Crystallography and Diffraction. Theory and Modern Methods of Analysis
Workshop 5
Rietveld Analysis
You will need to install the following software to carry out the refinements detailed
below:
GSAS and EXP-GUI download from http://www.ccp14.ac.uk/solution/gsas
Additionally you may find the following useful
FullProf http://www.ill.eu/sites/fullprof
You should download the following files from the course website
Ex1-xrd.gss
Ex2-xrd.gss
Ex3-xrd.gss
Ex4-xrd.gss
Ex4-neut.gss
xpert2.prm
Siemens.prm
poldec08.prm
Exercise 1 Le Bail refinement
You are provided with an X-ray data set for Bi3LaO6 compound. The data were
collected on Philips X’Pert Pro diffractometer in / reflection geometry using Cu-K
radiation. The crystal system is trigonal in space group R-3m with cell dimensions of
ca. a = 4.0 and c = 27.6 Å. Using a Le Bail approach obtain a set of refined cell
dimensions and quote the estimated standard deviation on each dimension.
Data set and parameter file required
Ex1-xrd.gss
xpert2.prm
Steps
1.
2.
3.
4.
5.
bank 1
Start the program and enter an experiment title (eg. Exercise 1)
Enter the phase information: phase name, space group and unit cell dimensions
Read in the histogram. Use the parameter file xpert2.prm with bank number 1
Set the maximum 2 to 120
Set the minimum 2 to 10 (do this by setting the first excluded region from 0
to 10
6. Under atom parameters enter the coordinates for a dummy atom of any type
and any name (coordinates should not be 0, 0, 0). E.g. La1 0.10 0.22 0.39 type
La. Set the fractional occupancy of this atom to 1.0 and give it the default
isotropic thermal parameter on 0.025
7. Under the least squares control menu change the extraction method for
histogram 1 to Lebail
8. Run Powpref and then Genles. Repeat Genles until Rwp stop reducing or
convergence is reached.
9. Plot the data using Liveplot or Powplot and see the quality of the fit.
10. Next vary the lattice parameters
11. After a few cycles of Genles re-run powpref and then Genles again
12. Once the Rwp value has stopped going down vary the peak shape parameters
GU and LX
13. You may need to run powpref again after a few cycles
14. Use Powplot or liveplot periodically to check the fit.
15. When things are stable vary the zero point correction under diffractometer
constants.
16. Finally change the background type to 1 and increase the number of
coefficients to 12 and carry on refining until the Rwp value remains stable.
17. delete the .LST file and run a final genles
18. read the refined lattice parameters from the LST file.
Exercise 2 Rietveld refinement using X-ray data
You are provided with an X-ray data set for Bi2V0.9Fe0.1O5.35. The data were collected
on in / reflection geometry using Cu-K radiation. The crystal system is Tetragonal
in space group I4/mmm and should be isostructural with Bi2V0.9Co0.1O5.35. Details of
the structure of Bi2V0.9Co0.1O5.35 are given below. Using these as a starting model,
with appropriate substitution of Co by Fe. Refine the structure of Bi2V0.9Fe0.1O5.3.
Quote the refined structural parameters with estimated standard deviations.
Data set and parameter file required
Ex4-xrd.gss
xpert2.prm
Starting Model
bank 1
Exercise 3 Multiphase refinement with X-ray data
You are provided with an X-ray data set for a composition 2Y2O3:P2O5. The data were
collected on in /2 reflection geometry using Cu-K radiation. The sample contains
two phases identified as Y2O3 and YPO4. Using the crystal data provided below for
these two phases, carry out a multi-phase Rietveld refinement and calculate the weight
and mole percentages of each compound in the sample.
Data set and parameter file required
Ex3-xrd.gss
Siemens.prm
bank 1
Starting model YPO4
Crystal system
Tetragonal
Cell dimensions
a = 6.88 Å c = 6.03 Å
Space Group
I41/amd
Atomic coordinates
Atom
x
y
Y1
0.0
0.75
P1
0.0
0.25
O1
0.0
0.4251
z
0.1250
0.3750
0.2147
Starting Model Y2O3
Crystal system
Cubic
Cell dimensions
a = 10.61 Å
Space Group
Ia-3
Atomic coordinates
Atom
x
Y1
0.4674
Y2
0.2500
O1
0.3907
z
0.2500
0.2500
0.3802
y
0.0000
0.2500
0.1519
Exercise 4 Combined refinement using X-ray and neutron data
You are provided with X-ray and neutron data sets for Bi3YO6. The X-ray data were
collected on in / reflection geometry using Cu-K radiation. The neutron data were
collected on the Polaris diffractometer at ISIS in back scattering mode. Using the
crystal data provided below for Bi3YO6, carry out a combined Rietveld refinement and
establish the oxide ion distribution.
Data and parameter files required
Ex4-neut.gss
poldec08.prm
Ex4-xrd.gss
xpert2.prm
Starting model Bi3YO6
Crystal System
Cubic
Space group:
Fm-3m
Cell dimension
a = 5.49 Å
Atomic coordinates
Atom
x
y
Bi/Y
0.0
0.0
O1
0.25
0.25
bank 2
bank 1
z
0.0
0.25
Alternative oxide ion positions to be tried as part of the exercise
Atom
x
y
z
O2
0.30
0.30
0.30
O3
0.50
0.20
0.20
occ
0.75/0.25
0.75
occ
?
?