Physical estimation of triplet phases--effects of different radiation sources and modes for profile scans

A comparative study has been made of the intensity profiles from three-beam experiments to estimate triplet phases using radiation from a conventional sealed-tube X-ray source and two different synchrotron sources. Synchrotron radiation, with its much smaller angular divergence, narrower spectral bandwidth and higher flux, distinctly improves the experimental conditions for physical phase estimation. Pure psi scans about the primary diffraction vector, such as can be made with a six-circle diffractometer, further improve the conditions compared with combined omega/psi scans with a four-circle instrument, where the rotation in psi is accomplished by combining rotations about the three axes omega, chi and varphi. Interference profiles collected by pure psi scans and unfocused synchrotron radiation have FWHM values reduced by factors in the range 20-35 relative to those obtained with combined omega-2theta/psi scans and radiation from a conventional source. As a consequence of these changes, which also involve greatly increased peak amplitudes, the 0/pi-type asymmetry of the profiles is exposed much more pronounced closer to the three-beam point, enabling unambiguous phase assignment for all triplets that were studied. The superiority of the pure psi scan will be even more important in studies of general phases for which the phase information lies also in the relative heights of the (sharp) interference maxima for a triplet and the Friedel-related triplet.