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Unit Cell And Backgrounds

First round of refinement

Manual adjustment

In a first step, you may want to adjust some parameters by hand (from one of the plots). Typically

• the incident intensity
• the unit cell parameters (for high pressure experiments)

and if your peaks are broad

• the isotropic crystallite size
• isotropic microstrains

Refinement cycles...

Here we go with the refinement cycle... At this point we'll try to get a nice background and reasonable cell parameters, what is presented here is just an option. There is no reciepe that will apply to all cases. Use your brain!

• in a first step, refine the main background parameters, the incident intensity, and the cell parameter
• in a second step, add 3 background parameters to all orientations, and refine the backgrounds only
• later on, try to refine your cell parameters and isotropic crystallite size and microstrain.

If you it fails, try to adjust those parameters by hand before starting the refinement...

Dataset before first round of adjusment	2-D plot after first round of adjusment

Do you have stress?

In the 2-D plot above, you can see that the position of the diffraction peaks change with orientation in the data, but not in the recalculated pattern. This can have two explanations:

• mis-calibration of beam center
• residual stresses

In most high pressure experiments, and certainly for radial diffraction, you will certainly have stress in your sample: that's what you are interrested in!

Refinement of stress will be discussed in the following section: Stress

Do you have texture?

Once again, in the 2-D plot above, you can see that the intensity of the diffraction peaks change with orientation in the data, but not in the recalculated pattern. This is due to texture or lattice preferred orientations.

Again, in most high pressure experiments, and certainly for radial diffraction, you will certainly have LPO in your sample...

Refinement of LPO should be done last, after the refinement of stress and it will be discussed in a later section: Lattice Preferred Orientations