Experimental: Structural Model Determination Details

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Experimental: Structural Model Determination Details

Given below are points which should be covered when describing how the starting structural model for the Rietveld refinement was determined. This includes sections on determination of the unit cell, pattern decomposition, determination of the space group and actual structure solution. All these procedures need to be described when a complete ab initio structure determination from powder data is presented. Some of the points discussed here will overlap with those in following sections; they need not be detailed more than once in an article. Again, for each point the most common options are listed for guidance.

1. Unit Cell Determination Details

If the unit cell is related to a known phase:

1. reference the article the unit cell constants were taken from.
If the unit cell is previously unknown, provide the following information:

1. scan details and whether and what internal standard (typically Si powder) was used.
2. reflections used for indexing - the number of reflections used; how the 2q position of the each reflection was obtained.
3. indexing - the program used; the figure of merit of the fit for the solution used subsequently; the values of the unit cell parameters themselves.
4. unit cell refinement prior to Rietveld refinement - the program used; the number of reflections used; the parameters refined (unit cell parameters with or without the zero point); the final values of the refined unit cell parameters.
2. Pattern Decomposition

If the pattern decomposition is for structure solution then give the following facts:

1. the pattern decomposition method (Le Bail or Pawley) and program used.
2. the reflection peak shape representation - the function used (Gaussian, Lorentz , pseudo-Voigt, etc) and give the formula for the 2q dependence of the function.
3. the final value of the R_p factor and its definition.
If pattern decomposition is performed for other reasons, such as, studies of crystallite size and inhomogeneous strain, then give, in addition to the above points, information on:

1. full width at half maximum - the smallest and largest values of FWHM; explain any large variation in FWHM from reflection to reflection or class of reflection to another class.
2. a plot of observed and final calculated intensity versus 2q (or d spacing for time-of-flight or energy dispersive studies). A plot of the difference in the former profiles (observed intensity - calculated intensity) versus 2q (or d spacing for time-of-flight or energy dispersive studies) should be plotted on the same plot. It is also usual to superimpose the position of the Bragg reflection markers on the plot.
3. deposit or publish a table of:
   

   d or 2q	Integrated Intensity	FWHM
   7.83	1000	0.1
   etc

3. Space Group Determination


1. state the systematic absences found (class and condition, for example, 0k0 k=2n+1) and the possible space groups derived from these absences.
2. explain why the space group used for structure solution was chosen above any of the other possibilities.
4. Structure Solution


1. state how the structure was solved (direct methods, Patterson function, Monte Carlo simulation, simulated annealing); the program used and if the program was written for single crystal or powder diffraction data.
2. state how many of the atoms (or which atom types) were found from the initial solution and how the remaining atoms were located (for instance, from a difference Fourier synthesis, by model building etc).

Interpretation

Again, look for relevant discussion of the points mentioned above in the text of the article you are reading. Generally, the structural model obtained is then refined and it is the discussion of the Rietveld refinement process and the final results that should be given particular attention. Certain other points may need careful consideration, such as, the choice of space group in cases where the space group is not uniquely determined by systematic absences; you should consider whether the reasons stated for the selection made are valid or not.

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