Low-temperature instability in [sup 242]Pu[sub 0.98]Ga[sub 0.02]. Page: 3 of 6
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The results of three types of refinement are presented in
this paper. In the first series of refinements, the symmetry of
the sample was cubic (Fm3m) and the profile function was
isotropic in crystal space. In the second series, the symmetry
remained cubic but the profile function of Stephens was used
to fit the observed peak shapes, which were found to be
anisotropic in crystal space. In a third series of refinements,
a tetragonal distortion was allowed (I4/mmm) whenever
significant line broadening was observed, but the peak-
shapes were isotropic in crystal space.
1. Observed diffraction patterns
We did not observe the expected a'-phase diffraction
pattern on the first cooling to low temperatures, but only the
diffraction pattern of 6-phase alloy. The diffraction patterns
that we observed are shown in Fig. 1.
Because the diffraction pattern of the monoclinic alpha
phase has many weak diffraction peaks, we make a special
Rietveld analysis of the low temperature data that included
10 at. % of the a'-phase. We estimated the lattice constants
of a' at low temperature by including corrections for the
anisotropic expansion observed as a - a' at room
temperature under pressure and the anisotropic thermal
contraction observed as a is cooled to low temperatures. The
estimated lattice constants for a'-Pu at 15K are a=6.142,
b=4.797 and c=10.915A with 3=101.850. In order to
enhance the visibility of weak diffraction peaks, the data
were compressed by a factor of 8 along the d-spacing axis.
0.5 1.0 1.5
2.0 2.5 3.0
Fig. 1. Neutron diffraction patterns from NPD of Pu 2 at. %
Ga at 300K (new sample) and 15K (first cool). The
observed strong reflections are from 6-phase Pu.
There are a few weak reflections for the vanadium
Fig. 2. Refined neutron diffraction pattern from Pu 2 at. % Ga at 15K.
phase Pu, vanadium, and a'-phase Pu.
The three sets of reflection markers correspond to S-
- Pu 2 at. % Ga -
_ 300 K - new sample
_ 15K - first cool
- Pu 2at. %Gaatl15K
imimm uuuuiuu 111111III IIII I**II 1111 111 I II II II lii I
Ilw n n n 1 I 1 1 1 1
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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/m1/3/: accessed November 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.