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PWDR Sample Parameters subdata tree item (Powder Diffraction)

This window shows the various sample-dependent parameters for the selected powder pattern. Some are related to the measurement conditions, such as diffractometer setting angles, or the data collection temperature. Others, such as the histogram scale factor, adjust the fit to match the collected data. The presence of a refine button indicates that a parameter can be refined (all others are fixed.) All values shown in this window can be edited.

Note that the last three parameters (named internally as FreePrmX, X=1,2,3), which are labeled by default as "Sample humidity", "Sample Voltage" and "Applied Load" are fields where the labels that can be changed. If changed in one histogram, the same label is used for all histograms. Thus, a total of six parametric variables are available, including the more common variable of temperature, pressure and time. Also, note when a label FreePrm is changed to "ParmName", the Comments tree item for each PWDR histogram is searched for a matching "*ParmName*=value" or "*ParmName*:value" pair (i.e. the string "ParmName" will be looked for within the label+value pairs in the comments, ignoring differences in letter case between the two label strings). Thus if the Sample Parameters label is specified as "focus", then a Comments string of "/Instrument/BeamFocus_at_sample=10" will be matched. When found, the value is converted to a float and saved as the value for the newly-labeled Sample Parameter. Also, note the similar action in the "Set from comments" command, below.

Be sure the correct instrument type is selected (Debye-Scherrer or Bragg-Brentano). Also, ensure the goniometer radius is correct (in mm) so that sample displacements are properly scaled (in μm).

What can I do here?

In this window you can change parameters associated with a histogram or set them to be refined.

  • The histogram scale factor is usually refined.

  • For Debye-Scherrer mode the "Sample X displacement" is also usually refined but the "Sample Y displacement" can only be refined when data are collected over a two-theta range that extends to much greater than 90 degrees (typically for CW Neutron). Note that while in the past it was common to vary the \(2\theta_0\) (Zero) Instrument Parameter, the "Sample X displacement" is almost always a much better parameter to fit. "Sample X displacement" should never be refined with Zero, with the possible exception of a calibrant where the lattice parameters are fixed.

  • For Bragg-Brentano mode the "Sample displacement" is the displacement of the sample from the diffractometer circle in \(\mu\)m. This almost always refined. The \(2\theta_0\) (Zero) Instrument Parameter is almost never refined with Bragg-Brentano data. For low-Z samples "Sample transparency" is usually refined instead of "Sample displacement". Only if data are collected over a very wide \(2\theta\) range should both be refined together.

  • Sample absorption should not be refined when all atomic displacement parameters (Uiso or Uij values) are varied, as the correlation is very high.

  • "Surface roughness" parameters are not usually refined, unless the sample has high absorption and negative Uiso values are obtained for heavy atoms.

The remaining parameters are non-refinable and may be needed for texture or parametric studies. They may be edited in the window or may be changed for all histograms with the menu commands described below.

"Command" Menu Commands

  • Set scale - Rescales a pattern by multiplying by the current scale factor. Scale factor is then set = 1.0.
  • Load - This loads sample parameters from a previously saved .samprm file.
  • Save - This saves the sample parameters to a file with the extension .samprm. A file dialog box will appear to ask for the name of the file to be written.
  • Copy - This copies the sample parameters shown to other selected powder patterns. If used, a dialog box (Copy parameters) will appear showing the list of available powder patterns, you can copy the sample parameters to any or all of them; select 'All' to copy them to all patterns. Then select 'OK' to do the copy; 'Cancel' to cancel the operation.
  • Copy selected... - This copies only the sample parameter that are selected to other selected powder patterns but is otherwise similar to "Copy".
  • Copy flags - This copies the sample parameter refinement flags shown to other selected powder patterns. If used, a dialog box (Copy refinement flags) will appear showing the list of available powder patterns, you can copy the sample parameter refinement flags to any or all of them; select 'All' to copy them to all patterns. Then select 'OK' to do the copy; 'Cancel' to cancel the operation.
  • Set one value - This is used to set a single selected sample parameter for a selected set of PWDR histograms. The same value can be used for all histograms or a dialog can be used to provide a table where you can set the values differently for each of selected histograms.

  • Load all - Reads a file containing a table of sample parameters and copies them to matching PWDR entries. The file will look something like the example below.

    Note that the first line(s) in the file can be a header, but each header line must start marked with a hash (#). A header is not required. "Columns" in the table are separated by one or more delimiters (which may be a comma, tab or space). Note that columns do not need to be aligned, as long as each entry is spaced by at least one delimiter. The first column in the table is used to look up PWDR entries where the initial space-delimited string after the PWDR tag ("myfile" in "PWRD myfile AZM=180...") must match the table. Subsequent columns can then be mapped to sample parameters or can be ignored, using a dialog window.

    #filename       temperature pressure ignore-me  humidity
    LaB6_dc250.tif      100          1      test       .2
    LaB6_dc300.tif      150          1      test       .25

  • Rescale all - Allows a series of selected PWDR histograms to be put on a common scale by integrating them over a specific two-theta region and then scaling them so that the integration range will match the first pattern.

  • Set from comments - The histogram comments section, depending on the importer used to read in the powder data (or image that data is derived from) often contains values that are recorded by the diffraction instrument. These are recorded in the comments as "Label=value" or "Label:value". This command can be used to scan the comments and transfer them to the sample parameters. This is done by selecting a sample parameter, as well as a "Label" found in the comments. One then specifies the histograms where the value will be located and the parameter will be set. Alternately, one can specify an equation to be used to set the chosen sample parameter as a function of one or more "Label" values. This command can be used to set any of the Sample Parameter values, but this is particularly useful for parametric variables and for sample setting angles. Note that a similar action is possible by changing the name of one of the "free parameters," as described above.