Structural transition of Friedel's salt 3CaO•Al2O3•CaCl2•10H2O studied by synchrotron powder diffraction

The structural phase transition occurring in Friedel's salt, the chlorinated compound 3CaO.Al2O3.CaCl2.10H(2)O (AFm phase), was studied by synchrotron and standard X-ray powder diffraction. The compound transforms at 35 degreesC from a rhombohedral (rh) high-temperature (HT) phase [R-3c; a=5.744(2) Angstrom, c=46.890(3) Angstrom] to a monoclinic (m) low-temperature (LT) phase [C2/c; a=9.960(4) Angstrom, b=5.7320(2) Angstrom, c =16.268(7) Angstrom, beta =104.471(2)degrees]. The LT and HT phases were refined with the Rietveld method from synchrotron data recorded at 20 and 40 degreesC. Variations of the lattice parameters as a function of temperature are reported between 8 and 48 degreesC. The rh --> m transition is characterized by a unit cell volume expansion of 1% and a movement of the interlayer species: a shift of 0.45 Angstrom of the Cl- anions along [010](h) and a shift of 0.25 Angstrom of the water molecules along [210](h) of the hexagonal cell. The m phase distortion is due to an ordering of the hydrogen bonds between chloride anions and H-atoms of the water molecules. (C) 2002 Elsevier Science Ltd. All rights reserved.