HYDROGEN HOPPING RATES AND THE ORDER-DISORDER TRANSITIONS IN SUB-STOICHIOMETRIC LANTHANUM TRIHYDRIDE

Extensive measurements of proton magnetic resonance lineshapes and spi n-lattice relaxation rates in sub-stoichiometric lanthanum trihydrides , prepared from highest purity La metal, furnish clear evidence for th e coexistence of ordered and disordered hydrogen sublattices over an e xtended 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 tempe rature transition in the hydrogen hopping rate appears to coincide wit h the onset of semiconducting behavior.