Tunable ferroelectric polarization of the bulk and free standing ATiO3 (A = Ba and Pb) thin films via unit-cell thicknesses and strain engineering

Ab-Initio calculations are used to investigate the spontaneous polarization in the bulk and free standing ATiO3 (A = Ba and Pb) thin films by increasing the corresponding unit-cell thickness in [001]-direction and by adjusting the bi-axial strain (both compressive and tensile) in ab-plane [110]. Our results exhibit that polarization gradually increases with the increase of unit-cell growth and compressive/tensile strain has a positive/negative impact on the spontaneous polarization for both bulk materials. For free standing thin films, it is found that polarization is strongly suppressed below a critical value of film thickness, due to depolarizing the intrinsic electric field induced by un-compensated dipoles at the surfaces. In BaTiO3 thin film, 8.5/6 unit-cells (32/26 Å) are predicted to be the critical thickness for BaO-BaO/BaO-TiO2 symmetric/asymmetric surface. In contrast, polarization gradually increases for TiO2-TiO2 symmetric surface upto 6.5 unit-cells and then raises sharply. In the case of PbTiO3 thin film, 6.5 and 6 unit-cells (23 and 22 Å) are estimated the critical thicknesses for symmetric [(PbO-PbO) and (TiO2-TiO2)] and asymmetric (PbO-TiO2) surfaces, respectively. Compressive and tensile strains display an increasing and decreasing trend for all thin film structures. However, the impact of compressive strain on the polarization is larger than tensile. Furthermore, we predict that asymmetric (AO-TiO2) surfaces in both films show higher polarization as compared to symmetric [(AO-AO) and (TiO2-TiO2)] surfaces due to large octahedral distortions.