Dislocation Modelling: Calculating EELS Spectra for Edge Dislocation in Bismuth Ferrite

Abstract

Recent advances in electron energy‐loss spectroscopy (EELS) triggered by the implementation of aberration correctors and novel spectrometers have enabled atomic resolution and single atom sensitivity. The energy‐loss near‐edge structure (ELNES) in core‐loss EELS provides insight into the electronic structure of individual atomic species containing information about their bonding characteristics such as, e.g., oxidation state, charge transfer and site coordination. Yet the electronic structure information is buried in the spectral fine structure which can be regarded as a “fingerprint” of the atom's bonding characteristics. In order to overcome this shortcoming of a purely experimental approach, the ELNES of core‐loss EELS ionization edges can be obtained from first‐principles electronic structure calculations. BiFeO 3 (BFO) is a multiferroic perovskite that exhibits antiferromagnetism coupled with ferroelectric order. Besides, because of their astounding electromechanical properties, BFO thin films are promising candidates for the replacement of lead‐based ceramics in microelectromechanical system devices. It is well known that performance of ferroelectric devices is reduced by the presence of crystal defects such as edge dislocations. This type of crystal defects within this material 1 are due to the lattice mismatch between the BFO film and the SrRuO 3 substrate as strain compensation mechanism. However, the electronic structure close to the dislocation core is not yet well understood. In this work, we investigate the influence of edge dislocations on the material's local electronic properties, using a combined experimental and theoretical strategy based on HAADF‐STEM, EELS and atomistic simulations. In this study the edge dislocation within BFO was modelled based on Peierls‐Nabarro Model (P‐N model) 2 theory. The initial guess was obtained from the P‐N model and further optimized using BVVS classical potentials for BFO 3 as incorporated in the LAMMPS package 4 . Fig 1 shows the final geometry of the edge dislocation model obtained after optimizing it with this potential. FEFF9 5 , a real space multi‐scattering Core level spectroscopies code will be used on the final optimized model to obtain the O K‐edge and Fe L‐edge spectra at the dislocation core. Fig 2 shows the experimentally recorded O K‐edge EELS on a line of atomic columns (points 1 to 9) crossing the dislocation core. The comparison of the experimental spectrum with the calculated EELS allows shedding atomistic insight on the EELS peak structure. In particular, it is possible to explain to which extent a defect in a bulk material (in this case, an edge dislocation) locally affects its electronic properties, thus enhancing the power of electron energy‐loss spectroscopy in the high‐resolution description of complex materials.