Multiferroic behavior of Aurivillius Bi4Mn3O12 from first-principles

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

Strain-gradient-induced switching of nanoscale domains in free-standing ultrathin films

We report first-principle atomistic simulations on the effect of local strain gradients on the nanoscale domain morphology of free-standing PbTiO$_3$ ultrathin films. First, the ferroelectric properties of free films at the atomic level are reviewed. For the explored thicknesses (10 to 23 unit cells), we find flux-closure domain structures whose morphology is thickness dependent. A critical value of 20 unit cells is observed: thinner films show structures with 90$^\circ$ domain loops, whereas thicker ones develop, in addition, 180$^\circ$ domain walls, giving rise to structures of the Landau-Lifshitz type. When a local and compressive strain gradient at the top surface is imposed, the gradient is able to switch the polarization of the downward domains, but not to the opposite ones. The evolution of the domain pattern as a function of the strain gradient strength consequently depends on the film thickness. Our simulations indicate that in thinner films, first the 90$^\circ$ domain loops migrate towards the strain-gradient region, and then the polarization in that zone is gradually switched. In thicker films, instead, the switching in the strain-gradient region is progressive, not involving domain-wall motion, which is attributed to less mobile 180$^\circ$ domain walls. The ferroelectric switching is understood based on the knowledge of the local atomic properties, and the results confirm that mechanical flexoelectricity provides a means to control the nanodomain pattern in ferroelectric systems.Comment: 9 pages, 6 figure

Mixed Formamidinium-Methylammonium Lead Iodide perovskite from first-principles: Hydrogen-bonding impact on the electronic properties

Hybrid perovskites with mixed organic cations such as methylammonium and formamidinium have attracted interest due to their improved stability and capability to tune their properties varying the composition. In this work we report on the local variation of the structural and electronic properties in mixed A-site cation MA/FA lead iodide perovskites FAXMA1-XPBI3 evaluated from static first-principles calculations in certain structures where the orientations of organic cations result from examining the energy landscape of some compositions. The cation replacement at the A-site to form the solid solution causes an increase tilting of the inorganic PbI6 octahedra: in the FA-rich compounds the replacement of FA by a smaller cation like MA is to compensate the reduced space filling offered by the smaller cation, whereas in the MA-rich compounds is to expand the space needed for the larger cation. In fact, the effect of octahedron tiltings exceeds that of unit-cell size in determining the band gap for these organic cation mixtures. Our calculations indicate that the key role played by hydrogen bonds with iodine anions in pure compounds is preserved in the cation mixed perovskites. It is found that MA-I bonds remain stronger than FA-I bonds throughout the composition range regardless of the unit-cell expansion as the FA content increases. Our calculations reveal how the hydrogen bonds stabilize the no-bonding I-5p orbitals, spatially perpendicular to the Pb-I-Pb bond axis, lowering their energy when the H-I interaction occurs, which would explain the well-known role of hydrogen bonding in the structural stabilization of hybrid perovskites. These results contribute to the understanding on the role played by cation mixing at A sites in the physics of lead halide perovskites.Comment: 10 pages, 15 figure

Size effects, local properties and morphology of the domains in free-standing ultrathin films of pbtio3

Los materiales ferroeléctricos poseen polarización eléctrica espontánea (P) que puede invertirse con la aplicación de un campo eléctrico. Presentan importantes efectos de tamaño, y distintos tipos de ordenamiento atómico pueden aparecer a medida que las dimensiones del sistema se reducen. Empleando un modelo atomístico ajustado con resultados de cálculos ab-initio, en este trabajo caracterizamos películas libres y ultradelgadas del ferroeléctrico PbTiO3 en condiciones eléctricas de circuito abierto, que por lo tanto exhiben estructuras de dominios. Nuestros resultados muestran un espesor crítico de 20 celdas unidad que define la morfología de los dominios: películas mas delgadas presentan un flujo cerrado de P alrededor de un centro donde confluyen cuatro paredes de dominios a 90º, mientras que aquellas de mayor espesor presentan dominios bien definidos del tipo Landau-Lifshitz, conformando bandas con P a 180º y dominios triangulares en la superficie, que cierran el flujo de P. Asimismo un análisis detallado de las propiedades locales estructurales y de P muestran que al igual que en la región superficial, en las cercanías de las paredes de dominios se producen gradientes locales de deformación de la red, y por lo tanto cambios en la P y en la rotación de los octaedros de oxígenos, con respecto a los valores en el volumen del material.Ferroelectric materials have spontaneous electric polarization whose direction can be reversed applying an electric field. These materials are sensitive to size effects, and different types of atomic order may arise as the dimensions of their structures are reduced. Using an atomic-level model fitted from ab-initio calculations, we study freestanding ultrathin films of the PbTiO3 ferroelectric under open-circuit boundary conditions which exhibit therefore domain structures. We find a critical thickness of ~ 20 unit cells that defines the morphology of the domains: thinner films show flux-closure structure of the polarization with domain walls at 90°, whereas thicker films present Landau-Lifshitz domain type with well-defined stripe domains at 180° plus triangular shaped domains at the surfaces that close the polarization flux. Furthermore, a detailed analysis of their polar and structural local properties show that similar to the superficial region, in the neighborhood of the domain walls local strain gradients exist affecting the polarization as well as the rotation of oxygen octahedral with respect to their values in the bulk material.Fil: Belletti, Gustavo Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Física del Litoral; ArgentinaFil: Tinte, Silvia Noemi. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Física del Litoral; Argentin

Phase transitions and antiferroelectricity in BiFeO3 from atomic-level simulations

The structural and polar properties of BiFeO3 at finite temperature are investigated using an atomistic shell model fitted to first-principles calculations. Molecular dynamics simulations show a direct transition from the lowerature R3c ferroelectric phase to the Pbnm orthorhombic phase without evidence of any intermediate bridging phase between them. The higherature phase is characterized by the presence of two sublattices with opposite polarizations, and it displays the characteristic double-hysteresis loop under the action of an external electric field. The microscopic analysis reveals that the change in the polar direction and the large lattice strains observed during the antiferroelectric-ferroelectric phase transition originate from the interplay between polarization, oxygen octahedron rotations, and strain. As a result, the induced ferroelectric phase recovers the symmetry of the lowerature R3c phase.Fil: Graf, Mónica Elisabet. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; ArgentinaFil: Sepliarsky, Marcelo Claudio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; ArgentinaFil: Tinte, Silvia Noemi. Universidad Nacional del Litoral; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Física del Litoral. Universidad Nacional del Litoral. Instituto de Física del Litoral; ArgentinaFil: Stachiotti, Marcelo Gabriel. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentin

Ab initio study of the phase diagram of epitaxial BaTiO3

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 (1998)], 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.Comment: 4 pages, with 4 postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/od_epi/index.htm

Coupling between Light and Terahertz-Frequency Acoustic Phonons in Ferroelectric BaTiO3 /SrTiO3 Superlattices

The acoustic phonons in epitaxial ferroelectric (BaTiO3 )n /(SrTiO3 )m superlattices (SLs) are investigated by high-resolution ultraviolet Raman scattering. The temperature dependence of the folded acoustic (FA) phonon Raman intensity through the ferroelectric transition is addressed. A comparison of this behavior between SLs with different number of ferroelectric BaTiO3 unit cells n and spacer SrTiO3 unit cells m is presented. A mechanism involving the strain modulation of the spatially varying ferroelectric polarization is introduced to explain the temperature dependence of the FA phonon scattering. The temperature dependence of the polarization can be derived from an analysis of the first-order optical phononspectra. Using this information, the observed temperature dependence of the whole set of SLs with different n can be consistently accounted for with the presented model. Atomistic shell-model simulations of the spatial pattern of the SL polarization are presented to explain the variation of the FA-spectral intensity for SLs with different m and the experimental fact that no high-order FA-replicas are observed. These results demonstrate the strong coupling between THz hypersound, charge, and light in these multifunctional nanoscale ferroelectrics.Fil: Bruchhausen, Axel Emerico. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. University of Konstanz; AlemaniaFil: Fainstein, Alejandro. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Tinte, Silvia Noemi. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química (i); ArgentinaFil: Soukiassian, A.. University Park; Estados UnidosFil: Schlom, D. G.. Cornell University; Estados UnidosFil: Xi, X. X.. Cornell University; Estados Unidos. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigaciones y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Constituyentes); Argentin

Anomalous enhancement of tetragonality in PbTiO3 induced by negative pressure

Using a first-principles approach based on density-functional theory, we find that a large tetragonal strain can be induced in PbTiO3 by application of a negative hydrostatic pressure. The structural parameters and the dielectric and dynamical properties are found to change abruptly near a crossover pressure, displaying a ``kinky'' behavior suggestive of proximity to a phase transition. Analogous calculations for BaTiO3 show that the same effect is also present there, but at much higher negative pressure. We investigate this unexpected behavior of PbTiO3 and discuss an interpretation involving a phenomenological description in terms of a reduced set of relevant degrees of freedom.Comment: 9 pages, with 9 postscript figures embedded. Uses REVTEX and epsf macros. Also available at http://www.physics.rutgers.edu/~dhv/preprints/st_pbti/index.htm

Vibrational effects on SrTiO3 Ti 1s absorption spectra studied using first-principles methods

We analyze the vibrational effects on the Ti 1s excited states in cubic SrTiO3 and related pre-edge x-ray absorption fine structure using first-principles methods. Ground-state, total-energy and electron-core hole Bethe-Salpeter calculations are performed for different atomic configurations related to eg-symmetry distortions of SrTiO3. From these, we can obtain normal-mode gradients of the electronic excited-state energy, i.e., of the excited-state Born-Oppenheimer surface. This yields the corresponding electron-phonon coupling coefficients that allow us to predict the spectral broadening induced by those vibrational modes.Fil: Tinte, Silvia Noemi. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; ArgentinaFil: Shirley, Eric. National Institute Of Standards And Technology; Estados Unido

Atomic-level simulation of perroelectricity in BaTiO3 ultrathin films

The question of how the ferroelectric phase transitions of bulk BaTiO3 are modified in ultrathin films is investigated using an atomic-level simulation approach based on a model with parameters obtained from first-principles calculations. This model correctly reproduces the ferroelectric phase behavior of the bulk material and its static surface properties. The phase transition sequence in the film is investigated through molecular dynamics simulations. The effect on ferroelectricity produced by the film-substrate lattice mismatch is investigated by simulating strained epitaxial films grown on cubic substrates. © 2001 Taylor & Francis.Fil: Tinte, Silvia Noemi. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; ArgentinaFil: Stachiotti, Marcelo Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentin