of wide UV transmittance in phosphates; meanwhile, it has large polarizability anisotropy, and theoretical calculation shows that the calculated birefringences are 0.023 and 0.019 for KLa(PO2F2)4 and NH4La(PO2F2)4, respectively. More importantly, this work will contribute to the structural and functional diversity of phosphate chemistry by the exploration of the fascinating difluorophosphates.