Structural phase transition and high temperature phase structure of Friedels salt, 3CaO center dot Al2O3 center dot CaCl2 center dot 10H(2)O

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 d iffraction and optical microscopy in transmitted light with crossed polaris ers on single crystals prepared by hydrothermal synthesis. The high tempera ture phase was determined at 37 degrees C from X-ray single crystal diffrac tion data. The compound crystallises in the space group R (3) over bar c wi th 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 reflection s with I > 2 sigma(I) led to a residual factor of 7.1%. The Friedels salt c an be described as a layered structure with positively charged main layers of composition [Ca2Al(OH)(6)](+) and negatively charged layers of compositi on [Cl-,2H(2)O]. The chloride anions are surrounded by 10 hydrogen atoms, o f which six belong to hydroxyl groups and foul to water molecules. The stru ctural phase transition may be related to the size of the chloride anions, which are not adapted to the octahedral cavity formed by bonded water molec ules. (C) 2000 Elsevier Science Ltd. All rights reserved.