An analysis of recent single crystal X-ray diffraction measurements on calcium fluoride

An analysis of recent single-crystal X-ray diffraction measurements on calcium fluoride has been carried out by comparing the observed data with those calculated from a theoretical model. A computer program was used which can refine parameters for extinction and anharmonic thermal vibrations in addition to the conventional parameters. It is shown that for the majority of the experiments in the recent ACA project the angle-dependent deviations between the experiments are consistent with those which would be expected due to a variation in the measured contributions from thermal diffuse scattering. There are discrepancies between experiments for the strongest reflexions, presumably because of errors associated with the use of attenuators, but if these reflexions are excluded the agreement between experiments, allowing for variations in the thermal diffuse scattering contributions, is much better than was indicated by previous analyses. The two remaining experiments and that reported by Zachariasen are in poor agreement with the others and this is attributed to the techniques used, namely an ω-scan with balanced filters in one case and a stationary-crystal, stationary-detector technique in the other two. Although the effects of anharmonicity on the X-ray intensities are rather small, the data from all experiments are consistent with the magnitude of an anharmonic thermal-vibration parameter, associated with the fluorine atoms, derived from neutron-diffraction measurements.