Ab initiostudy of the phase diagram of epitaxialBaTiO3

Abstract

Using a combination of first-principles and effective-Hamiltonian approaches, we map out the structure of BaTiO3 under epitaxial constraints applicable to growth on perovskite substrates. We obtain a phase diagram in temperature and misfit strain that is qualitatively different from that reported by Pertsev et al. [Phys. Rev. Lett. 80, 1988], who based their results on an empirical thermodynamic potential with parameters fitted at temperatures in the vicinity of the bulk phase transitions. In particular, we find a region of 'r phase' at low temperature where Pertsev et al. have reported an 'ac phase'. We expect our results to be relevant to thin epitaxial films of BaTiO3 at low temperatures and experimentally-achievable strains. PACS numbers: 77.55.+f, 77.80.Bh, 77.84.Dy, 81.05.Zx The perovskite oxide barium titanate (BaTiO 3 ) is a prototypical ferroelectric, an insulating solid whose macroscopic polarization can be reoriented by the application of an electric field [1]. In the perovskite ferroelectrics, it is well known both experimentally and theoretically that the polarization is also strongly coupled to strain [2], and thus that properties such as the ferroelectric transition temperature and polarization magnitude are quite sensitive to external stress.Experimentally, the properties of ferroelectrics in thin film form generally differ significantly from those in the bulk [3]. While many factors are expected to contribute to these differences, it has been shown that the properties of perovskite thin films are strongly influenced by the magnitude of the epitaxial strain resulting from latticematching the film to the substrate. For example, Yoneda et al.[4] used molecular-beam epitaxy (MBE) to grow BaTiO 3 (lattice constant of 4.00Å) on (001)-oriented SrTiO 3 (lattice constant of 3.91Å); they found that the ferroelectric transition temperature exceeds 600 • C, to be compared to the bulk Curie temperature of T C = 130 • C. Other studies have shown that the amount of strain in BaTiO 3 /SrTiO 3 superlattices on SrTiO 3 substrates strongly influences properties including the observed polarization, phase transition temperature, and dielectric constant [5,6,7,8].In a seminal paper, Pertsev, Zembilgotov and Tagantsev [9] introduced the concept of mapping the equilibrium structure of a ferroelectric perovskite material versus temperature and misfit strain, thus producing a "Pertsev phase diagram" (or Pertsev diagram) of the observable epitaxial phases. The effect of epitaxial strain is isolated from other aspects of thin-film geometry by computing the structure of the bulk material with homogeneous strain tensor constrained to match a given substrate with square surface symmetry [10]. In addition, short-circuit electrical boundary conditions are imposed, equivalent to ideal electrodes above and beneath the film [9]. Given the recognized importance of strain in determining the properties of thin-film ferroelectrics, Pertsev diagrams have proven to be of enormous interest to experimentalists seeking to interpret the ...

Electrostatic engineering of strained ferroelectric perovskites from first principles

“…This is analogous to the phase occurring in BaTiO 3 under similar conditions [44]. Finally, (iii) at a large enough value of the tensile strain a second phase transition occurs, where an AFD + z mode appears; as a consequence, the symmetry of the system reduces to P mc2 1 as recently reported in Refs.…”

Near room-temperature multiferroic materials with tunable ferromagnetic and electrical properties

“…This insensitivity of the polarization with strain is technologically useful since it implies that the polarization persists for a large choice of available substrates. It is also fundamentally interesting since it goes against the common knowledge that in-plane polarization of (001) films should rapidly decrease, and even disappear, when decreasing the in-plane lattice constant (that is, when the strain becomes compressive and large in magnitude) [27][28][29] . In fact, and as consistent with what was recently found for rare-earth orthochromates 20 , such a unusual and weak dependence of the in-plane polarization on the strain is due to the fact that a larger epitaxial compression tends to reduce the in-plane antiphase tilting o R while increasing the out-of-plane in-phase tilting o M .…”

Lattice screening of the polar catastrophe and hidden in-plane polarization in KNbO3/BaTiO3interfaces

“…IV B. We have not considered the influence of octahedral rotations over these interfaces as both BaTiO 3 and KNbO 3 are highly resistant to this kind of distortion, even when BaTiO 3 thin films are strained to match the SrTiO 3 lattice parameter, 52 or KNbO 3 films are grown under a higher 5% compressive in-plane strains. 53 Starting from ideal reference structures, built by piling up the corresponding unit cells of bulk strained materials without rumpling, the atomic coordinates are relaxed until the maximum component of the force on any atom was smaller than 0.01 eVÅ −1 .…”

“…52 In particular, when KNbO 3 or BaTiO 3 thin films are grown on top of a SrTiO 3 substrate, the in-plane polarization of these materials vanishes and the tetragonal c phase is stabilized. 60,61 However, contrary to current thought, in our calculations of the BaTiO 3 /KNbO 3 interfaces, we can observe in Fig.…”