Thermal decomposition of [(CH 3) 4N]M IIIF 4·H 2O compounds of iron and aluminum

Abstract

Within investigations about thermal dehydration and decomposition of fluorometallate hydrates also the thermal behaviour of [(CH 3) 4N]M IIIF 4·H 2O (M III= Fe, Al) has been studied by thermal analysis, X-ray, i.r., and EPR investigations. As it has been shown from the crystal structure determination the structure consists of centrosymmetrical dimeric anions [Fe 2F 8(H 2O) 2] 2− with fluorine double bridges [1]. Strong intramolecular and additional intermolecular H-bonds have been found. The thermal decomposition of the Fe compound starts at about 200°C with the liberation of water. The dehydrate phase [(CH 3) 4N]FeF 4 is stable up to 380°C and decomposes between 400–450° in two steps. The decomposition is accompanied by the reduction of Fe III to Fe II by the [(CH 3) 4N] cation. The end product of the thermal decomposition is FeF 2. The Al compound is dehydrated at lower temperatures (140°C) but the formed [(CH 3) 4N]AlF 4 phase is not so stable as the Fe compound respectively and rehydrates very fast on air. The decomposition of [(CH 3) 4N]AlF 4 proceeds between 450–500°C leading to α-AlF 3 and a new cubic form of AlF 3. Assumptions about the reaction mechanisms and the structural constitution of especially the [(CH 3) 4N]M IIIF 4 phases will be made.