Nuclear magnetic resonance study of the metal-nonmetal transition in lanthanum hydrides

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Chang, Chii-Tzong
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R. G. Barnes
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Physics and Astronomy

Proton spin-lattice relaxation time (T[subscript]1) measurements have been used to investigate the metal-nonmetal (MNM) transition in the lanthanum hydride system, LaH[subscript] x (2 ≤ x ≤ 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[subscript]1 through the electronic density-of-states at the Fermi level, N(E[subscript] F). In the pure samples, T[subscript]1 depends directly on N(E[subscript] F) at low temperatures through the Korringa relation, (T[subscript] 1 eT)[superscript]-1/2 ∝ N(E[subscript] F), where T[subscript] 1 e is the conduction electron contribution to T[subscript]1. These measurements over the temperature range 77 ≤ T ≤ 200 K show that N(E[subscript] F) follows free-electron behavior, N(E[subscript] F) ∝ (3 - x)[superscript]1/3, up to x = 2.8;In the Gd-doped samples, the Gd[superscript]3+ ion moment contributes an additional relaxation rate, R[subscript] 1 p = T[subscript] 1 p[superscript]-1. At low temperatures (7 ≤ T ≤ 150 K), R[subscript] 1 p reflects the temperature dependence of the spectral density function of the Gd[superscript]3+ ion spin fluctuations, characterized by the ion spin-lattice relaxation time [tau][subscript] i. For 2.50 ≤ x ≤ 2.87, the measurements show [tau][subscript] i ∝ T[superscript]-1, indicating that Gd[superscript]3+ ion spin relaxation is due to interaction with the conduction electrons, consistent with metallic behavior. At high temperatures (T > 250 K) R[subscript] 1 p reflects fast atomic diffusion of proton magnetization to the Gd[superscript]3+ ions, and R[subscript] 1 p ∝ [tau][subscript] i. For x ≥ 2.8, it was found that [tau][subscript] i ∝ T[superscript]-5, showing that Gd[superscript]3+ spin relaxation is governed by the two-phonon process characteristic of insulating solids. In contrast, for x 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.

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Fri Jan 01 00:00:00 UTC 1988