Computer simulations of functional solid oxides
Creators: name: Wu, Ji
Publisher: name: Imperial College London
Year: 2016

description: Several novel mixed ionic-electronic conductors (MIECs) based on the oxygen interstitial diffusion mechanism have been developed and studied for the past decade. This kind of materials has the potential to be used as the cathode of intermediate temperature solid oxide fuel cell (IT-SOFC) systems. Among them, the La2NiO4+δ Ruddlesden-Popper (RP) and the CeNbO4+δ fergusonite materials attract particular attention due to their interesting properties. These materials are capable to take extra oxygen and become oxygen ion conductor without any aliovalent doping. To understand more about their catalytic and diffusion behaviours, atomistic computer simulations using both classical pair potentials and density functional theory (DFT), as well as classical thermodynamics calculations, have been carried out on these materials. The La2NiO4+δ material has recently been discovered to exhibit a La-only surface. This observation disagrees with previous theoretical study which suggest a Ni-terminated surface and challenges the traditional belief that the oxygen reduction takes place on La2NiO4+δ's exposed Ni sites. Our hybrid DFT surface energy calculation suggests that the most stable (001) La2NiO4 surface is La terminated. Previous theoretical surface study's results may not appropriately reflect the room temperature La2NiO4 sample. To explain the La-only surface observed on the polycrystalline La2NiO4+δ samples, a thermodynamic decomposition based hypothesis is proposed. The evaluation of La2NiO4's stability with respect to the higher ordered RP materials (La3Ni2O7, La4Ni3O10 and LaNiO3) shows that the decomposition of La2NiO4 is thermodynamically favourable and supports our hypothesis. The defective La-Ni-O RP material's decomposition thermodynamics are estimated with the help of hybrid DFT calculations and agree with our hypothesis too. The decomposition hypothesis predicts a Ni-enriched subsurface layer and agrees with the experiments. The La vacancy formation energy and diffusion barriers are calculated with hybrid DFT methods, as these results may provide hints on why the thermodynamically favourable decomposition only limits to the surface of La2NiO4. Before these calculations, different input La2NiO4 phases and magnetic orderings are evaluated to check their impacts on simulation results' reliability. We have shown that the choice of input structure and magnetic ordering will have significant effect on the simulation results. Appropriate choice of the input setups is therefore very important to the quality of the simulation. Based on the evaluation, La2NiO4's room temperature Fmmm phase and gx type anti-ferromagnetic ordering are selected for the DFT calculations. The La vacancy formation energy and diffusion barrier calculated are compared to the oxygen defect formation energy and related diffusion barrier. Possible relations between the La2NiO4 decomposition reaction and the La vacancy's formation/diffusion energetics are also discussed. Similar to the La2NiO4+δ, the CeNbO4+δ fergusonite material has a range of different oxygen enriched phases and is able to conduct oxygen ions. Its characteristic complex modulated structures also attracts much attentions. The details of its modulated oxygen-rich structures remained unclear for a long time. Recently the CeNbO4.25 modulated structure has been fully solved and opens opportunity for further theoretical studies. Molecular dynamics simulations using classical pair potentials are carried out on the stoichiometric CeNbO4 and the modulated CeNbO4.25 materials to study the oxygen diffusion pathways for the first time. The calculated oxygen ions' mean-squared displacements clearly show that the stoichiometric CeNbO4 exhibits no oxygen self-diffusion up to 1773 K. In addition, the oxygen diffusion coefficients in the CeNbO4.25 are calculated and plotted over a range of temperatures. Three regions with different oxygen diffusion behaviours are found from the graph and the oxygen diffusion activation energies are calculated for each of the regions. The differences and similarities between the computed oxygen diffusion behaviours and the experimentally observed oxygen diffusion behaviours are explored and discussed. Finally, the oxygen diffusion trajectory snapshots are taken from the MD simulation. The exact oxygen diffusion pathways and possible diffusion anisotropicities of the CeNbO4.25 material are analysed and discussed based upon the diffusion trajectories.