<div>Y-doped BaZrO<sub>3</sub> is an ion conductor under intense research for application in medium temperature solid oxide fuel cells. The conductivity is maximized at ~20% doping, and the decrease with further doping has often been attributed to the association effect, or the trapping of ionic charge carriers by the dopant. This seems like a reasonable conjecture since the dopant and carrier are charged in opposite polarities</div><div>and should attract each other. However, at such high doping concentrations, many-body interactions between nearby dopants and carriers are likely to modify such a simple two-body attraction picture. Thus, in this work, we employ a large-scale first-principles thermodynamic sampling scheme to directly examine the configuration of dopants and charge-compensating defects at realistic doping concentrations under processing conditions. We find that although there is, indeed, a clear Y<sub>Zr</sub> – V<sub>O </sub>association effect at all doping concentrations examined, the magnitude of the effect actually decreases with increasing dopant concentration. We also find that Y<sub>Zr</sub>–Y<sub>Zr </sub>and V<sub>O</sub> –V<sub>O </sub>interactions cannot simply be understood in terms of two-body Coulomb attraction and repulsion, highlighting the importance of many-body effects in understanding the defect chemistry</div><div>in heavily doped oxides. Finally, we examine the dopant configurations and successfully explain the conductivity maximum based on a percolation vs. trapping picture that has gained attention recently.</div>
First principles calculation of the potential energy surface for the lowest-quartet state of H3 and modelling by the double many-body expansion method
