Proton Motion, Knight Shifts, and Quadrupolar Effects in the Lanthanum-Hydrogen System

Abstract

The nuclear magnetic resonance of both the lanthanum and hydrogen nuclei in the lanthanum-hydrogen system has been studied as a function of hydrogen concentration and temperature. The concentrations ranged from 0.4 H/La to 2.85 H/La and temperatures -197°C to 400°C. The existence of two phases, part La metal and part La${\mathrm{H}}_{2.0}$ for concentrations with less than 2 H/La, is confirmed. Measurements of the proton linewidth and thermal relaxation time $T_1$ unambiguously demonstrate that proton self-diffusion takes place at moderate temperatures. Activation energies and attempt frequencies for the proton self-diffusion, which are determined as a function of hydrogen concentration, decrease abruptly at ≈ 2 H/La from 23 kcal/mole and ${10}^{14}$ ${\sec }^{-1\mathrm{}}$, respectively, to 3 kcal/mole and ${10}^{11}$ ${\sec }^{-1\mathrm{}}$ at 2.85 H/La. The proton static linewidths vary continuously from 7.8 g at 2 H/La to 12.4 g at 2.85 H/La and the proton $T_1$ has a characteristic self-diffusion induced minimum of ≈ 5 to 8 msec and a maximum of ≈ 100 msec where spin diffusion to paramagnetic impurities dominates. The self-diffusing protons have a pronounced effect, via a quadrupole interaction, on the La resonances. For hydrogen concentrations slightly greater than 2 H/La, a broadening and then narrowing again of the La linewidth, and a decrease with a subsequent recovery of the La Knight shift is observed as the proton self-diffusion rate increases with temperature. For concentrations greater than 2.4 H/La, no La resonance is observed until a sufficiently high proton self-diffusion rate is attained to average out the quadrupolar effects. A detailed semiquantitative analysis incorporating the proton resonance data is made of these quadrupolar effects. At 400°C the La Knight shift is found to decrease from 0.23% for 2 H/La to 0.10% for 2.85 H/La while no Knight shift is observed for the proton resonance at any concentration or temperature. The thermal relaxation time of the La resonance in La${\mathrm{H}}_{2.0}$ is found to be the result of a conduction electron hyperfine interaction with $T_{1}T=11.3\mathrm{}$ sec °K when the protons are static. With proton motion the La $T_1$ decreases exponentially to a value somewhat greater than the La $T_2$ or about 100 μsec at 400°C. A schematic picture of the band structure of the hydride consistent with the available data is suggested, based on an ionized hydrogen atom or proton, whose electron goes into a conduction band localized on the La ion.