Thermal behaviour of the nitrated AFm phase Ca4Al2(OH)12(NO3)2•4H2O and structure determination of the intermediate hydrate Ca4Al2(OH)12(NO3)2•2H2O

The nitrated AFm phase Ca4Al2(OH)(12)(NO3)(2). 4H(2)O, which belongs to the layered double hydroxide family, is studied by thermogravimetric measurements and Raman microspectrometry as a function of temperature. Its structure at room temperature was determined previously. The existence of an intermediate hydrate around 70 degrees C with composition Ca4Al2(OH)(12)(NO3)(2). 2H(2)O was detected and its structure resolved by single crystal X-ray diffraction at that temperature. The structure at 70 degrees C is described and has the rhombohedral space group R (3) over bar c with cell parameters a = 5.731(2) Angstrom and c = 48.32(1) Angstrom and Z = 3. It can be considered as a pillared layer structure, be cause adjacent main layers [Ca2Al(OH)(6)](+) are bridged by nitrate groups. The remaining two water molecules are space filling only, and are not connected to the main layers. The structure exhibits dynamic disorder due to free rotation of nitrate groups around trigonal axes, in agreement with the Raman spectra. (C) 2000 Elsevier Science Ltd. All rights reserved.