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. Synchrotro n radiation, with its much smaller angular divergence, narrower spectral ba ndwidth and higher flux, distinctly improves the experimental conditions fo r physical phase estimation. Pure psi scans about the primary diffraction v ector, such as can be made with a six-circle diffractometer, further improv e the conditions compared with combined omega/psi scans with a four-circle instrument, where the rotation in psi is accomplished by combining rotation s about the three axes omega, psi and to. Interference profiles collected b y pure psi scans and unfocused synchrotron radiation have FWHM values reduc ed by factors in the range 20-35 relative to those obtained with combined o mega-2 theta/psi scans and radiation from a conventional source. As a conse quence of these changes, which also involve greatly increased peak amplitud es, 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 b e even more important in studies of general phases for which the phase info rmation lies also in the relative heights of the (sharp) interference maxim a for a triplet and the Friedel-related triplet.