Nuclear Magnetic Resonance Study of the Metal - Transition in Lanthanum Hydrides.

Proton spin-lattice relaxation time (T _1) measurements have been used to investigate the metal-nonmetal (MNM) transition in the lanthanum hydride system, LaH_{rm x} (2 <=q x <=q 3), on samples prepared from highest purity Ames Laboratory lanthanum and also from the same lanthanum doped with controlled low levels of gadolinium. The dependence of hydrogen diffusion parameters on hydrogen concentration x was also determined. The temperature and composition dependence of the MNM transition in this system affects the proton T _1 through the electronic density -of-states at the Fermi level, N(E_{ rm F}). In the pure samples, T _1 depends directly on N(E_ {rm F}) at low temperatures through the Korringa relation, (T_{rm 1 e}T)^{-1/2} ~ N(E_ {rm F}), where T_ {rm 1 e} is the conduction electron contribution to T_1. These measurements over the temperature range 77 <=q T <=q 200 K show that N(E _{rm F}) follows free-electron behavior, N(E_{rm F}) ~ (3 - x)^{1/3}, up to x = 2.8. In the Gd-doped samples, the Gd^ {3+} ion moment contributes an additional relaxation rate, R_{rm 1 p} = T_{rm 1 p}^{-1}. At low temperatures (7 <=q T <=q 150 K), R_{rm 1 p} reflects the temperature dependence of the spectral density function of the Gd^{3+} ion spin fluctuations, characterized by the ion spin-lattice relaxation time tau_ {rm i}. For 2.50 <=q x <=q 2.87, the measurements show tau_{rm i} ~ T^{-1 }, indicating that Gd^{3+ } ion spin relaxation is due to interaction with the conduction electrons, consistent with metallic behavior. At high temperatures (T > 250 K) R_{rm 1 p} reflects fast atomic diffusion of proton magnetization to the Gd^{3+} ions, and R_{rm 1 p} ~ tau_ {rm i}. For x >=q 2.8, it was found that tau_ {rm i} ~ T^{-5}, showing that Gd ^{3+}^in relaxation is governed by the two-phonon process characteristic of insulating solids. In contrast, for x < 2.7 it was found that tau_{rm i} follows approximately Korringa-like behavior, i.e., tau_{rm i} ~ T^{-1 }, showing that LaH_{rm x} remains metallic at high temperatures for these x-values, in agreement with earlier measurements of the ^{139}La NMR Knight shift. These results are summarized as follows: (a) LaH _{rm x} is metallic at all temperatures at all x-values up to 2.8 < x < 2.9, (b) LaH_ {rm x} is at least weakly metallic at all x-values at low temperatures (T < 150 K), and (c) for x >=q 2.87 LaH _{rm x} is nonmetallic at high temperatures (T > 250 K). ftn*DOE Report IS-T 1378. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy.