Etude par RMN de la structure dynamique de l'alanate Na 3AlH 6

Abstract

A low temperature wide-line NMR study has allowed a determination of the AI-H bond lengths in cryolite-type Na 3AlH 6. It was found from the thermal behavior of the proton lines that the |AIH 6| 3− octahedra reorient around a C 4 axis. Starting below 170 K, this rotation can be hindered by lattice defects. Above room temperature it becomes isotropic, and a quick protonic exchange appears. The thermal narrowing of the linewidth and the T 1 and T 1 ρ relaxation times lead to activation energies of about 0.38 eV for axial rotation and 0.51 eV for protonic exchange. The minimum in T 1 is in good agreement with the exchange model. Absorption phenomena, as well as partial decomposition of Na 3AlH 6 during the heat treatments, explain the presence of small amounts of mobil hydrogen.