| Abstract
| - The structure of the phases obtained upon dehydration and decomposition of Friedel'ssalt [Ca2Al(OH)6]Cl·2H2O, also known as hydrocalumite, was investigated by severalexperimental techniques, in particular, high-temperature in situ XRD measurements, whichallowed the detection of a metastable intermediate phase. Thermogravimetric analyses showthat Friedel's salt, like most of the layered double hydroxides, undergoes a three-stepdecomposition on heating (dehydration, dehydroxylation, and anion expulsion) over thefollowing temperature ranges: 25−280, 280−400, and >400 °C. Sharp phase transitions areobserved as a result of the ordered distribution of Ca and Al atoms in the hydroxide layerand the well-ordered interlayer structure. Upon cooling to room temperature and exposureto the atmosphere, the dehydrated phase obtained by calcination between 80 and 280 °Cwas found to recover the basal spacing characteristic of hydrated galleries (7.81 Å) withina few minutes. The structural determination of this thermally metastable phase based onX-ray powder diffraction data recorded at 116 °C reveals a quasi-pillared layer structurewith chloride anions situated midway in the interlamellar space at only 2.904(3) Å from Caatoms of adjacent hydroxide layers. Friedel's salt becomes amorphous at ca. 400 °C, andabove 750 °C, it crystallizes into a mixture of CaO and mayenite Ca12Al14O33. Exposure ofthe amorphous residue obtained at 400 °C to aqueous solutions of KCl led to reconstruction−intercalation phenomena.
- The reversibility of the gallery dehydration process for Friedel's salt between 80 and 280 °C was unequivocally established by means of in situ techniques. The resulting thermally metastable phase exhibits a quasi-pillard layer structure with chloride anions located midway in the interlamellar space.
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