[1] Inst Max Von Laue Paul Langevin, F-38042 Grenoble 9, France
[2] Max Planck Inst Phys Komplexer Syst, Dresden, Germany
[3] Max Planck Inst Chem Phys Fester Stoffe, Dresden, Germany
[4] Tech Univ Darmstadt, D-64287 Darmstadt, Germany
[5] Univ Paris 11, CNRS, Lab Chem Solides UA446, F-91405 Orsay, France