ON THE BREAKDOWN OF FRIEDEL LAW FOR COHERENT MICRODIFFRACTION FROM AMORPHOUS MATERIALS

Electron microdiffraction from amorphous solids using nanometric probe s in FEG-STEMs frequently shows breakdown of Friedel's Law, i.e. the d iffraction pattern is no longer centrosymmetric. We have previously si mulated the effect using simple models for uniform disordered material s. Here, we present an analytical demonstration for the case of a weak phase object with a finite probe of coherent illumination. This shows that the breakdown is due to lens aberrations and/or mixing, in recip rocal space, of the broadened straight-through beam with the diffracte d beams. In our analysis these appear as cross-terms which are normall y zero for an infinite incident plane wave. The sensitivity of this ef fect on parameters such as probe size and thickness are investigated w ith the aid of simulations for our model disordered system.