Oxygen vacancy formation energies in PbTiO3/SrTiO3 superlattice

Abstract

The defect stability in a prototypical perovskite oxide superlattice consisting of SrTiO3 and PbTiO3 (STO/PTO) is determined 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 the 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 (a 0 a 0 c -) in the FE superlattice have a significant effect on the 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 via controlling the constituent materials of superlattices and interface engineering.

Emergent Ferroelectricity in Otherwise Nonferroelectric Oxides by Oxygen Vacancy Design at Heterointerfaces

“…In ABO 3 perovskite oxide heterostructures, point defects can occupy different atomic sites, which will impose different impacts on crystal symmetry, e.g., oxygen vacancy in the BO 2 layer vs oxygen vacancy in the AO layer at interfaces . Achieving desired properties usually requires good control of oxygen vacancy occupying a desired atomic site. , Furthermore, the as-grown sample will experience a high-temperature thermodynamics process during growth, which favors disordering defect structures .…”

DFT computations combined with semiempirical modeling of variations with temperature of spectroscopic and magnetic properties of Gd3+-doped PbTiO3

“…For the gas phase, the pressure is nonzero, but this case is not considered here, since we deal with solid phase only. We have computed the formation energies of the defects, E D f , as follows: 88–90 E D f ≡d = E (Pb 0.875 Gd 0.125 TiO 3 ) − E (PbTiO 3 ) + E(Pb) − E(Gd), E D f ≡Gd = E (PbTi 0.875 Gd 0.125 O 3 ) − E(PbTiO 3 ) + E(Ti) − E(Gd), E D f ≡V O = E (Pb 0.875 Gd 0.125 TiO 3− δ ) − E (Pb 0.875 Gd 0.125 TiO 3 ) + E(O), E D f ≡V O = E (PbTi 0.875 Gd 0.125 O 3− δ ) − E (PbTi 0.875 Gd 0.125 O 3 ) + E (O),where D stands for either Gd dopant or oxygen vacancy V O , the first terms in eqn (3)–(6) are the total energies derived from a supercell calculation with dopant or defect doped in the cell and the second terms are the total energies of the ideal supercell. The third and the last terms of eqn (3) and (4)) are the ground state total atomic energies of Pb and Gd (Ti and Gd) atoms, respectively.…”

“…Although taking the O 2 molecule as reference has led to bit cumbersome and more expensive computations, we have considered it to increase the accuracy of the computations (see ref. 88).…”

“…For the gas phase, the pressure is nonzero, but this case is not considered here, since we deal with solid phase only. We have computed the formation energies of the defects, E D f , as follows: 88-90…”

“…Although taking the O 2 molecule as reference has led to bit cumbersome and more expensive computations, we have considered it to increase the accuracy of the computations (see ref. 88). Furthermore, in addition to the APW + lo code, we have computed the atomic energies using a modified version of the relativistic atomic LSDA code originally written by J. P. Desclaux and Y.-K. Kim.…”