“Energy selection channels” for high-performance electrolyte : anion-Frenkel defect pair as dominant source for O ion conductions in pyrochlore-type lanthanide hafnium oxides SOFC

Abstract

The excellent ion conductivities of pyrochlore-type materials are believed to be based on oxygen anion transportations caused by the intrinsic defects, in which the anion Frenkel (a-Fr) pair (VO+IO) defect is the most stable one that lacks detailed study. The partially disordered pyrochlore with formation of the a-Fr pair defect will result in more disorder in local pyrochlore structure and increase number of possible migration paths for oxygen anions, which could further improve the ion conductivities of materials. Hence, we studied the formation of a-Fr defect pairs in La2Hf2O7 as a representative pyrochlore structure by density functional theory (DFT) calculations. Three types of defect migration sites were discovered with the ability to incorporate interstitial oxygen atoms from 48f sites and form a-Fr defect pairs (IO+VO (48f)). Besides the most stable vacant 8a sites with lowest defect formation energy of 3.49 eV/pair, two other novel migration sites have been first reported with ability to form a-Fr pair defect with formation energies of 6.53 and 8.49 eV/pair, respectively. These two new types of migration path, as intermediate sites, could construct a diffuse channel with vacant 8a site for interstitial oxygen anions diffusion in the lattice and significantly decrease the distance and barrier of each jump for oxygen atoms. In contrast with the oxygen interstitial defects, the formation of a-Fr pair defect shows higher priority because of much lower formation energies. Since oxygen anions could be easier to generate and diffuse in the pyrochlore structure, the a-Fr pair defect can be explained as the origin of excellent ion conductivities of pyrochlore materials. This work provides a detailed understanding of relationship between intrinsic defects and electronic properties, which enable us to predict electronic properties of other pyrochlore-type materials in the future study.