44 research outputs found

    Toroidal ferroelectricity in PbTiO3 nanoparticles

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    We report from first-principles-based atomistic simulations that ferroelectricity can be sustained in PbTiO3 nanoparticles of only a few lattice constants in size as a result of a toroidal ordering. We found that size-induced topological transformations lead to the stabilization of a ferroelectric bubble by the alignment of vortex cores along a closed path. These transformations, which are driven by the aspect ratio of the nanostructure, change the topology of the polarization field, producing a rich variety of polar configurations. For sufficiently flat nanostructures, a multi-bubble state bridges the gap between 0D nanodots and 2D ultra-thin films. The thermal properties of the ferroelectric bubbles indicate that this state is suitable for the development of nanometric devices.Comment: 16 pages, 4 figures. Accepted in Phys. Rev. Let

    Multiferroic behavior of Aurivillius Bi4Mn3O12 from first-principles

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    The multiferroic behavior of the hypothetical Aurivillius compound Bi4Mn3O12 has been explored on the basis of density functional calculations. We find that the tetragonal paraelectric phase of this material is ferromagnetic, showing ferroelectric and antiferrodistortive instabilities similar to the ones observed in its ferroelectric parent compound Bi4Ti3O12 . Our results indicate, however, that the presence of Mn+4 ions at the B-sites shrinks the cell volume and consequently the unstable polar mode, associated with the ferroelectric polarization, is overcame by an antiferrodistortive distortion. In this way, Bi4Mn3O12 exhibits incipient ferroelectricity at its equilibrium volume. We show that the ferroelectric state can be favored by strain or partial substitution of Mn with Ti.Comment: 6 pages, 5 figure

    Interface effects in ferroelectric PbTiO3_3 ultrathin films on a paraelectric substrate

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    Interface effects on the ferroelectric behavior of PbTiO3_3 ultrathin films deposited on SrTiO3_3 substrate are investigated using an interatomic potential approach with parameters fitted to first-principles calculations. We find that the correlation of atomic displacements across the film/substrate interface is crucial for the stabilization of the ferroelectric state in films a few unit-cells thick. We show that the minimum film thickness for the appearance of a spontaneous polarized domain state is not an intrinsic property of the ferroelectric film but depends on the polarizability of the paraelectric substrate. We also observe that the substrate displays an induced polarization with an unusual oscillatory behavior.Comment: 4 page

    Relative phase stability and lattice dynamics of NaNbO3_3 from first-principles calculations

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    We report total energy calculations for different crystal structures of NaNbO3_3 over a range of unit cell volumes using the all-electron full-potential (L)APW method. We employed both the local-density approximation (LDA) and the Wu-Cohen form of the generalized gradient approximation (GGA-WC) to test the accuracy of these functionals for the description of the complex structural behavior of NaNbO3_3. We found that LDA not only underestimates the equilibrium volume of the system but also predicts an incorrect ground state for this oxide. The GGA-WC functional, on the other hand, significantly improves the equilibrium volume and provides relative phase stability in better agreement with experiments. We then use the GGA-WC functional for the calculation of the phonon dispersion curves of cubic NaNbO3_3 to identify the presence of structural instabilities in the whole Brillouin zone. Finally, we report comparative calculations of structural instabilities as a function of volume in NaNbO3_3 and KNbO3_3 to provide insights for the understanding of the structural behavior of K1x_{1-x}Nax_xNbO3_3 solid solutions.Comment: Accepted for publication in Physical Review

    Temperature-driven phase transitions in SrBi2_2Ta2_2O9_9 from first-principles calculations

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    The phase transition sequence of SrBi2_2Ta2_2O9_9 is investigated using a shell model with parameters fitted to first-principles calculations. We show that the complex interplay between polar and nonpolar instabilities leads to the presence of two phase transitions, corroborating the existence of an intermediate orthorhombic paraelectric phase. This phase is characterized by the rotation of the TaO6_6 octahedra around the a-axis. We show that this phase can be also detected from the dielectric response of the material. The present approach constitutes a powerful tool for a theoretical prediction of intermediate phases, not yet observed experimentally, in other Aurivillius compounds

    Effects of the antiferrodistortive instability on the structural behavior of BaZrO3_3 by atomistic simulations

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    Recently, the possibility of a low-temperature non-cubic phase in BaZrO3_3 has generated engaging discussions about its true ground state and the consequences on its physical properties. In this paper, we investigate the microscopic behavior of the BaZrO3_3 cubic phase by developing a shell model from ab initioab~initio calculations and by performing molecular dynamics simulations at finite temperature and under negative pressure. We study three different scenarios created by tuning the intensities of the antiferrodistortive (AFD) instabilities, and consequently, the sequence of phase transitions with temperature. From a detailed analysis of the cubic phase at atomic scale, we find that oxygen octahedra are barely distorted, present rotation angles that may oscillate with significant amplitudes, are AFD correlated with their closest neighbors on the plane perpendicular to the pseudocubic rotation axis exhibiting (00a)(0 0 a^-)-type ordering, and form instantaneous, dynamic and unstable domains over time. Our simulations support the existence of nanoregions with short-range ordering in cubic BaZrO3_3 associated with experimentally observed anomalies and unveil that they can exist regardless of whether or not structural phase transitions related with AFD distortions occur at lower temperatures.Comment: 10 pages, 8 figure

    Surface reconstruction and ferroelectricity in PbTiO3_3 thin films

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    Surface and ferroelectric properties of PbTiO3_3 thin films are investigated using an interatomic potential approach with parameters computed from first-principles calculations. We show that a model developed for the bulk describes properly the surface properties of PbTiO3_3. In particular, the antiferrodistortive surface reconstruction, recently observed from X-ray scattering, is correctly reproduced as a result of the change in the balance of long-range Coulombic and short-range interactions at the surface. The effects of the surface reconstruction on the ferroelectric properties of ultrathin films are investigated. Under the imposed open-circuit electrical boundary conditions, the model gives a critical thickness for ferroelectricity of 4 unit cells. The surface layer, which forms the antiferrodistortive reconstruction, participates in the ferroelectricity. A decrease in the tetragonality of the films leads to the stabilization of a phase with non-vanishing in-plane polarization. A peculiar effect of the surface reconstruction on the in-plane polarization profile is found.Comment: 6 pages, 5 figure

    Microscopic scale investigation of piezoelectric properties of lead-free alkaline niobates

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     A first-principles-based technique is used to investigate the electromechanical properties oflead-free alkaline niobates. In particular, we study the structural and piezoelectric properties of LixK1-x NbO3 which is taken as a prototypical solid solution. We show that the system develops an orthorhombic to tetragonal polymorphic phase transition at room temperature for a Li concentration of 4.5%. We showed that not only the phase coexistence region is relevant for the enhancement of the piezoelectric properties but also the off-center Li impurities play an important role in the improvement of the electromechanical response.Fil: Machado, Rodrigo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); ArgentinaFil: Sepliarsky, Marcelo Claudio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); ArgentinaFil: Stachiotti, Marcelo Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentin
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