M. Sepliarsky

Atomistic modelling of BaTiO3 based on first-principles calculations

Interatomic potentials are determined in the framework of a shell model used to simulate the structural instabilities, dynamical properties, and phase transition sequence of BaTiO3. The model is developed from first-principles calculations by mapping the potential energy surface for various ferroelectric distortions. The parameters are obtained by performing a fit of interatomic potentials to this energy surface. Several zero-temperature properties of BaTiO3, which are of central importance, are correctly simulated in the framework of our model. The phase diagram as a function of temperature is obtained through constant-pressure molecular dynamics simulations, showing that the non-trivial phase transition sequence of BaTiO3 is correctly reproduced. The lattice parameters and expansion coefficients for the different phases are in good agreement with experimental data, while the theoretically determined transition temperatures tend to be too small.

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.

Ferroelectric properties of BaxSr1−xTiO3 solid solutions obtained by molecular dynamics simulation

Classical shell-model potentials for describing the complex ferroelectric behaviour of barium titanate and strontium titanate are developed and used to simulate BaxSr1?xTiO3 solid solutions. The temperature versus composition phase diagram is very well described and the local behaviour of the structure and polarization is analysed. It is shown that the ferroelectric properties of the solid solution can be understood in terms of the effects of average density and the local chemical environment. The experimentally observed static dielectric peak broadening around Tc at low x is reproduced in the simulation and seems to be related to the average volume rather than to the local chemical environment.

Atomic-level simulation of ferroelectricity in perovskite solid solutions

Building on the insights gained from electronic-structure calculations and from experience obtained with an earlier atomic-level method, we developed an atomic-level simulation approach based on the traditional Buckingham potential with shell model which correctly reproduces the ferroelectric phase behavior and dielectric and piezoelectric properties of KNbO3. This approach now enables the simulation of solid solutions and defected systems; we illustrate this capability by elucidating the ferroelectric properties of a KTa0.5Nb0.5O3 random solid solution.Building on the insights gained from electronic-structure calculations and from experience obtained with an earlier atomic-level method, we developed an atomic-level simulation approach based on the traditional Buckingham potential with shell model which correctly reproduces the ferroelectric phase behavior and dielectric and piezoelectric properties of KNbO3. This approach now enables the simulation of solid solutions and defected systems; we illustrate this capability by elucidating the ferroelectric properties of a KTa0.5Nb0.5O3 random solid solution.

Ferroelectric properties of KNbO3/KTaO3 superlattices by atomic-level simulation

We use atomic-level simulation methods to determine the zero-temperature structure and properties of coherent KNbO3/KTaO3 superlattices. We find that the in-plane behavior is essentially bulk-like with an abrupt jump in the polarization at the interfaces. By contrast, the polarization in the modulation direction is continuous through the interfaces with the interior of the KTaO3 layers remaining polarized for modulation lengths of up to 160 unit cells. The zero-frequency dielectric constant in the modulation direction has a similar magnitude to that of KNbO3. An investigation of the switching behavior shows that for modulation lengths greater than 24 unit cells, each KNbO3 layer behaves essentially independently. For modulation lengths of less than 12 unit cells, the KNbO3 layers interact so strongly with each other that the superlattice essentially behaves as a single artificial structure.

Surface reconstruction and ferroelectricity in PbTiO 3 thin films

Surface and ferroelectric properties of PbTiO

Polarization reversal in a perovskite ferroelectric by molecular-dynamics simulation

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Relative phase stability and lattice dynamics of NaNbO3from first-principles calculations

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

Temperature-driven phase transitions in SrBi2Ta2O9 from first-principles calculations

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

. 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.