A neutron diffraction study of non-stoichiometric Mn/sub 1-x/O Page: 3 of 7
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controlled in-situ by flowing CO/CO 2 gas mixtures over the sample. The
thermogravimetric data of Dieckmann et al. [191 were used to determine the sample
composition for the various gas mixtures employed. All tank gases were commercially
available, non-certified grade. Seven different sample compositions in the range
0<x<0.02 were studied, but only that nearest stoichiometry (x=0.001) will be reported
here. To ensure sample equilibrium, powder profiles were taken in two hour increments
and refined for the cubic lattice parameter. When the lattice parameter for successive
data sets fluctuated by 0.0002 A or less (equilibrium was reached) another series of runs
totaling 8 to 10 hours was collected. Then the gas mixture was changed. Equilibrium
was fast for sample compositions near stoichiometry; typically less than 2 hours. After
summing together the equilibrium profiles for each composition, a profile of measured
background scattering from the empty furnace was scaled and subtracted. The scale
factor used was the ratio of total incident neutrons measured in the incident beam
monitor for the raw and background data sets. First order thermal diffuse scattering was
calculated based on the method of Cole and Windsor [241, scaled same as above, and
subtracted.
Standard Rietveld [25] analysis techniques were used to refine the summed powder
profiles. The measured profile, namely the background,lattice parameter and peak
shape, were refined first. Then the structural model incorporating Debye-Waller
factors, site occupations, extinction and absorption, was refined. At elevated
temperature, diffuse scattering around Bragg reflections could not adequately be treated
by the available background fitting function. To remove the diffuse background we used
the Fourier filtering technique of Richardson [261. (Refinements with and without
filtering on profiles of Fe 1-x O at 1323'K showed filtering had no effect on refined site
occupations.) The primary effect of filtering the data is to improve both background and
Bragg peak fitting with the available analytic functions, and thereby reduce the
magnitude of profile fitting statistics.
Two different refinements were made, one for ideal MnO and one with
interstitial Mn occupation at the 1/4,1/4,1/4 position. For the ideal MnO the site
occupation for Mn on octahedral sites was constrained to the known composition and
isotropic Debye-Waller factors were used for both Mn and 0. After completing
refinement for octahedral Mn occupation only, the tetrahedral site was added to the
structural model and the sum of the occupations for octahedral and tetrahedral sites was
constrained to the known composition. The Debye-Waller factor for the tetrahedral site
was constrained to equal that of the octahedral site, and the profile refined again.
RESULTS
The refined powder pattern of Mn 0.999 0 is shown in Figure 1. Displayed are the
measured (+) points, calculated profile (solid line), and difference pattern. Despite some
minor peak fitting errors the overall fit is quite good. Refined structural parameters and
their standard deviations are given in Table 1. Included are values and statistics for
refinements with and without tetrahedral occupation. From the calculated statistics of
each fit and the number of points that comprise Bragg peaks a significance test, as
described by Hamilton [27], can be performed. The tested hypothesis is:
H 0 : There is no interstitial occupation in MnO
Using the weighted profile R values, the number of points in Bragg peaks, 486 , and
the number of variables, 14, the test statistic is:
R 1,474,0.01 1.007
and the observed ratio is:
R - 1.0836The hypothesis is easily rejected at the 1% level.
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Radler, M.; Cohen, J.B. & Faber, J. Jr. A neutron diffraction study of non-stoichiometric Mn/sub 1-x/O, article, February 1, 1988; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1203181/m1/3/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.