ANALYSIS OF TIME-RESOLVED POWDER DIFFRACTION DATA USING A PATTERN-DECOMPOSITION METHOD WITH RESTRAINTS

Whilst the analysis of time-resolved powder diffraction data from simp le high-symmetry crystals may be straightforward, the kinetic data fro m more complex materials such as molecular crystals can present a chal lenge. For such materials, the reflection density in the diffraction p attern tends to be high, giving rise to severe peak overlap between re flections of the different phases present. This problem can be further compounded by the poor spatial resolution inherent in time-resolved s tudies. Additional complications can arise owing to the use of a scann ing detector in the dynamic studies. The latter can cause apparent var iation of the relative integrated intensities of the reflections and i ntroduce distortion into the diffraction pattern with respect to the 2 theta axis. These complications, if present, rule out the use of curre ntly available pattern-decomposition methods. In view of this, a new m ethod has been developed. The method is based on the profile-fitting a pproach utilizing lattice parameters. The problems of peak overlap and correlation in the fitted intensities are overcome by restraints; the restraints attempt to maintain the relative integrated intensities of the reflections of each component (phase) in the mixed sample at the values observed for the pure component. The method is ideally suited t o the analysis of time-resolved powder diffraction data and enables th e reduction of data to a high precision.