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

Abstract

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.