Atomistic Structure Determination by Incoherent Electron Scattering
Transmission electron microscopy offers two physically different routes to achieve an imaging of materials structures at atomic resolution: Coherent and incoherent imaging. Coherent high-resolution imaging in the fixed beam mode (HRTEM) is a well-established method to obtain atomically resolved images of materials and their defects (e.g. grain boundaries). However, the HRTEM images are difficult to interpret quantitatively, as the interference of the low-angle scattered electron waves suffers highly from coherency effects and therefore the information in the image is strongly mixed. Incoherent imaging in the scanning mode by means of high-angle scattering originating from a strongly focussed probe (HAADF-HRSTEM) overcomes this drawback, as the information originating from one atomic column is highly localised in the image. Additionally, the number of electrons scattered into high angles is proportional to Z², which makes image interpretation easier. The example shown here demonstrates the successful application of this new methodical capability at the IFW Dresden.
Using the HAADF-HRSTEM technique the atomistic structure of Cs(Nb,W)5O14 was imaged in an orientation that reveals ring-like structural features. Analysis of the obtained images clearly reveals 6-fold and 7-fold rings, which possess a fully and only partially occupied centre, respectively. The direct interpretability of the image reveals also, that the centre atoms inside the 7-fold rings are slightly shifted to an “off-centre” position.