Interplay of couplings between antiferrodistortive, ferroelectric, and strain degrees of freedom in monodomain PbTiO3_{3}/SrTiO3_{3} superlattices

We report first-principles calculations on the coupling between epitaxial strain, polarization, and oxygen octahedra rotations in monodomain (PbTiO$_{3}$)$_{n}$/(SrTiO$_{3}$)$_{n}$ superlattices. We show how the interplay between (i) the epitaxial strain and (ii) the electrostatic conditions, can be used to control the orientation of the main axis of the system. The electrostatic constrains at the interface facilitate the rotation of the polarization and, as a consequence, we predict large piezoelectric responses at epitaxial strains smaller than those that would be required considering only strain effects. In addition, ferroelectric (FE) and antiferrodistortive (AFD) modes are strongly coupled. Usual steric arguments cannot explain this coupling and a covalent model is proposed to account for it. The energy gain due to the FE-AFD coupling decreases with the periodicity of the superlattice, becoming negligible for $n \ge 3$.Comment: 5 pages, 4 figure

Structural and energetic properties of domains in PbTiO 3 /SrTiO 3 superlattices from first principles

We report first-principles calculations, within the density functional theory, on the structural and energetic properties of 180 ∘ stripe domains in (PbTiO 3 ) n /(SrTiO 3 ) n superlattices. For the explored periodicities (n=3 and 6), we find that the polydomain structures compete in energy with the monodomain phases. Our results suggest the progressive transition, as a function of n , from a strong to a weak electrostatic coupling regime between the SrTiO 3 and PbTiO 3 layers. Structurally, they display continuous rotation of polarization connecting 180 ∘ domains. A large offset between [100] atomic rows across the domain wall and huge strain gradients are observed. The domain wall energy is very isotropic, depending very weakly on the stripe orientation.The authors thank P. Zubko for the valuable discussion and his careful reading of themanuscript. Thisworkwas supported by the Spanish Ministery of Science and Innovation through the MICINN Grant FIS2009-12721-C04-02, by the Spanish Ministry of Education through the FPU fellowship AP2006- 02958 (PAP), and by the European Union through the project EC-FP7, Grant No. CP-FP 228989-2 “OxIDes.” The authors thankfully acknowledge the computer resources, technical expertise, and assistance provided by the Red Española de Supercomputación. Calculations were also performed at the ATC group of the University of Cantabria

Lattice screening of the polar catastrophe and hidden in-plane polarization in KNbO3/BaTiO3 interfaces

We have carried out first-principles simulations, based on density functional theory, to obtain the atomic and electronic structure of (001) BaTiO3/KNbO3 interfaces in an isolated slab geometry. We tried different types of structures including symmetric and asymmetric configurations and variations in the thickness of the constituent materials. The spontaneous polarization of the layer-by-layer non-neutral material (KNbO3) in these interfaces cancels out almost exactly the “built-in” polarization responsible for the electronic reconstruction. As a consequence, the so-called polar catastrophe is quenched and all the simulated interfaces are insulating. A model, based on the modern theory of polarization and basic electrostatics, allows an estimation of the critical thickness for the formation of the two-dimensional electron gas between 33 and 36 KNbO3 unit cells. We also demonstrate the presence of an unexpected in-plane polarization in BaTiO3 localized at the p-type TiO2/KO interface, even under in-plane compressive strains. We expect this in-plane polarization to remain hidden due to angular averaging during quantum fluctuations unless the symmetry is broken with small electric fields

Effect of intrinsic defects on the thermal conductivity of PbTe from classical molecular dynamics simulations

Despite being the archetypal thermoelectric material, still today some of the most exciting advances in the efficiency of these materials are being achieved by tuning the properties of PbTe. Its inherently low lattice thermal conductivity can be lowered to its fundamental limit by designing a structure capable of scattering phonons over a wide range of length scales. Intrinsic defects, such as vacancies or grain boundaries, can and do play the role of these scattering sites. Here we assess the effect of these defects by means of molecular dynamics simulations. For this we purposely parametrize a Buckingham potential that provides an excellent description of the thermal conductivity of this material over a wide temperature range. Our results show that intrinsic point defects and grain boundaries can reduce the lattice conductivity of PbTe down to a quarter of its bulk value. By studying the size dependence we also show that typical defect concentrations and grain sizes realized in experiments normally correspond to the bulk lattice conductivity of pristine PbTe

Ferromagneticlike Closure Domains in Ferroelectric Ultrathin Films: First-Principles Simulations

We simulate from first principles the energetic, structural, and electronic properties of ferroelectric domains in ultrathin SrRuO3/BaTiO3/SrRuO3 ferroelectric capacitors in short circuit. The domains are stabilized down to two unit cells at zero temperature, adopting the form of a domain of closure, common in ferromagnetic thin films. The domains are closed by the in-plane relaxation of the atoms in the first SrO layer of the electrode, which behaves more like SrO in highly polarizable SrTiO3 than in metallic SrRuO3. Even if small, these lateral displacements are very important to stabilize the domains and might provide some hints to explain why some systems break into domains while others remain in a monodomain configuration. An analysis of the electrostatic potential reveals preferential points of pinning for charged defects at the ferroelectric-electrode interface, possibly playing a major role in film fatigue

Domain walls in a perovskite oxide with two primary structural order parameters: First-principles study of BiFeO3

We present a first-principles study of ferroelectric domain walls (FE-DWs) in multiferroic BiFeO3 (BFO), a material in which the FE order parameter coexists with antiferrodistortive (AFD) modes involving rotations of the O6 octahedra. We find that the energetics of the DWs are dominated by the capability of the domains to match their O6 octahedra rotation patterns at the plane of the wall, so that the distortion of the oxygen groups is minimized. Our results thus indicate that, in essence, it is the discontinuity in the AFD order parameter, and not the change in the electric polarization, that decides which crystallographic planes are most likely to host BFO's FE-DWs. Such a result clearly suggests that the O6 rotational patterns play a primary role in the FE phase of this compound, in contrast with the usual (implicit) assumption that they are subordinated to the FE order parameter. Our calculations show that, for the most favorable cases in BFO, the DW energy amounts to several tens of mJ/m2, which is higher than what was computed for other ferroelectric perovskites with no O6 rotations. Interestingly, we find that the structure of BFO at the most stable DWs resembles the atomic arrangements that are characteristic of low-lying (meta)stable phases of the material. Further, we argue that our results for the DWs of bulk BFO are related with the nanoscale-twinned structures that Prosandeev et al. [Adv. Funct. Mater. (2012)] have recently predicted to occur in this compound, and suggest that BFO can be viewed as a polytypic material. Our work thus contributes to shape a coherent picture of the structural variants that BFO can present and the way in which they are related