Direct phase determination from EM images

Abstract

EM images contain phase information, in contrast to X-ray and electron diffraction patterns. The phases are extracted from the Fourier transform (FT) of the EM image. However, there has been doubts as to what exactly this phase information is. Are the EM phases the same as those of the crystallographic structure factors F(hkl) as defined in x ray crystallography? Or are the phases so distorted by dynamical scattering that very sophisticated procedures are needed for phase retrieval, as has been suggested.We have investigated this question, by comparing phases obtained from EM-images with those of the x ray structure factors from two accurately refined barium niobium oxide crystals.EM images of Ba4Nb14O23 (orthorhombic Cmmm, a=20.782, b=12.448, c=4.148 Å) and Ba3Nb16O23 (orthorhombic Cmmm, a=20.930, b=12.478, c=4.162 Å) were taken with a Philips CM 30 300kV and a JEOL 200CX 200kV electron microscope. Thin areas near the edge were analyzed by our newly developed image processing system, CRISP.