are provided with diffraction data for a sample that contains a mixture
of SrPrO3 and SrPr2O4, SrPr03_riet.xy.
Data were collected using the Siemens D-500, Cu Ka
radiation, in 0.01° steps, with a dwell time of 15 s/step. You
are to perform refinement of the preliminary structural data to obtain
relevant crystallographic data (e.g. lattice parameters, atomic coordinates,
atomic displacement factors, and phase fractions). If you have a material
relevant to your own research, you are highly encouraged to collect
diffraction data from that material instead.
performing the refinement, you can chose to use the XPert Pro software,
or a freeware program entitled Rietica
and available from ANSTO (Australian Nuclear Science and Technology
Organization). Detailed instructions for using Rietica are provided
below. For instructions on using XPert Pro for Rietveld refinement
see your TA, Mikhail Kislitsyn.
atomic coordinates for SrPrO3 are given in the cif file, SrPrO3.cif.
Starting atomic coordinates for SrPr2O4 are given in a paper
by Fiscus and zur Loye. The 'cif' file is an ascii file with crystallographic
data compiled in a standard format that many programs can import.
using your own sample, collect diffraction data over an appropriate
2 range. Record the instrument settings.
the data for analysis into the program you are using (again, see
below for detailed instructions for Rietica) and also enter the
crystallographic data for the two phases using the information provided.
The cif file can be imported to provide the crystallographic data
for SrPrO3, whereas the data for SrPr2O4 will have to be entered
Sequentially turn on parameters for refinement. Do not use the automatic
sequence option offered by XPert Pro.
you will have to your judgment as to whether to keep a parameter turned
on for refinement as you proceed to turn on the following parameters,
or to turn it off to prevent the refinement from 'blowing up.' Simultaneous
refinement of highly correlated parameters causes this problem and
must be avoided. For example, occupancies and displacements parameters
should not be simultaneously refined. Similarly, the overall scale
factor should not be refined at the same time as the occupancy factors
of all of the atoms. One amongst these parameters must be fixed. Also,
it is important to refine the parameters most uncorrelated with other
parameters first. The refinement strategy suggested in Young's "Rietveld
Method" (p. 35) is
factor - stable
2. Specimen displacement - stable
3. Flat background - stable
4. Lattice parameters - stable
5. More background - stable
6. W, in the Caglioti function for peak widths-poorly stable
7. x, y, z (atomic coordinates) - fairly stable
8. Occupancies and isotropic (thermal) displacement parameters - not
9. U, V, (in the Caglioti function) and other profile parameters -
not generally stable
10. Aniostropic (thermal) displacement parameters - not generally
11. Zero point - stable
cases it is not very meaningful to refine anisotropic displacement
parameters using conventional X-ray diffraction data. If you must
limit the refinement to isotropic parameters in order to prevent the
displacements of the atoms from becoming 'non-positive definite' then
proceed with the refinement, you must periodically examine the difference
plot (plot of Icalc - Iobs). This will provide you with insight as
to what parameters might be misbehaving and whether you have selected
the appropriate peak profile function. If appropriate, you may chose
to change the values of parameters such as the scale factor and flat
background to more closely match the data and get the refinement 'jump-started.'
Bragg R-factor, goodness-of-fit (GOF), Rp, and Rwp
of calculated and measured I(2Q), as
well as difference plot
and weight percentages of phases with standard deviations
group and cell parameters of both phases with standard deviations
2Q, Icalc, Iobs (integrated intensities)
positions, site multipicities, atomic coordinates and displacement
parameters for atoms in SrPrO3 with standard deviations. For coordinates
fixed by the Wycoff position, standard deviations are meaningless
and not reported.
distances and angles in SrPrO3 with standard deviations
number of refined parameters and their roles (e.g. 4 background
parameters, 6 profile parameters, etc.)
as to which profile function was selected
the write-up, use the abbreviated lab report format described for
lab 5. In this case, however, references are required.