Identification of Mn site in Mn-doped SrTiO3: First principles study

Manganese can incorporate into perovskite SrTiO3 at either on the Sr-site (A-site) or Ti-site (B-site) depending on the growth conditions. However, there is no clear explanation on what conditions lead to A-site or B-site occupations and what are the local structures around Mn. In this work, first-principles calculations were carried out to study Mn in SrTiO3 under different growth conditions. The calculations showed that Mn prefers to substitute for Ti site under a wide range of growth conditions; indicating that the substitution it is difficult to avoid even when one intentionally tries to dope Mn into the A-site. Due to a large size difference, Mn cannot fit in the Sr site. Instead, a charged interstitial Mn has rather low formation energy, especially under O-poor conditions. In addition, Mni can bind with Sr vacancy which has the compensating charge with a very high binding energy to form Mni-VSr complex. Mni-VSr has an equivalent chemical composition to MnSr. Therefore, one might view Mni-VSr as an off-center MnSr. In this view point, Mn A-site has a large cation off-centering consistent with experimental observations. We further relate our local structure models with the experiments by calculating the Mn K-edge x-ray absorption near-edge spectroscopy (XANES) spectra associated with MnTi, MnSr (on regular Sr lattice), and Mni-VSr or off-center MnSr defects to compare with the measured Mn K-edge XANES spectra. Our results show good agreement with the measured spectra.