Low-temperature instability in [sup 242]Pu[sub 0.98]Ga[sub 0.02]. Page: 2 of 6
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Low Temperature Crystallographic Instability in 2Puo.98Gao.o2
A. C. LAWSON, B. MARTINEZ, J. A. ROBERTS and R. B. VON DREELE
Los Alamos National Laboratory
Los Alamos, NM 87545 USA
We have made an unsuccessful attempt to make a detailed characterization by neutron powder diffraction of the S-a'
transformation that is known to occur at low temperatures in dilute Pu-Ga alloys. The a'-structure is a variant of the
monoclinic a a'-phase, except that the lattice constants are a bit different, with some (or all) of the Pu sites partially
occupied by Ga. This transformation is easily detected by dilatometric methods because of the 19% volume difference
between the 6 and a'-phases. In our study, we used 242Pu in a Puo 9sGao 02 alloy to minimize neutron absorption and
cooled the sample very slowly to maximize the degree of transformation. We did not detect any transformation to the a'-
phase, but instead observed a slight line broadening effect that was visible in the raw data. The onset temperature of the
line broadening (150K) coincides with observations of the S-a' transformation. We have developed two alternative
explanations for the line broadening effect: (1) the occurrence at low temperatures of a cubic-tetragonal transformation in
Ga-stabilized alloy or (2) the occurrence at low-temperature of a spontaneous anisotropic microstrain that is supported by
the highly anisotropic elastic constants of S-Pu.
KEYWORDS: plutonium, plutonium alloy, neutron diffraction, phase transformation
It has been known for a long time that marginally
stabilized Pu-Ga alloys transform at low temperatures from
the FCC 6-phase to a monoclinic structure called the a'-
phase.) This phase is very closely related to the a-phase that
is always observed at room temperature and below for
unalloyed element, but differs because some Ga is retained at
the monoclinic atom sites.
The transformation appears to be martnezitic,2) as allowed
by the symmetry relations between the phases. This is
somewhat surprising in view of the large difference in
atomic volume between the phases: about 25% smaller for
a'! In view of the large volume change, it is important to
understand in detail the conditions that permit the occurrence
of this transformation and their relationship to the aging of
The experiments described in this paper were undertaken
to determine the crystal structure of the a'-phase by high-
resolution neutron diffraction measurements. It turned out
that the 6-a' transformation was not observed in our
experiments, but rather a subtle line-broadening effect was
observed instead. The line-broadening effect is the subject
of this paper.
II. Experimental Procedure
The Puo098Gao.02 sample was prepared from electrorefined
metal that was 99.85 wt.% plutonium with 95% 242Pu,
required to minimize absorption of neutrons; all other
impurity levels were less than 100 ppm. Alloys were
prepared by arc melting. Rods were cast either in a casting
furnace or in the hearth of the arc furnace and machined to
the final dimensions. The Pu alloy was heat-treated at 4500C
for 200 hours, and then doubly encapsulated in thin-walled
vanadium tubes for radiological protection.
2. Neutron diffraction
We made neutron powder diffraction measurements over
a wide range of temperatures using the pulsed neutron
diffraction technique at the Los Alamos Neutron Science
Center (LANSCE) at Los Alamos National Laboratory. The
NPD powder diffractometer has a path length of
approximately 32m, and a resolution in d-spacing of about
0.15%. Measurements were made over the range 15-773K,
using a closed cycle helium refrigerator to obtain low
temperatures and a specially constructed furnace for high
The thermal history of the sample is important. The data
presented in this paper were obtained from three separate
experiments. In the first experiment, the freshly prepared
sample was cooled very slowly to low temperatures, warmed
to 150K, cooled again to 15K, and finally warmed again to
room temperature. The cooling rate for the first cooling was
1 K/min. and about 2K/min. in the second experiment. In
the third experiment, the sample was heated to 800K and
furnace cooled slowly to room temperature.
3. Data analysis
Data were analyzed using the Rietveld analysis package
in GSAS.4) The Pu data are the same as those discussed in
previous publications.5'6> For each temperature, the refined
parameters include lattice constants, scale factors,
background, diffuse background, and Debye-Waller factor,
and peak profile parameters. The absorption factor was
fixed at pR=0.3; this high value reflects the resonant
absorption by isotopic impurities, primarily 239Pu, in our
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Lawson, A. C. (Andrew C.); Martinez, B. S. (Barbara S.); Roberts, J. A. (Joyce A.) & von Dreele, R. B. (Robert B.). Low-temperature instability in [sup 242]Pu[sub 0.98]Ga[sub 0.02]., article, January 1, 2001; United States. (digital.library.unt.edu/ark:/67531/metadc933298/m1/2/: accessed November 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.