Oxygen vacancy formation energies in PbTiO3/SrTiO3 superlattice

The defect stability in a prototypical perovskite oxide superlattice consisting of SrTiO3 and PbTiO3 (STO/PTO) is determined by using first principles density functional theory calculations. Specifically, the oxygen vacancy formation energies Ev in the paraelectric and ferroelectric phases of a superlattice with four atomic layers of STO and four layers of PTO (4STO/4PTO) are determined and compared. The effects of charge state, octahedral rotation, polarization, and interfaces on Ev are examined. The formation energies vary layer by layer in the superlattices, with Ev being higher in the ferroelectric phase than that in the paraelectric phase. The two interfaces constructed in these oxide superlattices, which are symmetrically equivalent in the paraelectric systems, exhibit very different formation energies in the ferroelectric superlattices and this can be seen to be driven by the coupling of ferroelectric and rotational modes. At equivalent lattice sites, Ev of charged vacancies is generally lower than that of neutral vacancies. Octahedral rotations (a0a0c-) in the superlattices have a significant effect on Ev, increasing the formation energy of vacancies located near the interface but decreasing the formation energy of the oxygen vacancies located in the bulk-like regions of the STO and PTO constituent parts. The formation-energy variations among different layers are found to be primarily caused by the difference in the local relaxation at each layer. These fundamental insights into the defect stability in perovskite superlattices can be used to tune defect properties by controlling the constituent materials of superlattices and interface engineering.