Crystal structure, Mössbauer spectroscopy, and magnetic properties of a new potassium iron oxyphosphate K 11Fe 15(PO 4) 18O related to the Langbeinite-like compounds

Abstract

A new potassium iron phosphate K 11Fe 15(PO 4) 18O has been synthesized by the flux method and characterized by single-crystal X-ray diffraction. The compound crystallizes in the cubic system with the space group P2 13 and the cell parameter a=9.917(2) Å. The unit cell contains one formula unit K 7.33Fe 10(PO 4) 12O 0.66. The structure is closely related to that of the Langbeinite-like compounds, the main difference being the existence of a four-coordinated iron atom in the structure of the oxyphosphate. K 11Fe 15(PO 4) 18O adopts a complex three-dimensional network involving corner- and edge-linkage between iron polyhedra, and corner-sharing between phosphorus tetrahedra and iron polyhedra. The Mössbauer spectral parameters are consistent with the structure and reveal the presence of octahedral and tetrahedral sites for the iron(III) in the expected 4:1 ratio. The thermal variation of the magnetic susceptibility shows that the title compound is antiferromagnetic with a Néel temperature T N=6 K. At high temperature, the susceptibility follows a Curie–Weiss law with C=58.82 emu K mol −1 and θ p=−88.39 K. The relationship between the structure of K 11Fe 15(PO 4) 18O and the previous reported trivalent vanadium phosphate K 11V 15(PO 4) 18O will be discussed.