Affiliation: | a Laboratoire de Chimie du Solide Minéral, UMR 7555, Université Henri Poincaré, Nancy I, F-54506 Vandoeuvre les Nancy, France b Institut für Mineralogie, Kristallographie und Strukturchemie Technische Universität Wien, Getreidemarkt 9, A-1060 Wien, Austria c Département de Chimie Minérale, Analytique et Appliquée Université de Genève, 30 quai Ernest Ansermet, CH-1211 Genève 4, Switzerland |
Abstract: | Friedels salt, the chlorinated compound 3CaO · Al2O3 · CaCl2 · 10H2O (AFm phase), presents a structural phase transition at about 30°C from a monoclinic to a rhombohedral phase. It has been studied by X-ray powder diffraction and optical microscopy in transmitted light with crossed polarisers on single crystals prepared by hydrothermal synthesis. The high temperature phase was determined at 37°C from X-ray single crystal diffraction data. The compound crystallises in the space group R
c with lattice parameters of a = 5.7358(6)Å and c = 46.849(9)Å (Z = 3 and Dx = 2.111 g/cm3). The refinement of 498 independent reflections with I > 2σ(I) led to a residual factor of 7.1%. The Friedels salt can be described as a layered structure with positively charged main layers of composition Ca2Al(OH)6]+ and negatively charged layers of composition Cl−,2H2O]. The chloride anions are surrounded by 10 hydrogen atoms, of which six belong to hydroxyl groups and four to water molecules. The structural phase transition may be related to the size of the chloride anions, which are not adapted to the octahedral cavity formed by bonded water molecules. |