Structural phase transition and high temperature phase structure of Friedels salt, 3CaO • Al2O3 • CaCl2 • 10H2O

Friedels salt, the chlorinated compound 3CaO . Al2O3 . CaCl2 . 10H(2)O (AFm phase), presents a structural phase transition at about 30 degrees 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 degrees C from X-ray single crystal diffraction data. The compound crystallises in the space group R (3) over bar c with lattice parameters of a = 5.7358(6)Angstrom and c = 46.849(9)Angstrom (Z = 3 and D-x = 2.111 g/cm(3)). The refinement of 498 independent reflections with I > 2 sigma(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-,2H(2)O]. The chloride anions are surrounded by 10 hydrogen atoms, of which six belong to hydroxyl groups and foul 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. (C) 2000 Elsevier Science Ltd. All rights reserved.