Low-temperature instability in [sup 242]Pu[sub 0.98]Ga[sub 0.02]. Page: 4 of 6
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300K - new sample
- - - 0- - 300K - after first cool -
2.30 2.31 2.31
Fig. 3. 111 and 200 reflections for Pu 2 at. % Ga at 300K: new sample and sample warmed to 300K after first cooling.
This refinement is shown in Fig. 2. Three sets of
reflection markers are shown: 6-phase Pu, vanadium
(radiological containment), and the hypothetical a'-phase.
As expected, the most of the calculated a' peaks do not show
up in the calculated pattern, but a few exceptions are found
in the vicinity of d=1.6 and 2.4A Since even these peaks are
not observed, we can say that the phase fraction of a'-phase
is less than 10 at. %. In fact, the refined value for the phase
fraction of a' is 0.2+0.4 at. % - essentially zero. However, it
must be admitted that there is some uncertainty associated
with the extrapolation of the a' lattice constants to low
temperature and that our estimate of the phase fraction will
0 200 400
Fig. 4. Microstrain versus temperature fort
isotropic model. (Figure redrawn fr
be invalid if very small particles of a' are involved.
There is observable peak broadening in the 6-phase at low
temperatures that remains when the sample is warmed again.
This is shown in Fig. 3, for the 111 and 200 reflections.
Since the degree of peak broadening differs between the two
reflections, we can say that the peak broadening is
anisotropic in crystal space.
2. Results of Rietveld refinement.
First we mention that the refined lattice constant of the
nominally 2 at. % alloy studied here is 4.625 A. This means
that the alloy composition, determined independently from
its lattice constant," is 1.8 at. % Ga.
first cool (1) Cubic structure - isotropic microstrain
warm The average microstrain was determined as a function of
second cool temperature using a Rietveld model for which the crystal
symmetry is held cubic and the peak width is assumed to be
isotropic in crystal space. The term "microstrain' signifies
the normalized variance of the lattice constant, under the
assumption that the diffraction patterns is formed from an
ensemble of grains with a Gaussian distribution of lattice
constants. Figure 4 shows the microstrain is plotted versus
The initial value of the microstrain was 500x10-6, or
simply 500 in the usual "micro" units, at 300K. This
represents the initial state of the sample. During the first
slow cooling, the microstrain increased very little until the
temperature of 150K was reached, and then it increased very
rapidly to a value of 2500 as the sample was cooled to 15K.
he cubic The sample was then warmed to 175K, but the microstrain
m Ref. 6).) recovered only partially to a value of 2000, and reverted to
2500 when cooled again to 15K. When the sample was
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300K - new sample
- - - - - 300K - after first cool -
or * W~ -
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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/m1/4/: accessed December 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.