Acceptor doping in the proton conductor SrZrO₃

Perovskite zirconates such as SrZrO₃ exhibit improved proton solubility and conductivity when doped with trivalent cations substituting at the Zr site. In this work, we present a detailed study of Sc and Y dopants in SrZrO₃ based on first-principles, hybrid density-functional calculations. When substituting at the Zr site (Sc_Zr, Y_Zr), both dopants give rise to a single, deep acceptor level, where the neutral impurity forms a localized hole polaron state. The ε(0/-) charge transition levels are 0.60 eV and 0.58 eV above the valence-band maximum for Sc_Zr and Y_Zr, respectively. Under certain growth conditions, Sc and Y will form self-compensating donor species by substituting at the Sr site (Sc_Sr, Y_Sr), and this is detrimental to proton conductivity. Due to its larger ionic radius, Y exhibits a greater tendency than Sc to self-compensate at the Sr site. We also investigated the proton-dopant association. The binding energy of a proton to a negatively charged acceptor impurity is 0.41 eV for Sc and 0.31 eV for Y, indicating that proton transport is limited by trapping at impurity sites.