Mild hydrothermal synthesis, crystal structure, thermal behavior, spectroscopic and magnetic properties of the (NH{sub 4})[Fe(AsO{sub 4}){sub 1-x}(PO{sub 4}){sub x}F] (x=0.3, 0.6, 0.8) series. Thermal transformation of (NH{sub 4})[Fe(AsO{sub 4}){sub 0.7}(PO{sub 4}){sub 0.3}F] into the textural porous orthorhombic Fe(AsO{sub 4}){sub 0.7}(PO{sub 4}){sub 0.3}

The (NH{sub 4})[Fe(AsO{sub 4}){sub 1-x}(PO{sub 4}){sub x}F] (x=0.3, 0.6, 0.8) series of compounds has been synthesized under mild hydrothermal conditions. The compounds crystallize in the orthorhombic Pna2{sub 1} space group, with the unit-cell parameters a=13.1718(1), b=6.5966(6), c=10.797(1) A for x=0.3; a=13.081(1), b=6.5341(6), c=10.713(1) A for x=0.6 and a=13.0329(9), b=6.4994(4), c=10.6702(6) A for x=0.8, with the volumes 938.6(1), 915.7(1) and 903.8(1) A{sup 3}, respectively, with Z=8. Single crystals of (NH{sub 4})[Fe(AsO{sub 4}){sub 0.7}(PO{sub 4}){sub 0.3}F] heated under air atmosphere at 465 deg. C remain as single crystals, changing the composition to Fe(AsO{sub 4}){sub 0.7}(PO{sub 4}){sub 0.3}. This later phase belongs to the orthorhombic Imam space group, with the unit cell parameters a=13.328(2), b=6.5114(5), c=10.703(1) A, V=928.9(2) A{sup 3} and Z=12. The crystal structure of the ammonium phases consists of a KTP three-dimensional framework constructed by chains formed by alternating Fe(2)O{sub 4}F{sub 2} or Fe(1)O{sub 4}F{sub 2} octahedra and As/P(2)O{sub 4} or As/P(1)O{sub 4} tetrahedra, respectively. These octahedra and tetrahedra are linked by a common oxygen vertex. The chains run along the 'a' and 'b' crystallographic axes. The crystal structure of Fe(AsO{sub 4}){sub 0.7}(PO{sub 4}){sub 0.3} is a three-dimensional skeleton derived from that of the precursor, formed from (100) sheets stacked along the [001] direction, and interconnected by chains of alternating Fe(2)O{sub 6} octahedra and As/P(2)O{sub 4} tetrahedra sharing a vertex in the 'a' direction. Transmission electronic microscopy of this compound indicates the existence of unconnected external cavities with a BET surface area of 3.91(3) m{sup 2} g{sup -1}. The diffuse reflectance spectra in the visible region show the forbidden electronic transitions characteristic of the Fe(III) d{sup 5}-high spin cation in slightly distorted octahedral geometry, for all the compounds. The ESR spectra for all the compounds, carried out from room temperature to 4.2 K, remain isotropic with variation in temperature; the g-value is 1.99(1). Magnetic measurements indicate the predominance of antiferromagnetic interactions, with Neel temperatures near to 70.0 and 50.0 K for the ammonium phases and Fe(AsO{sub 4}){sub 0.7}(PO{sub 4}){sub 0.30}, respectively. At low temperatures a spin canting phenomenon for Fe(AsO{sub 4}){sub 0.7}(PO{sub 4}){sub 0.30} is detected. - Graphical abstract: The relations