CCP14
Methods, Problems and Solutions
Powder Diffraction Structure Solution Pathways
Solving an Inorganic Structure from Powder Diffraction Data
The CCP14 Homepage is at http://www.ccp14.ac.uk
This example uses Li0.92Ti2.88O6
as published in (I.E. Grey, L. M. D. Cranswick, C. Li,
L. A. Bursill, and J. L. Peng, "New Phases Formed in the Li-Ti-O System under
Reducing Conditions", Journal of Solid State Chemistry, 138, 74-86 (1998)).
It should be noted that this structure was not solved in the following
software pathway but gives an example of what modern options are available.
Structure Solution Pathway
- Get the raw data in the right data format
- Profile the peaks using XFIT
in Fundamental Parameters mode to
obtain accurate peak positions at low angle
- Powder Indexing to get the cell and spacegroup using the
combination of CRYSFIRE Powder Indexing suite
and Chekcell
[Example of auto mode]
- Perform a Unitcell refinement (also inside Chekcell) to get the best cell possible
prior to structure solution
- Use Le Bail extraction in an appropriate Rietveld package
(or the latest Powder Cell software)
to confirm that the obtained cell and spacegroup looks good using a whole profile structureless fit.
- Use EXPO (sequel to Sirpow) to
solve the structure using direct methods
- This structure could have also been solved with
Le Bail extracted data. After being processed with
the Overlap software
of Armel Le Bail, single crystal structure solution packages could have been used to
solve the structure. An obvious first choice
would have been to use the Patterson based DIRDIF structure solution software. An example would be the
single crystal solving programs that are in the WinGX
Suite and System S for UNIX.
- The structure is then refined in the Rietveld package of personal choice.
- Ton Spek's Platon could
then be used to validate/check the structure by doing such things as
using Platon's Addsym to find missing Symmetry
and possibly correctly assign the spacegroup based on the atom positions
- While the above located the Titaniums and Oxygens. To confidently locate the Lithiums,
a similar related phase of known structure was synthesized and refined. Constant wavelength
neutron powder diffraction data was also collected on both the Lithium titanate and the known related
phase. Much use was made of difference electron density Fourier contour maps
- Many of the single crystal suites such as ORTEX, Powder Cell,
System S/Platon and WinGX (which includes Ortep-3 and GUI WinSTRUPLO) also allow
convenient structure visualisation and output; including photorealistic rendering of the structure.
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