Structural aspects of amorphous iron(III) fluorides

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Abstract
Different forms of amorphous FeF3 (a-FeF3) are obtained either by vapour deposition or fluorination techniques. A structural investigation by extended X-ray absorption fine-structure spectroscopy and diffraction techniques (neutrons and X-rays) was undertaken in order to confirm and refine previous Mossbauer data. The latter are summarised and compared with the data for the three crystalline forms of FeF3. For both amorphous compounds, structural results agree with Mossbauer results; the trivalent iron ion is surrounded by weakly distorted fluorine octahedra and the Fe-F distances are estimated to be 1.95(1) AA. The a-FeF3 obtained by fluorination is built up from only corner-sharing octahedra while the a-FeF3 obtained by vapour deposition exhibits corner-sharing but perhaps also edge-sharing octahedra. Both compounds show speromagnetic behaviour evidenced by high-field Mossbauer spectroscopy. In comparison with the magnetic structures of the three crystalline phases of FeF3, and in particular with their degree of frustration, one can describe 'ideal' a-FeF3 as consisting of random corner-sharing octahedra where iron sites belong to odd- and even-membered cycles.