Ionic Conductors: Characterisation of Defect Structure
Workshop 5
Defect structure analysis from powder neutron data
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
ORTEP-3 for Windows http://www.chem.gla.ac.uk/~louis/software/ortep3/index.html
(This requires you to obtain a license which is free for academic use)
You should download the following files from the course website
BIMG.EXP
HRPD97.PRM
R18350.GSS
(GSAS experiment file)
(Instrument parameter file)
(HRPD neutron diffraction data)
Create a directory where you will do the work and copy the files into this directory.
You may also find it useful to refer to the original pare on this work, which you can
download from the course website.
Part 1 Structure Refinement
You are provided with a GSAS experimental file which already contains the basic structure
of Bi2Mg0.1V0.8O5.35-. The aim of the first part of the exercise is to locate the oxygen
atoms in the vanadate layer and refine their positions and occupancies, using neutron
data.
The model is in space group I4/mmm with cell dimensions: a = 3.935 Å, c = 15.449 Å
The atomic positions given are:
Atom
Bi
V
Mg
O1
x
0.0
0.5
0.5
0.0
y
0.0
0.5
0.5
0.5
z
0.16923
0.0
0.0
0.25
Occupancy
1.0
0.9
0.1
1.0
1. Carry out an initial refinement:
To do this first run Powpref and then Genles.
Record your Rwp (R-factor) for this refinement and include it in your report.
2. Enter the trial position for O(2).
x
y
z
Atom
Occupancy
O2
0.5
0.5
0.11
1.0
This can be done in ExpGui or using EXPEDT. Leave the Uiso value at the default of
0.025Å3. Allow the fractional occupancy to refine and record the resulting Rwp value for
your report. You will not need to run Powpref again.
3. Enter the trial position for O(3).
x
y
z
Atom
Occupancy
O3
0.5
0.0
0.03
1.0
Allow the fractional occupancy to refine and record the resulting Rwp value for your report.
4. Enter the trial position for O(4).
x
y
z
Atom
Occupancy
O4
0.0
0.32
0.10
1.0
Allow the fractional occupancy to refine and record the resulting Rwp value for your report.
5. Tie the Uiso parameters for O(2) and O(4) in the constraints menu. Allow the Uiso
values for all O atoms to refine and carry out a refinement. Record your Rwp value.
6. Allow the fractional coordinates to vary for O(2), O(3) and O(4). Continue refining until
convergence is reached. Record your Rwp value and the values for the fractional
occupancies for these atoms.
7. O(2) is a 4-fold site and O(4) is a 16 fold site. They cannot be simultaneously occupied.
Therefore there total must add up to 4 or less. Your refinement should indicate a total
value close to this. To apply a total occupancy constraint to the occupancies of O2 and
O(4), you will need to enter the constraints menu. The shifts in the fractional occupancies
of O2 and O4 are related by:

Frac(O4) = -4Frac O(2)
If O(2) is atom 5 and O(4) is atom 7 then in EXPEDT you would enter
1 frac 7 1
1 frac 5 -4
Carry out refinements until you reach convergence. Once you have then carry out a final
Powpref and Final Genles. Run Disagl to calculate bond distances and Powplot to
produce a plot of the fitted diffraction profile (including difference plot and reflection
markers options d and m).
The final output is recorded in the .LST file. Use the information in the file to produce a set
of final refined parameters for your report.
Your report should include:
The Rwp values at each stage
a. The refined unit cell parameters and volume with estimated standard deviations.
b. A table of refined parameters with estimated standard deviations.
c. A plot of the fitted diffraction profile.
d. A Table of significant contact distances.
Part 2. Estimation of cation coordination environments
Using ORTEP read in the experiment file.
Using the sphere of enclosure option plot the average environment for (V/Mg). You may
save this as a postscript file or paste directly into your report (Press Printscreen to save
the current screen into the paste buffer and Ctl-V to paste it into your word document).
By excluding selected atoms show a possible octahedral geometry for (V/Mg) and a
possible tetrahedral environment for (V/Mg). Include these diagrams in your report.
Part 3. Calculation of other structure parameters
Using your refined parameters calculate (equations in your lecture notes)
a. The fraction of octahedral sites
b. The fraction of tetrahedral sites
c. the value of 
Include these values in your report.
Make sure your name is on the report and Email your report to: abrahams@if.ed.pl by
10:00 a.m. on Monday 28th March 2011