Hydrocalumite-type materials: 2. Local order into Ca2Fe(OH)6(CrO42−)0.5·nH2O in temperature studied by X-ray absorption and Mössbauer spectroscopies

Abstract Local order around the cations Fe 3+ into Ca 2 Fe(OH) 6 (CrO 4 2− ) 0.5 · n H 2 O is studied by XAS and Mossbauer spectroscopies. The hydrocalumite phase Ca 2 Fe(OH) 6 (CrO 4 2− ) 0.5 · n H 2 O presents a Mossbauer spectrum which is refined with two distinct sites Fe 3+ , and two different distances Fe–Ca are necessary for the refinement of the XAS fine structure at Fe K edge. Upon calcination up to 250 °C where the inorganic phase turns amorphous, the cations Fe 3+ is found to change from octahedrally to tetrahedrally coordinated. The corresponding isomeric shift (IS) is shifted from 0.35 to 0.2 mm/s, accompanied with an increase of the quadrupolar splitting. This is corroborated by a change observed on the absorption edge. Conversely, the local environment of the interlayered chromate oxoanions is unmodified in this range of temperature.