Refinement of the λ/2 contribution to CCD detector data

A new method is proposed to improve structure modeling by taking into account the half-wavelength contribution (λ/2) to X-ray data measured on CCD and image-plate diffractometers. Such an effect arises because the reflection intensities contain contributions associated with both the λ and the λ/2 wavelengths. The ratio w of these two contributions is a new parameter refined using the full-matrix least-squares techniques. The new method does not require any additional measurement or information. According to the tests performed, proper account of the λ/2 contribution lowers the wR₂(|F|²) factors by a relative percent difference of 4–14% and the atomic displacement parameters by ∼1–4 s.u. Other effects are analyzed that might correlate with the λ/2 contribution, such as absorption, charge-density redistribution, thermal diffuse scattering and extinction. Strong correlation was found between w and the other parameters, but a fixed λ/2 correction reduces this difficulty. The value of w is found to be both characteristic of the diffractometer and a general indicator of the refinement quality (similar to R factors).