Diffracted beam crystal centering and its application to high-pressure crystallography

Deviations in the position of a crystal from the center of a goniostat and diffractometer alignment errors can be calculated from the observed angles for diffracted radiation. The present study includes a derivation of equations for the general setting (ω ≠ 0) and a scheme for transformation of these equations into other diffractometer coordinate systems. This method is illustrated by determination of centering errors for FeS in a diamond-anvil cell. In addition, unit-cell parameters for this crystal at high pressure are precisely determined with angular data collected by this technique.