The coefficient of self-diffusion of 7Li in liquid lithium has been measured with nuclear magnetic resonance. The diffusion coefficient was determined from observations of the dependence of the spin-echo signal upon the amplitude of a pulsed magnetic field gradient. It is found that at the melting point D = 6.8 ± 0.3 × 10−5cm2 / sec and (T / D) (dD / dT) = 2.8 ± 0.3. In terms of an activation model, D = 0.94 ± 0.4 × 10−3 exp [ − (2300 ± 300cal) / RT]cm2 / sec. Resultsare compared with the Cohen–Turnbull free-volume model and the Ascarelli–Paskin dense-gas model for liquid diffusion. From the free-volume model, the effective hardsphere diameter is determined for Li and, in addition, for Na and K as well. The results compare favorably with effective hard-sphere diameters determined from neutron-scattering data.
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 LaH 2 .o for concentrations with less than 2 H/La, is confirmed. Measurements of the proton linewidth and thermal relaxation time T\ unambiguously demonstrate that proton self-diffusion takes place at moderate temperatures. Activation energies and attempt frequencies for the proton selfdiffusion, which are determined as a function of hydrogen concentration, decrease abruptly at ^2 H/La from 23 kcal/mole and 10 14 sec -1 , respectively, to 3 kcal/mole and 10 11 sec -1 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\ 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-duffusion 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 LaH2.o is found to be the result of a conduction electron hyperfine interaction with TiT-11.3 sec °K when the protons are static. With proton motion the La Ti decreases exponentially to a value somewhat greater than the La T 2 or about 100 yusec 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.All of the tetrahedral sites are occupied in the dihydride. As more hydrogen is added, the octahedral sites randomly fill while the tetrahedral sites remain fully occupied. The dihydride exhibits a metallic conductivity 1/100 that of the metal 6 and a paramagnetic susceptibility not much less than the metal. At concentrations near 3 H/La, the hydrides have semiconductor-like electrical conductivity and a diamagnetic susceptibility. A review of other metal hydrides has been prepared by Libowitz. 7 Previous nuclear magnetic resonance (NMR) investigations of the other metallic hydrides 7-" 14 have been studies of the proton ...
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