Seminars Archive

Quantitative Cs corrected Scanning Transmission Electron Microscopy of crystal structures and defects

Abstract
In the seminar some recent results from the structural analysis of selected materials using probe Cs-corrected STEM will be displayed and the influence of the atomic level structure on the material properties will be discussed. The main emphasis will be on the planar crystal defects, such as twin boundaries, stacking faults, inversion domains and ferroelectric domain boundaries. One of the examples will be ferroelectric (Ba,Sr)TiO₃ based materials that undergo at a Curie temperature (Tc) a phase transition from a non-centrosymmetric polar ferroelectric phase to a paraelectric phase. This phase is centrosymmetric (cubic) and the polarisation is lost. However, recently it have been shown that cubic BaTiO₃ phase exhibits breaking of nominal centric symmetry and exhibits polarization which is probably linked to the presence of polar nano-regions. Our aim was a direct visualisation of polar nano-regions in paraelectric (Ba_0.6Sr_0.4)TiO₃ phase based on oxygen atoms displacement measured from ABF STEM images. The chemical composition fluctuations (Ba/Sr ratio) in BST was correlated with appearance of polar nano-regions. The methodology, error estimation and results obtained from ABF and HAADF images, acquired with Cs probe-corrected STEM will be explained and discussed. Another example will be multiferroic bismuth ferrite (BiFeO₃) with Neel and Curie temperatures well above room temperature that is a prospective material for high-temperature piezoelectric devices. A serious drawback is its high electrical conductivity, which has recently been discovered to be localized at domain walls (DWs). While the origin of this local conduction was theoretically explained in terms of accumulation of charged defects at domain walls, no experimental evidence of these defects has been provided. Recently, we have reported that in contrast to the usually assumed oxygen vacancies, the dominant defects in BiFeO3 are bismuth vacancies along with compensating electron holes, associated with the presence of Fe4+. Novel method for local vacancy concentration determination will be explained and discussed. In the field of catalysis the structure and defects in Cu₃Pt nanoparticles and graphene-Fe-N clusters used for proton exchange membrane fuel cells will be addressed and correlated with the properties.