H-1-NMR STUDY OF ZRNIALHX AND YNIALHX

1H CW and pulsed NMR studies on the hydrides ZrNiAlH x, with hydrogen concentration x=0.33 and 0.53, and YNiAlH x, with hydrogen concentration 0.12<or=x<or=1.17, have been performed in the temperature range 150-400 K. In the case of the Zr compounds, only a single resonance line has been observed; the linewidth remains almost unchanged over the whole temperature range studied. A superposition of two lines, one broad and one narrow, has been observed in the case of the Y compounds with x>or=0.75 in the temperature range 220-320 K, whereas only a single line is observed for hydrides of YNiAl with x<or=0.66 at all temperatures studied. The narrow component for the former and the single line for the latter exhibit similar features of motional narrowing due to hydrogen diffusion. Measurements of T1 of the protons reveal that the relaxation process in YNiAlHx at low temperatures (below 200 K) and in ZrNiAlHx at all temperatures is dominated by interactions of 1H nuclei with the unpaired conduction electrons at the Fermi level. In the high-temperature region, 200-400 K, T1 for the narrow lines for the Y compounds at all hydrogen concentrations is characteristic of a diffusionally dominated relaxation mechanism. Motion of hydrogen in the broad component is also revealed through relaxation measurements, although it is not clearly observed in the CW experiments. The pressure-composition isotherms, and the fact that below 200 K only a single T1 is observed for the samples with x>0.75, indicate that YNiAlHx is a single-phase hydride. Hydrogen atoms occupy two types of interstitial sites in YNiAl, in which the mobility of hydrogen in one of the sites is higher than that in the other. The activation energies for these motions have been estimated to be 0.24 eV/atom and 0.33 eV/atom.