Cone-angle dependence of ab-initio structure solutions using precession electron diffraction

Precession electron diffraction (PED) is a technique which is gaining increasing interest due to its case of use and reduction of the dynamical scattering problem in electron diffraction, leading to more direct structure solutions. We have performed a systematic study of the effect of precession angle for the mineral andalusite on kinematical extinctions and direct methods solutions where the semiangle was varied from 6.5 to 32 mrad in five discrete steps. We show that the intensities of kinematically forbidden reflections decay exponentially as the precession semiangle (phi) is increased and that the amount of information provided by direct methods increases monotonically but non-systematically as phi increases. We have also investigated the zeolite-framework mineral mordenite with PED and have found a direct methods solution where the 12-ring is clearly resolved for the first time.