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.

(a) Atomistic structure of a Cs(Nb,W)<sub>5</sub>O<sub>14</sub> unit cell in [001]-projection showing Nb, W in black, Cs in pink, and Oxygen in red. (b) HAADF-HRSTEM image of the structure with an inset of the unit cell which showsthe one to one correspondence of image contrast to the position of the cations. The arrow points to the “off-centre” position inside the 7-fold ring.
(a) Atomistic structure of a Cs(Nb,W)<sub>5</sub>O<sub>14</sub> unit cell in [001]-projection showing Nb, W in black, Cs in pink, and Oxygen in red. (b) HAADF-HRSTEM image of the structure with an inset of the unit cell which showsthe one to one correspondence of image contrast to the position of the cations. The arrow points to the “off-centre” position inside the 7-fold ring.