K4(PO2F2)2(S2O7): first fluorooxophosphorsulfate with mixed-anion [S2O7]2−and [PO2F2]−groups

The first fluorooxophosphorsulfate with mixed-anion building units [S₂O₇]²⁻and [PO₂F₂]⁻was synthesized. The introduction of an F atom into phosphate–sulfate improved the polarization anisotropy and increased the birefringence.