1. LGIT, University of Grenoble and CNRS, B.P. 53, 38041 Grenoble Cedex 9, France, Institut Laue-Langevin, B.P. 156, 38042 Grenoble Cedex 9, France, Departament of Chemistry, University of Girona, Campus de Montilivi, 17071 Girona, Spain, INFM-GILDA c/o ESRF, B.P. 220, 38043 Grenoble Cedex 9, France, and Eduardo Torroja Institute for Construction Sciences, Spanish Council of Scientific Research, Serrano Galvache 4, 28033 Madrid, Spain