Localized electronic states induced by defects and possible origin of   ferroelectricity in strontium titanate thin films
  Several defect configurations including oxygen vacancies have been investigated as possible origins of the reported room-temperature ferroelectricity of strontium titanate (STO) thin films [Appl. Phys. Letts. 91, 042908 (2007)]. First-principles calculations revealed that the Sr-O-O vacancy complexes create deep localized states in the band gap of SrTiO3 without affecting its insulating property. These results are in agreement with electronic structural changes determined from optical transmission and X-ray absorption measurements. This work opens the way to exploiting oxygen vacancies and their complexes as a source of ferroelectricity in perovskite oxide thin films, including STO. 
Yong Su Kim, J. Kim, S. J. Moon, W. S. Choi, Y. J. Chang, J.-G.Yoon,   J. Yu, J.-S. Chung, and T. W. Noh