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

The ionic conduction mechanism in M²⁺-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 M²⁺ ions were the major point defects in M²⁺-doped LaOBr, while Br^- ion vacancies and antisite O^2- ions at Br sites were the major defect types in pure LaOBr. In the relaxed point defect models, doped Mg²⁺ and Zn²⁺ ions were displaced from the initial positions of the La³⁺ 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^2- ions was 0.92 eV, implying that Br^- ion migration via a vacancy system was more probable than O^2- ion migration. The calculated association energies between M_La and V_Br 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 M²⁺-doped LaOBr.