The Golden Standard for X-ray Data Collection

Bruker AXS recently announced XRDWizard V2.9 support for Variable Counting Time (VCT), a data collection scheme for X-ray powder diffraction, where the measurement time is systematically increased towards higher 2theta angles. As a result, the natural steep intensity fall towards higher 2theta angles is compensated for, leading to dramatically improved data quality for all kinds of profile fitting applications.

VCT data typically results in: greatly enhanced chance for success in structure determination. Atomic coordinates, occupancy factors and (anisotropic) thermal parameters are better determined, especially in the case of light atoms; more stable refinement of atomic coordinates and thermal parameters of light atoms, including hydrogen; substantially lower RWP indicating a better overall fit; and a GOF much closer to unity, indicating a better fit of the structural and profile models to the data and that the weights are properly applied.

The concept of a Variable Counting Time (VCT) strategy for X-ray powder diffraction data acquisition was introduced by Madsen & Hill (1992, 1994) and David (1992). A VCT strategy is based on a function that increases the counting time used at each step in the scan in a manner that is inversely proportional to the decline in reflection intensity that inevitably results from the combined effects of the porentz-Polarisation (LP) factor, scattering factors f2 (form factor fall-off) and thermal vibration of atoms. The highly dominating factor of these is the sample / structure independent LP factor, leading to an intensity decrease by a factor of about 400 as the diffraction angle increases up to about 100° 2theta; above this angle, the intensities slowly increase again.

The contribution by scattering factors and thermal vibration of atoms is typically an order of magnitude smaller and can therefore be neglected. In the XRDWizard V2.9 implementation of VCT, the measurement time depends on a function approximately compensating the dominating effect of the LP factor. This allows for a very user-friendly push-button creation of VCT schemes, as no crystal structure information is required.