1. Contribution from the Department of Inorganic, Physical and Materials Chemistry, NIS Centre of Excellence, and Centro di Riferimento INSTM, University of Turin, Via P. Giuria 7, I-10125, Torino, Italy, DiSCAFF and NIS Centre of Excellence, Via Bovio 6, 28100 Novara, Italy, and ESRF, 6 rue Jules Horowitz, BP220, F-38043, Grenoble CEDEX, France