Hydrogen hopping rates and the order-disorder transitions in sub-stoichiometric lanthanum trihydride

Extensive measurements of proton magnetic resonance lineshapes and spin-lattice relaxation rates in sub-stoichiometric lanthanum trihydrides, prepared from highest purity La metal, furnish clear evidence for the coexistence of ordered and disordered hydrogen sublattices over an extended temperature range terminating with the onset of semiconducting behavior. This phenomenon persists even at the stoichiometric limit, LaH3. Analysis of resonance lineshape intensities yields H-t=0.080+/-0.020 eV for the enthalpy of the order-disorder transition in LaH2.99, and analysis of intensities in relaxation rate measurements yields H-t=0.066+/-0.020 eV for this enthalpy in LaH2.90. A second, higher temperature transition in the hydrogen hopping rate appears to coincide with the onset of semiconducting behavior.