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

Abstract

Mixed-anion compounds are among the most promising systems to design functional materials with enhanced properties. Among the phosphate–sulfate species, the [SO₄]²⁻ and [PO₂F₂]⁻ tetrahedra are known and give rise to structural versatility. However, to date, the crystal structures of phosphate–sulfates with the coexistence of two distinct anion groups ([S₂O₇]²⁻ and [PO₂F₂]⁻) in one compound are unknown. Here, a novel type of fluorooxophosphorsulfate, K₄(PO₂F₂)₂(S₂O₇) (KSPOF), is designed and synthesized via a high-temperature method in a closed system. The crystal structure is derived from single-crystal X-ray diffraction (C2/c, a = 13.000(10) Å, b = 7.5430(10) Å, c = 19.010(10) Å, β = 130.070(10)°, and Z = 4). It is the first fluorooxophosphorsulfate with mixed-anion building units ([S₂O₇]²⁻ and [PO₂F₂]⁻) and a unique drum-like cluster was found, which enriches the diversity of structures for fluorooxophosphorsulfate systems. Furthermore, the theoretical calculations indicate that KSPOF possesses moderate birefringence, which mainly originates from the distorted [PO₂F₂]⁻ tetrahedron.