Percolation Transition in Hole-Conducting Acceptor-Doped Barium Zirconate

Abstract

Acceptor-doped perovskite oxides are highly attractive materials for renewable energy applications such as protonic ceramic fuel cells. Cathode materials with mixed protonic and electronic conductivity are then highly desirable. Here, we consider acceptor-doped BaZrO3 as a potential mixed conductor using the density functional theory. We study the effect of the dopant atom on the formation of electronic hole polarons and establish that doping of BaZrO3 with Fe and Y, respectively, leads to completely different polaron configurations, in which the polaron is centered on the dopant when BaZrO3 is Fe-doped and on an oxygen ion when BaZrO3 is Y-doped. These differences are attributed to the difference in covalency between Fe-O and Y-O bonds, with the former having a much more covalent character. The stronger covalency also results in a stronger association of the polaron with Fe than Y. With a large Fe concentration of 25%, the acceptor state is significantly smeared in energy with a near closing of the electronic band gap. At this concentration level, Fe-doped BaZrO3 is thus on the verge of becoming metallic along the Fe-clusters, supporting the experimental finding of percolation conductivity in Fe-doped BaZrO3 [Kim et al., Solid State Ionics 2014, 262, 875]. In contrast, the effect of increased dopant concentration on the electronic structure in Y-doped BaZrO3 is only minor. Our results provide new atomistic insights into hole transport in acceptor-doped BaZrO3 and highlight the importance of the chemical nature of the dopant atom on the material's electronic properties.