Crystal chemistry of synthetic hercynite (FeAl (sub 2) O (sub 4) ) from XRD structural refinements and Moessbauer spectroscopy

Both the crystal chemistry and thermal behaviour of synthetic hercynite produced hydrothermally and by sintering oxides at high T were studied, the samples being annealed at different T under controlled O (sub 2) atmospheres in a gas mixture furnace using CO (sub 2) /CO gas mixtures, and subsequent quenching. The inversion parameter, x, increases from 0.101 at 500 degrees C to 0.187 at 900 degrees C as determined by XRD, or from 0.162 to 0.19 respectively, as determined by Mossbauer spectroscopy. The increase in x is accompanied by a similar smooth decrease of the oxygen positional parameter, u. The thermal behaviour is anomalous compared to previously studied simple oxide spinels, perhaps due to the extreme sensitivity of the defect chemistry of hercynite to variations in oxidation state, leading to the formation of ferric iron and vacancies in solid solution at elevated T.