1. Department of Inorganic Environmental Chemistry, Chalmers University of Technology, S-412 96 Göteborg, Sweden, ISIS Department, CCLRC Rutherford Appleton Laboratory, Chilton Didcot, Oxfordshire, OX11 0QX, U.K., and European Synchrotron Radiation Facility (ESRF), BP 220 F-38043 Grenoble, France