Lattice instabilities of BaTiO₃/SrTiO₃ artificial superlattice

We have investigated lattice instabilities of ultra-short period BaTiO ₃VSrTIO₃ (BTO/STO) artificial superlattice (double-perovskite 10-atom unit cell) as a function of epitaxial strain using first-principles density functional theory (DFT) calculations based on pseudopotentials and a plane-wave basis. We find a structural transition from the tetragonal phase at compressive strain to the monoclinic phase at tensile strain. In the tetragonal phase, the Ti ions displacement patterns of the BTO/STO superlattice show ferrielectric phase behavior. Likewise, those of the monoclinic BTO/STO superlattice along [001] direction are very similar to the tetragonal BTO/STO superlattice. However, the displacement patterns in the xy-plane BTO/STO, superlattice exhibit different behavior which both Ti ions displace parallely along [110] direction and against direction of oxygen displacements. It means that the monoclinic phase of BTO/STO superlattice is ferrielectric along [001] direction similar to tetragonal BTO/STO superlattice, but is ferroelectric in the xy-plane.