Mild hydrothermal synthesis, crystal structure, thermal behaviour, spectroscopic and magnetic properties of (NH{sub 4}){sub 0.80}Li{sub 0.20}[Fe(AsO{sub 4})F]

The (NH{sub 4}){sub 0.80}Li{sub 0.20}[Fe(AsO{sub 4})F] compound has been synthesized under mild hydrothermal conditions. The compound crystallize in the orthorhombic Pna2{sub 1} space group, with cell parameters a=13.352(9), b=6.7049(9), c=10.943(2) A and Z=8. The compound belongs to the KTiO(PO{sub 4}) structure type, with chains alternating FeO{sub 4}F{sub 2} octahedra and AsO{sub 4} tetrahedra, respectively, running along the 'a' and 'b' crystallographic axes. The diffuse reflectance spectrum in the visible region shows the forbidden electronic transitions characteristic of the Fe(III) d{sup 5}-high spin cation in slightly distorted octahedral geometry. The Moessbauer spectrum at room temperature is characteristic of iron (III) cations. The ESR spectra, carried out from room temperature to 200 K, remain isotropic with variation in temperature; the g-value being 1.99(1). Magnetic measurements indicate the predominance of strong antiferromagnetic interactions. - Graphical Abstract: Three-dimensional structure of (NH{sub 4}){sub 0.80}Li{sub 0.20}[Fe(AsO{sub 4})F], a fluoroarsenate containing lithium and ammonium in the structural cavities. Highlights: > (NH{sub 4}){sub 0.80}Li{sub 0.20}[Fe(AsO{sub 4})F] has been synthesized by mild hydrothermal technique. > The compound exhibits a three-dimensional structure. > Moessbauer spectrum indicates the existence of Fe(III) cations. > Visible spectroscopy confirms the hexacoordination of Fe(III). > Magnetic measurements indicate the existence of a global antiferromagnetic ordering.