STRUCTURE-CONDUCTIVITY RELATION IN OXYGEN ION CONDUCTORS: DOPED CERIA AND LA-MELILITES

摘要

Oxygen ion conductors are fundamental materials for various electrochemical applications such as solid oxide fuel cells. On the microscopic level, the transport of individual ions is determined by the migration barriers and the interactions between defects, which depend on the structure of the material and the response to local disorder. Density functional theory (DFT) calculations allow the study of both structures and defect energies. In this paper, the relation between structural details and defect energies is investigated on the basis of two examples: The effect of strain on vacancy migration and interaction in biaxially strained, doped ceria and the structural distortions during interstitialcy migration in melilite structured La_(1+x)Sr_(1-x)Ga_3O_(7+x/2). The migration barriers and interactions in ceria with small (Lu~(3+)), medium sized (Gd~(3+)) or large (La~(3+)) dopant ions are calculated for different strain states. Results show that the migration energies are affected by both the strain and the ionic radius of the cations along the migration path, which can be combined in a critical radius model. The interactions between vacancies and dopants are primarily determined by the dopant radius with an additional contribution due to the applied strain that cannot be explained by simple electrostatic interactions. In the melilites the site energies and migration barriers of oxygen interstitials depend on the occupation of surrounding cation sites by La~(3+) and Sr~(2+) ions. Calculations show an increasing stability of the interstitials with increasing number of La~(3+) ions. The transport of interstitial ions by an interstitialcy mechanism in the a/b-plane is facilitated by the flexible framework of corner-shared GaO_4 units and the relaxation of the GaO_4 tetrahedrons allow migration barriers of 0.15 eV and below. The DFT derived energies are applied in Kinetic Monte Carlo simulations to obtain the ionic conductivity, thus relating the microscopic processes with the macroscopic transport properties.