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

Thumbnail Image
Date
1988
Authors
Chang, Chii-Tzong
Major Professor
Advisor
R. G. Barnes
Committee Member
Journal Title
Journal ISSN
Volume Title
Publisher
Altmetrics
Authors
Research Projects
Organizational Units
Organizational Unit
Journal Issue
Is Version Of
Versions
Series
Department
Physics and Astronomy
Abstract

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.

Comments
Description
Keywords
Citation
Source
Subject Categories
Copyright
Fri Jan 01 00:00:00 UTC 1988