Predicting the Electronic Structure of CeO₂ Grain Boundaries for Comparison with Atomic Resolution EELS

Abstract

Intermediate temperature solid oxide fuel cells are an attractive energy source for converting hydrocarbon fuels into electrical energy. However, the highly resistive grain boundaries (GB) in these polycrystalline electrolytic materials leads to a significant decrease in the overall ionic conductivity. One of the most popular theories for the origin of this increased resistivity across electrolytic GB's is an intrinsic resistance arising from a so-called space charge potential region surrounding the GB core. Recent electron energyloss spectroscopy (EELS) analysis on doped CeO2 shows that grain boundary dopant segregation may also be a significant factor influencing oxygen migration energy [1,2]. To gain a fundamental understanding of the underlying GB charge transport mechanisms in ceria-based oxides, we are modelling the relationship between GB character, composition, electronic structure and conductivity. Initially, density functional theory was employed to determine the bonding for a select number of stoichiometric CeO2 symmetric tilt boundaries. Preliminary calculations are compared with experimentally determined EELS from CeO2.All computations were performed using the local density approximation (LDA) exchange-correlation functional with spin-polarization considered. Within LDA formalism, the Kohn-Sham equations were solved through the implementation of projector augmented wave (PAW) method using a plane-wave basis set as implemented in VASP [3]. The energies were converged within 1 meV with k-point sampling during all optimization carried out on a Monkhorst-Pack grid of 8x8x8 for bulk calculations and 1x5x5 for the GB supercells with a plane-wave cut-off of 400 eV. For a more accurate representation of the Fermi level and partial occupancies, the tetrahedron method with Blӧchl corrections and the Davidson-block iteration scheme was used.To validate the choice of pseudopotential, a comparison of simulated and experimental EELS oxygen Kedge was carried out for bulk CeO2. Figure 1 shows a good qualitative match between experiment and theory. The first peak in the simulated oxygen energy-loss spectrum is due to the 1s4f type transition. Our current DFT model fails to correctly cancel out the self-interaction energy leading to delocalization of the Ce 4f electrons which shifts the 4f levels down in energy. The simulate edge shape matches most of the predictions from other DFT methods seen in literature which show similar relative peak height trends [4]. Grain boundary structures were modelled using the method proposed by Braithwaite et al [5]. Our study has focused on the Σ3(210)/[001] symmetric tilt GB of stoichiometric CeO2 and the system was allowed to relax in three dimensions. Significant restructuring of the GB core was observed with atoms moving as much as 1 Å, as shown in Figure 2. To understand the electronic and local bonding changes associated with the GB, angular momentum site-projected density of states (DOS) was calculated for oxygen atoms at the bulk, the GB core and the GB side as depicted in F...