Small polarons and point defects in LaFeO$_3$

The proton-conductive perovskite-type LaFeO$_3$ is a promising negative-electrode material for Ni/metal-hydride (Ni-MH) batteries. It has a discharge capacity up to 530 mAhg$^{-1}$ at 333~K, which is significantly higher than commercialized AB$_5$-type alloys. To elucidate the underlying mechanism of this performance, we have investigated the structural and electronic properties of bulk LaFeO$_3$, as well as the effect of point defects, using hybrid density functional methods. LaFeO$_3$ is antiferromagnetic in the ground state with a band gap of 3.54~eV. Small hole and electron polarons can form through self- or point-defect-assisted trapping. We find that La vacancies and Sr substitutional on La sites are shallow acceptors with the induced holes trapped as small polarons, while O and Fe vacancies are deep defect centers. Hydrogen interstitials behave like shallow donors, with the donor electrons localized on nearby iron sites as electron polarons. With a large trapping energy, these polarons can act as electron or hole traps and affect the electrical performance of LaFeO$_3$ as the negative electrode for Ni-MH batteries. We acknowledge DOE for financial support.