Mechanisms of point defect formation and ionic conduction in divalent cation-doped lanthanum oxybromide: first-principles and experimental study.

The ionic conduction mechanism in M-doped (M: Mg, Ca, Zn, and Sr) lanthanum oxybromide (LaOBr) was investigated theoretically and experimentally. Formation energy calculations of point defects revealed that Br ion vacancies and substitutional M ions were the major point defects in M-doped LaOBr, while Br ion vacancies and antisite O ions at Br sites were the major defect types in pure LaOBr. In the relaxed point defect models, doped Mg and Zn ions were displaced from the initial positions of the La ions, and this was experimentally supported by crystal structural analysis. These significant atomic shifts were probably due to the strong interactions between Br and the dopant ions. First-principles calculations and experimental analyses using X-ray photoelectron spectroscopy and X-ray absorption fine-structure spectroscopy also suggested the existence of strong interactions. The migration energy of Br ions was calculated to be 0.53 eV, while the migration energy of O ions was 0.92 eV, implying that Br ion migration a vacancy system was more probable than O ion migration. The calculated association energies between M and V were 0.4-0.6 eV, suggesting that the association needed to be disrupted for Br ion conduction. The sum of the association and migration energies was comparable to the experimental association energies of M-doped LaOBr.