Graphical Interface for Powder Pattern Diffraction data

NEW: it is now possible to use auto-indexing and profile-fitting in Fox.

To know where to begin, make sure you have read the tutorials first !

powder.jpg

Powder Menus

Powder Pattern graph

PowderGraph.png

The powder pattern graph window displays the calculated (in red) and observed (in blue, with small grey bars to indicate the observed esd's) powder patterns. The gray line which increases from left to right is the cumulative Chi^2 which can be used to find which parts of the patterns present the worst fit. For a perfect fit (with only statistical errors remaining) it should be a straight line (though in Fox the cumulative Chi2 uses the integrated profiles and therefore looks like a bar chart).

The mouse and keyboard can be used to get more information from the graph:

Right-clicking on the graph will open a popup menu, from which you can:

Export

This feature is available since Fox revision #1029, and is still being tested. Only export to Fullprof and constant-wavelength data is available yet (no TOF).

The "Export" menu can be used to create a .pcr and a .dat file suitable for Fullprof. Notes about the .pcr file:

Sorry, no export to GSAS yet...

Radiation

Here you can describe the radiation corresponding to your experiment (neutron, X-Ray, tube,...). The wavelength is in Angstroem as usual. Set the polarization rate between 0.95 and 1 for synchrotron experiments, and 0 for an X-Ray tube (actually if you use a monochromator it should be above 0, but a precise value is not critical for a global optimization).

You can also select Neutron Time Of Flight. You will need to enter the DIFA and DIFC parameters corresponding to the instrument the data was recorded on.

2theta correction parameters

These parameters allow to correct for experimental errors in the 2theta positions of reflections, due to a wrong zero, or a misplacement of the sample holder or its transparency. For most data sets, using the 2theta zero is sufficient. After each change, you can right-click on the powder pattern graph to update it.You should always check that all peaks are correctly and precisely indexed before launching an optimization.

Maximum sin(theta)/lambda

Use this field to limit the extent of the calculations on the powder pattern. It is usually a very good idea to do that, since for a global optimization, high-angle data is useless. A good value is 0.25 Angstroem-1 (corresponding to a resolution of 2 Angstroems), although you can go higher to 0.4 A-1 if you feel there are not enough reflections...

Powder Pattern Background

The background can be modelized using either linear or cubic spline interpolation between user-chosen 2theta (or tof) values. The easiest way to create the background is to use the automatic Bayesian (Sivia-David) background estimation: use the "Components->Add Bayesian Background (automatic". You will be prompted to choose the number of interpolation points (10 to 20 should be OK, depending on how much structure there is in the background). FOX will then spend a few seconds finding the best background

To input the (2theta, intensity) points you can also create a text file with a list of "2theta intensity" on each line (2theta in degrees), and load it using the menu.

If you want to change the points (either the intensity or the position), just edit the values in the grid.

There is currently no limit to the number of points, but even for complex backgrounds 20 should be enough.

PowderPatternDiffraction: Crystalline phase contribution to the powder pattern

This allows to describe the contribution of a crystalline phase to the powder pattern. It is possible to save the calculated structure factors using the menu (note that structure factors above the chosen sin(theta)/lambda limit are not calculated).

Texture (Preferred Orientation) using the March-Dollase Model

FOX supports optimization of Preferred orientation, usign the March-Dollase model. To add one phase, use the "Phases" local menu. You can then enter the fraction, March coefficient (>1 for needles, <1 for plate-like crystallites), and HKL coordinates for the preferred orientation.

Note: it is very important to use this only as a last resort, or if you know for sure that you have preferred orientation. Taking one more day to prepare carefully a non-textured sample is definitely worth it, since even if a solution is found, it will take much longer - preferred orientation reduces the information available in some direction of the crystal.

If you finally decide to search for preferred orientation parameters, you must input as much information as possible: you should be able to know what kind of preferred orientation to expect (plates or needles), so that you can restrict the March coefficient to be either >1 or <1 (e.g. use limits [.5;1] or [1;2] - never go below .1 or above 10, that would just slow things). If you know what the preferred orientation vector is, that's even better.

Note that it is possible to use several preferred orientations, but I strongly discourage to do this for a global optimization.

None: Manual/Powder (last edited 2009-01-06 21:30:44 by VincentFavreNicolin)