G. Akcay

Influence of mechanical boundary conditions on the electrocaloric properties of ferroelectric thin films

A thermodynamic analysis of the electrocaloric (EC) effect in BaTiO3 ferroelectric thin films has been carried out under differing mechanical boundary conditions. It is shown that both the magnitude of the electrocaloric effect and temperature at which it is maximized depend not only on the extent of the applied field change but also on the value of the initial field. For initial fields smaller than a critical value the EC effect is largest at the phase transition temperature but the effect is a strong function of temperature. For external electrical fields larger than this value, conversely, the EC effect is the largest at a higher temperature and is a weak function of temperature. Perfect lateral clamping transforms the first-order phase transition into a second-order transition, lowering the magnitude of the electrocaloric effect and dependence on temperature. Compressive and tensile misfit strains also alter the nature of the phase transition and affect the electrocaloric properties in an analogous wa...

Magnitude of the intrinsic electrocaloric effect in ferroelectric perovskite thin films at high electric fields

Monodomain contributions to the electrocaloric effect in mechanically free (bulk) and laterally clamped (thin film) BaTiO3 are computed using a thermodynamic analysis. The authors show that the intrinsic electrocaloric coefficient at electric fields sufficient to destroy the discontinuous ferroelectric phase transition results in an adiabatic temperature change of 8K that agrees closely with the giant value of 12K recently observed experimentally for lead zirconate titanate thin films [A. S. Mischenko et al., Science 311, 1270 (2006)]. Perfect lateral clamping transforms the discontinuous transition into a continuous transition, which decreases the magnitude of the electrocaloric effect by 20% but reduces its sensitivity to temperature.

Ferroelectric solid solutions with morphotropic boundaries: Vanishing polarization anisotropy, adaptive, polar glass, and two-phase states

The generic case of a ferroelectric solid solution is considered wherein different symmetry phases located at opposing ends of the diffusionless phase diagram are separated by a morphotropic boundary (MB). It is shown that the Landau theory of weak first-order phase transformations automatically predicts vanishing of the anisotropy of polarization, continuity of thermodynamic properties, and a drastic decrease in domain wall energy near the MB line that results in the formation of adaptive ferroelectric nanodomain states. Low-resolution diffraction from these adaptive states acquired at the coherence lengths of elastic x-ray or neutron scattering probes will produce the same diffraction pattern as attributed to monoclinic (MA,MB,MC) phases. It is further shown that the electric- or stress-field-induced reconfiguration of these adaptive nanodomain states results in a softening of the piezoelectric, elastic, and dielectric properties near the MB line. In addition, the spherical degeneration of the polarization direction, reflecting the decoupling of the polarization from the crystal lattice at the MB, also predicts the formation of a polar glass state whose properties should be similar to the special properties of amorphous ferromagnets. In particular, the vanishing of the polarization anisotropy at the MB should result in ferroelectric domains with irregular shapes exhibiting high configurational sensitivity to external forces. The theory further predicts that two tricritical points will occur on the line of paraelectric→ferroelectric transitions and it is shown that all two-phase equilibrium lines of the diffusionless phase diagram—including the MB line—must be replaced by two-phase fields. Within these two-phase fields, the adjacent ferroelectric-ferroelectric and paraelectric-ferroelectric phases coexist. Possible topologies of the equilibrium MB phase diagram illustrating these two-phase equilibrium fields are computed and discussed.The generic case of a ferroelectric solid solution is considered wherein different symmetry phases located at opposing ends of the diffusionless phase diagram are separated by a morphotropic boundary (MB). It is shown that the Landau theory of weak first-order phase transformations automatically predicts vanishing of the anisotropy of polarization, continuity of thermodynamic properties, and a drastic decrease in domain wall energy near the MB line that results in the formation of adaptive ferroelectric nanodomain states. Low-resolution diffraction from these adaptive states acquired at the coherence lengths of elastic x-ray or neutron scattering probes will produce the same diffraction pattern as attributed to monoclinic (MA,MB,MC) phases. It is further shown that the electric- or stress-field-induced reconfiguration of these adaptive nanodomain states results in a softening of the piezoelectric, elastic, and dielectric properties near the MB line. In addition, the spherical degeneration of the polarizat...

Interface effects in ferroelectric bilayers and heterostructures

We study the role of interlayer interfaces on the polarization response of ferroelectric-paraelectric bilayers using a nonlinear thermodynamic model. We carry out a numerical analysis for prototypical BaTiO3–SrTiO3 bilayers ranging from 40to800nm total thickness as a function of SrTiO3 fraction. There exists a critical fraction of SrTiO3 at which the polarization is suppressed due to the depolarization field arising from the interlayer coupling and a large dielectric response is predicted. It is shown that this critical fraction decreases with decreasing total bilayer thickness, indicating that the interfacial effects are more pronounced in thinner bilayers.

Dielectric and pyroelectric properties of barium strontium titanate films on orthorhombic substrates with (110)//(100) epitaxy

The role of anisotropic misfit strains on the spontaneous polarization, dielectric properties, and pyroelectric response of (110) oriented Ba0.6Sr0.4TiO3 (BST 60/40) thin films on (100) orthorhombic substrates is analyzed theoretically. The anisotropic in-plane strain state and the rotation of the elastic and the electrostrictive constants of the BST 60/40 films result in strongly directional and unique properties, different from BST 60/40 films on cubic substrates with (100)BST//(100)substrate epitaxy. The thermodynamic formalism also incorporates the thickness dependence of the internal stress state due to the anisotropic relaxation of epitaxial stresses through the formation of misfit dislocations along the two in-plane directions. In particular, the model is applied to (110) BST 60/40 ferroelectric films on (100) NdGaO3 orthorhombic substrates. A more generalized analysis treating the in-plane misfit strains as parameters shows that ferroelectric phases that cannot be observed in single-crystal perovs...

Strain induced internal potentials of compositionally graded epitaxial ferroelectric thin films

Compositionally graded ferroelectrics produce internal potentials that manifest themselves as asymmetric hysteresis in polarization versus applied field plots. The direction and magnitude of the built-in fields depend on the sense of the chemical gradient in unclamped graded ferroelectrics. A thermodynamic model was employed to understand how these potentials would be affected by an inhomogeneous internal strain in epitaxial compositionally graded ferroelectric structures. The analysis indicates that the strain field can be employed to control both the magnitude and direction of the internal potentials. Indeed, the sense of the polarization gradient may be completely reversed with the appropriate strain gradient.

Dielectric tunability of (110) oriented barium strontium titanate epitaxial films on (100) orthorhombic substrates

We develop a thermodynamic model to determine the polarization, dielectric permittivity, and tunability of epitaxial ferroelectric films on orthorhombic substrates. In particular, we study the film thickness dependence of the tunability in epitaxial (110) Ba0.6Sr0.4TiO3 films on (001) NdGaO3 substrates. The analysis takes into account the thickness dependence of the internal stress state due to the anisotropic relaxation of epitaxial stresses through the formation of misfit dislocations along the two in-plane directions. We predict significant improvement in the tunability along both in-plane and out-of-plane directions of Ba0.6Sr0.4TiO3 films with increasing film thickness compared to the similar films on cubic substrates.

Electrocaloric response of the ferroelectrics

Monodomain contributions to the electrocaloric effect in mechanically free (bulk) and laterally strained (thin film) BaTiO3 are computed using a thermodynamic analysis. We show that the intrinsic electrocaloric coefficient at electric fields sufficient to destroy the discontinuous ferroelectric phase transition results in an adiabatic temperature change of 8 K that agrees closely with the giant value of 12 K recently observed experimentally for lead zirconate titanate thin films [Science 311, 1270 (2006)]. It is also demonstrated that the magnitude, the temperature-dependence and the maxima temperature (where the effect is maximum) of the electrocaloric response can be modified as a function of misfit strain.

PHASE TRANSFORMATION CHARACTERISTICS OF BARIUM STRONTIUM TITANATE FILMS ON ANISOTROPIC SUBSTRATES WITH (001)//(001) EPITAXY

ABSTRACT The role of anisotropic misfit strains on the spontaneous polarization of (100) oriented Ba0.6Sr0.4TiO3 thin films on (100) orthorhombic substrates is theoretically analyzed. A modified thermodynamic model is utilized to evaluate the equilibrium polarization values as a function of the anisotropic misfit strains. Results show that ferroelectric phases that cannot be observed in single-crystal Ba0.6Sr0.4TiO3 can be stabilized due to the reduction in the symmetry induced by the anisotropic strain state.

LOW-TEMPERATURE MONOCLINIC PHASE IN EPITAXIAL (001) BARIUM TITANATE ON (001) CUBIC SUBSTRATES

ABSTRACT The possibility of the existence of a low temperature monoclinic phase in epitaxial (001) BaTiO3 films on a (001) compressive substrate is analyzed theoretically and compared to recent experimental data from literature. There is good agreement between the theoretical findings and the experimentally observed behavior. The formation of the monoclinic phase arises from the point group reduction due to the rotation of the polarization vector commensurate with the variations in the in-plane strain state.