Motion of point defects and monitoring of chemical reactions in sodium aluminium hydride

Anelastic spectroscopy experiments (elastic modulus and energy dissipation) were carried out for the first time with Ti-doped and undoped NaAlH4. We found that the various decomposition reactions taking place in the alanates are most sensitively monitored by the dynamic Young modulus variations. We also found that, during one of the decomposition reactions occurring at higher temperature, a new point-defect complex is formed, very likely involving hydrogen, which has fast dynamics and gives rise to a thermally activated relaxation process at 70 K in the kHz range, with an activation energy of 0.126 eV. The occurrence of this process suggests that any model concerned with the decomposition mechanism should take into account the hydrogen mobility in the crystal lattice. (c) 2005 Elsevier B.V. All rights reserved.