The Anomalous Spectral Density Function for Diffusive Motion of Hydrogen in LaNi 5 H 7
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331 THE ANOMALOUS SPECTRAL DENSITY FUNCTION FOR DIFFUSIVE MOTION OF HYDROGEN IN LaNi5 H 7*
H. CHANG and I. J. LOWE Physics Department, University of Pittsburgh, R. J. KARLICEK, JR.t Chemistry Department,
Pittsburgh, PA
15260
University of Pittsburgh, Pittsburgh, PA
15260
ABSTRACT Measurements of TI (B = 0.94T), and T as a function 1 0 ir of temperature have been carried out on a LaNi5H7 sample at 4 different rotating magnetic field values.
The T1
and Tlr data are consistent with earlier data by Karlicek and Lowe [3,4],
in which an asymmetry in
-i
log Tlr vs.
T
plot was found.
the slopes of the
The new data has been
analyzed assuming a spectral density function J(W,T C ) of form J(-,r
c)
= A( c)B(SI)F(w-r c),
with Tc = T
exp(E a/kT).
This assumption leads to a spectral density function that fits all our data well, with E = 39 KJ/gm-Atom H, and ---- 1.35 . a J(W) W w in the high frequency limit. This Ea agrees well with the E
a
obtained from diffusion constant measurements.
INTRODUCTION Hydrogen motion in
the intermetallic hydride 8-LaNi5 Hxx
"
6 to 7)
has
been studied in recent years by both nuclear magnetic resonance [1-5] and neutron scattering techniques [6]. The data of Karlicek and Lowe [3,4] could be interpreted as suggesting the existence of two activation energies that influenced hydrogen motion. They measured the diffusion constant of hydrogen in LaNi 5 H6 . 331K to 375-K) using an alternating pulsed field gradient (APFG) technique
[5],
log D vs. T
yielding an activation energy E plot.
laboratory frame (B = 1.4T) and T 0
from 77'K to 375 K.
of 40 KJ/gm-Atom H in
They also measured NMR relaxation times T
The log Tlr vs.
in T
in
the
the
the rotating frame (B1 = 1.0 mT) plot was found to be asymmetrical
with Ea 's of 39KJ/gm-Atom H on the short correlation time side of Tlr minimum and 20KJ/gm-Atom H on the long correlation time side. A similar plot for T1 after the electronic contribution was removed, gave E a of 20 KJ/gm-Atom H ' in the long correlation time region. The observed T and Tlr minima, occur-
*Supported by NSF Grant DMR 78-15441 fNow at Bell Telephone Laboratories, Murray Hill, N.J.
332 ring at W T c=1 and 2w 1rc=l respectively,
scaled according to Ea of
39 KJ/gm-Atom H. In order to investigate the asymmetrical non-BPP-type behavior of T and Tlr in T
ý-LaNi
5
hydride, we have carried out further measurements of
(Bo = 0.94T) and T r for four different rotating magnetic field values
from 120*K to 320*K. This data has been analyzed,using a frequency temperature superposition technique [7], to fit one empirical spectral density function of the form J (w, -re)
with
c
and one activation energy.
(1)
= A(- c )B(92)F(w-rc
T
(2)
c
We report here the results of such an analysis.
NMR RELAXATION TIMES MEASUREMENTS 8-LaNi
5
hydride used in this experiment was prepared in a sealed quartz
Hydrogen concentration tube using the technique described by Karlicek [8]. was determined to be 7.0 H/La by the drop in pressure due to absorption of
104
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