R. L. Migoni

Ferroelectric phase transitions and dynamical behavior in KNbO3/KTaO3 superlattices by molecular-dynamics simulation

The phase transitions and dynamical behavior of superlattices consisting of equal-thickness layers of a perovskite ferroelectric (KNbO3) and a perovskite paraelectric (KTaO3) are explored using molecular-dynamics simulation. We find that the response in the plane and in the modulation direction are essentially decoupled. The Curie temperature for the transition from a polarized to unpolarized state in the modulation direction decreases approximately linearly with modulation length, Λ, for Λ>12; for smaller modulation lengths, it is essentially constant. The Curie temperature in the plane appears to be only weakly modulation-length dependent. We relate our results to experimental findings on the same system.The phase transitions and dynamical behavior of superlattices consisting of equal-thickness layers of a perovskite ferroelectric (KNbO3) and a perovskite paraelectric (KTaO3) are explored using molecular-dynamics simulation. We find that the response in the plane and in the modulation direction are essentially decoupled. The Curie temperature for the transition from a polarized to unpolarized state in the modulation direction decreases approximately linearly with modulation length, Λ, for Λ>12; for smaller modulation lengths, it is essentially constant. The Curie temperature in the plane appears to be only weakly modulation-length dependent. We relate our results to experimental findings on the same system.

Surface reconstruction and ferroelectricity in PbTiO 3 thin films

Surface and ferroelectric properties of PbTiO

Order-disorder, local structure and precursor effects in BaTiO3

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Surface and Substrate Effects on the Ferroelectric Properties of PbTiO3 Ultrathin Films

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 PbTiO

Ab initio supported model simulations of ferroelectric perovskites

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Ferroelectricity and Domain Wall Motion in Ultrathin Perovskite Epitaxial Films

. 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.

Molecular dynamics simulation of KNbO3: Relaxational dynamics and precursor domains

Abstract Interatomic potentials are determined in the framework of the nonlinear oxygen polarizability model to describe the structural behavior of BaTiO3To this purpose, the adiabatic potential is evaluated for different ferroelectric distortions and the model potential parameters are obtained by comparing with LAPW total energy calculations. The phase diagram as a function of temperature is obtained through constant pressure molecular dynamics simulations, and the non-trivial phase sequence is correctly reproduced. Calculations of the pair distribution function indicate that order-disorder is the dominant dynamics at all transitions. Precursor clusters with local polarizations correlated along Ti-O chains are determined to persist several hundreds of degrees above the para-ferroelectric transition.

Order-disorder behavior in KNbO3 and KNbO3/KTaO3 solid solutions and superlattices by molecular-dynamics simulation

The effects of the antiferrodistortive surface reconstruction and the interface with a SrTiO 3 substrate on the ferroelectric properties of PbTiO 3 ultrathin films are investigated using an interatomic potential approach with parameters determined from first-principles results. First we show that a shell model developed for the bulk describes properly the surface properties of PbTiO 3 , in particular the antiferrodistortive surface reconstruction recently observed from x-ray scattering. We investigate also the effects of the surface reconstruction on the polarization behavior of ultrathin films under open-circuit electrical boundary conditions, showing that the surface layer participates in the ferroelectricity. Finally, film-substrate interface effects on the ferroelectric properties are investigated simulating PbTiO 3 ultrathin films on a SrTiO 3 substrate.

Atomic-level simulation of ferroelectricity in perovskttes

While ferroelectric KNbO 3 and BaTiO 3 present the cubic-tetragonal-orthorhombic-rhombohedral phase sequence with decreasing T, their related compounds KTaO 3 and SrTiO 3 are incipient ferroelectrics which exhibit the ferroelectric soft mode but remain paraelectric up to 0 K. Despite the large amount of research attracted by the above phenomenology since long ago, quite a few realistic model simulations are available. Many lattice dynamical calculations have been performed for each isolated material, but the only unified view of their ferroelectric behavior has been provided by the nonlinear oxygen polarizability model. Although initially based on phonon data for the cubic phase, recent ab initio calculations confirm that the model is basically correct with regards to the energetics involved in the various ferroelectric distortions in KNbO 3 . Our molecular dynamics simulations show the crossover from a soft mode to an order-disorder dynamics in the cubic phase of KNbO 3 , the appearance of its various ferroelectric phases, and the soft-mode behavior of KTaO 3 .

Particle dynamics in the vicinity of a ferroelectric phase transition

The effect on ferroelectricity produced by the film-substrate lattice mismatch is investigated by atomic-level simulations on strained BaTiO 3 ultrathin films. We show that a polydomain ferroelectric phase with an out-of-plane orientation of polarization is stabilized in films as thin as 20 Å. We also get insight into the microscopic mechanism responsible for domain wall motion in the film.