Interface-induced enhancement of piezoelectricity in the (SrTiO3)m/(BaTiO3)M−m superlattice for energy harvesting applications

Abstract

We present the results of a detailed first principles study of the piezoelectric properties of the (SrTiO₃)_m/(BaTiO₃)_M−m heterostructure using the 3D STO_m/BTO_M−m superlattice model. The atomic basis set, hybrid functionals and slabs with different numbers of STO and BTO layers were used. The interplay between ferroelectric (FE_z) and antiferrodistortive (AFD_z) displacements is carefully analyzed. Based on the experimental data and group theoretical analysis, we deduce two possible space groups of tetragonal symmetry which allow us to reproduce the experimentally known pure STO and BTO bulk phases in the limiting cases, and to model the corresponding intermediate superlattices. The characteristic feature of the space group P4mm (#99) model is atomic displacements in the [001] direction, which allows us to simulate the FE_z displacements, whereas the P4 (#75) model besides FE_z displacements permits oxygen octahedra antiphase rotations around the [001] direction and thus AFD_z displacements. Our calculations demonstrate that for m/M ≤ 0.75 layer ratios both models show similar geometries and piezoelectric constants. Moreover, both models predict an approximately 6-fold increase of the piezoelectric constant e₃₃ compared to the BaTiO₃ bulk value, albeit at slightly different layer ratios. The obtained results clearly demonstrate that piezoelectricity arises due to the coordinated collective FE_z displacements of atoms in both STO and BTO slabs and interfaces and reaches its maximum when the superlattice approaches the point where the tetragonal phase becomes unstable and transforms to a pseudo-cubic phase. We demonstrate that even a single or double layer of BTO is sufficient to trigger FE_z displacements in the STO slab, in P4mm and P4 models, respectively.