Theoretical study of native defects and positron annihilation states in BiOBr

In this work, we calculated the electronic properties and formation energies of various defects in BiOBr using first principles calculations based on density functional theory. The calculated formation energies of Bi, O and Br vacancies are 9.85 eV, 3.66 eV and 1.9 eV, respectively, which suggests that the Br vacancy has the highest formation probability. We also calculated the positron trapping states of BiOBr in the perfect bulk state and vacancy trapping state. The positron bulk lifetime in BiOBr crystal is 221 ps, and the positron wave function is distributed in the layer gap. Positrons are insensitive to O vacancies, with lifetime the same as the bulk lifetime, and the positron wave function is delocalized and distributed in the layer gap region. However, the positron lifetimes in Bi and Br vacancies are 234 ps and 265 ps, respectively, and the positron wave function is localized at vacancy sites.