A two-circle powder diffractometer for synchrotron radiation with a closed loop encoder feedback system

A high-angular-resolution two-circle powder diffractometer equipped with long diffracted-beam collimators has been built at Daresbury Laboratory. The diffractometer has encoders mounted directly on the 2θ and ω axes. These give a nominal angular resolution of 0.1 and 1.0 mdeg respectively. Repeated scans of single powder peaks have demonstrated a reproducibility of 0.1 mdeg 2θ. Measurements on five independent peaks of tungsten give a self consistency of 1(1) × 10⁻⁵ Å. An example data set from synthetic olivine Mg₂SiO₄ has been refined using the Rietveld method and the results compare very well with single-crystal structure refinements.