Optical signatures of defects in BiFeO3

Optical absorption in rhombohedral ${\mathrm{BiFeO}}_{3}$ starts at photon energies below the photoemission band gap of $\ensuremath{\approx}3$ eV calculated from first principles. A shoulder at the absorption onset has so far been attributed to low-lying electronic transitions or to oxygen vacancies. In this paper, optical spectra are calculated ab initio to determine the nature of the optical transitions near the absorption onset of pristine ${\mathrm{BiFeO}}_{3}$, the effect of electron-hole interaction, and the spectroscopic signatures of typical defects, i.e., doping (excess electrons or holes), intrinsic defects (oxygen and bismuth vacancies), and low-energy structural defects (ferroelectric domain walls).