During the last decades, increasing progresses have been made on the theoretical study of ferroelectric materials since the discovery of the barium titanate (BaTiO3). The ab-initio methods are based on quantum mechanics, which takes into account only fundamental physical constants. This work aims the study of the electronics properties of perovskite oxides based on BaTiO3, considering the inclusion of rare-earth ions as impurities into the structure, by using the density functional theory (DFT). The presence of structural defects, such as vacancies, as well as their influence on the physical properties, has been investigated.