12.2 Sample displacement parameters

After a good fit is obtained for the unit cell constants, noted by the tick marks now well aligned to the reflection positions, this should be optimized by refinement of an appropriate sample parameter, but do not do this until the reflection positions are well fit.

• For Bragg-Brentano Diffraction: for all types of samples refine the “Sample displacement” value. Note that the units for this correction are in microns (\(1 \times 10^{-6} m\)). For samples composed of low-Z materials, you should also refine the sample transparency correction (see §5.2.)
• For Debye-Scherrer X-ray Diffraction: one should always refine the “Sample X displ. perp. to beam” (sample displacement perpendicular to beam) value.
• For Debye-Scherrer Neutron Diffraction: one should always refine the “Sample X displ. perp. to beam” (sample displacement perpendicular to beam) value. If data are collected well above \(120^\circ \ 2\theta \) one can also refine the “Sample Y displ. —— to beam” (sample displacement parallel to beam) in addition. In theory, this second parameter could be refined with x-rays as well, but it is so rare to have much data observed at very high angles.
• For TOF diffraction, the sample placement can affect the effective distance of the sample from the source. Refining the DIFC Instrument Parameter can adjust for this, but it is usually not necessary.

Note that with older software, one would vary the \(2\theta \) or TOF zero correction. This correction is available in GSAS-II but does not properly apply the correct shift for a misplaced sample and should not normally be used.

There is a special sample displacement parameter, which is found as an HAP parameter (see §14.7), rather than in the sample or instrumental data tree entries. This is called the “Layer displacement parameter,” which is a value in \(\mu \)m. This would be used in a material where the sample components are not mixed homogeneously, and thus the phases are not all at the same location with respect to the sample surface. This means that the sample displacement is dependent on the histogram and on the phase. An example of this would be a layered sample, where each phase will have a slightly different sample displacement.