I was wondering if it is possible to analyze unknown phases using Rietveld Refinement.
Please comment
Dear Maninder Singh ,
It is possible to analyze unknown phases with Rietveld analysis. Naturally, it is necessary to do a Pawley extraction; It is typically used with powder patterns derived from a crystal structure whose unit cell contents are not or insufficiently known.
In classic Rietveld refinement, each Bragg reflection's intensities are calculated from the atoms' scattering intensities in the unit cell.
By contrast, the Pawley procedure uses the Bragg intensities as freely varying refinement parameters and disregards the underlying atomic structure.
The extracted integrated intensities can, in theory, be used in traditional methods of structure determination.
Pawley refinement of an experimental powder pattern is an essential step before attempting crystal structure determination.
If the lattice parameters and space group are unknown, indexing is necessary to determine possible lattice candidates.
The procedure helps identify the most likely lattices and space group settings corresponding to the experimental pattern.
Once the powder experiment is indexed, it provides the proposed set of lattice parameters' fundamental values for the experimental data.
At this stage, perform refining of the integrated Pawley intensities for exporting the integrated intensities correcting the scale factor and base line contributions of the unknown phase.
These procedures can determine the cell parameters and a bare low symmetric space group for the unknown by observing diffraction peak absences.
Once partially characterized, there are no atoms in the symmetric site description; the known phase is fitted, and the unknown lattice is characterized for further atoms inclusions and association.
In case of a completely unknown phase, some other methods are possible to be applied, such as Patterson's method to extract the heavier atoms positions in the first place and fulfill the asymmetric unit step by step.
Best regards,
WNM
How is it possible to identify the unknown phase. I think the unassigned peaks that do no match after the refinement you can treat as an impurity.
Dear Maninder Singh ,
It is possible to analyze unknown phases with Rietveld analysis. Naturally, it is necessary to do a Pawley extraction; It is typically used with powder patterns derived from a crystal structure whose unit cell contents are not or insufficiently known.
In classic Rietveld refinement, each Bragg reflection's intensities are calculated from the atoms' scattering intensities in the unit cell.
By contrast, the Pawley procedure uses the Bragg intensities as freely varying refinement parameters and disregards the underlying atomic structure.
The extracted integrated intensities can, in theory, be used in traditional methods of structure determination.
Pawley refinement of an experimental powder pattern is an essential step before attempting crystal structure determination.
If the lattice parameters and space group are unknown, indexing is necessary to determine possible lattice candidates.
The procedure helps identify the most likely lattices and space group settings corresponding to the experimental pattern.
Once the powder experiment is indexed, it provides the proposed set of lattice parameters' fundamental values for the experimental data.
At this stage, perform refining of the integrated Pawley intensities for exporting the integrated intensities correcting the scale factor and base line contributions of the unknown phase.
These procedures can determine the cell parameters and a bare low symmetric space group for the unknown by observing diffraction peak absences.
Once partially characterized, there are no atoms in the symmetric site description; the known phase is fitted, and the unknown lattice is characterized for further atoms inclusions and association.
In case of a completely unknown phase, some other methods are possible to be applied, such as Patterson's method to extract the heavier atoms positions in the first place and fulfill the asymmetric unit step by step.
Best regards,
WNM
Wagner Da Nova Mussel has provided an excellent answer. I just want to add that, as one can find Rietveld's method is a 'refinement' method. For it, to apply, you should have at least some information regarding the crystal system, lattice parameters, unit cell, Bragg planes etc from other methods, which has to be refined in order to match the experimental diffraction pattern.
Once, the rudimentary crystallographic information is obtained, one has to consider a few space groups, which are suspected to be the correct or closely correct one. Then all the close structures have to be refined one by one, until the best fit is obtained. That gives the complete CIF of the material.
Strictly speaking, no... Rietveld analysis is a structure refinement technique; in other words, a reasonable structure model should be determined (or thought) prior to the use of refinement. However, there are some techniques (Pawley and/or LeBail methods of intensity extraction, followed by direct and/or Patterson methods of structure solutions) which can help determine unknown structures from powder diffraction data.
Anyway, I agree that the answer given by Wagner Da Nova Mussel is excellent.
Best wishes!
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