Wavelength Refinement

Wavelength Refinement

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Wavelength Refinement

For angle-dispersive powder diffraction using non-laboratory radiation sources, it is often the wavelength of the incident radiation that requires calibration. This can be readily achieved by using the standard samples described previously. Instead of refining the lattice parameters, these are left fixed at the known pre-calibrated values and the wavelength itself is refined. Note that it is not possible to refine the wavelength and the unit-cell parameters simultaneously; it is a chicken and egg situation where one variable can be refined from knowing the other and vice versa.

The data below shows a neutron diffraction pattern of nickel powder (a = 3.5238 Å) collected with λ ≈ 1.9 Å (on the instrument D1A at the ILL, Grenoble). Nickel has a face-centred cubic structure and 5 Bragg peaks were measurable at this wavelength as shown in the figure below:

You can click on the icon to get a list of reflections for nickel powder. It is a relatively simple procedure to match them to the peaks as shown in the figure above.
From the 2θ positions of the five reflections the incident neutron wavelength can be refined.

Click on the icon and submit the pre-completed form without editing any of the values in order to see the output from the refinement program. In the output you should see the refined values for both the wavelength and the 2θ_zero error. You should note that this instrument was run with a much larger 2θ_zero error than that normally tolerated for a laboratory X-ray diffractometer! As with a unit-cell refinement using data collected to high scattering angle, the precision of the wavelength is relatively high. If only low-angle data had been used, the precision would have been much worse.

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Author(s): Jeremy Karl Cockcroft