Structural characteristics of ferroelectric phase transformations in single-domain epitaxial films

Abstract

Structural characteristics of phase transformations in epitaxial ferroelectric films are analyzed via a Landau-Devonshire thermodynamic formalism. It is shown that the phase transformation temperature, the lattice parameters, and the order of the phase transformation are a strong function of the misfit strain and are considerably different compared to unconstrained, unstressed single crystals of the same composition. Depending on the internal stress state, it is possible that the structural aspects of the paraelectric-ferroelectric phase transformation may be completely obscured in the presence of epitaxial strains. The thickness dependence of epitaxial stresses due to relaxation by misfit dislocations during film deposition is incorporated into the model using an ''effective'' substrate lattice parameter. There is a good quantitative agreement between the theoretical analysis and experimental observations reported in the literature on the variations in the lattice parameters and the phase transformation temperature with film thickness in epitaxial BaTiO 3 films.

Impact of layer defects in ferroelectric thin films

“…Due to the coupling of the stress field of the dislocation and the polarization, there is expected a drastic variation of the polarization near to the dislocation. The compressive regions enhance the polarization and increase Curie temperature whereas tensile stresses decrease polarization with a commensurate drop in the Curie temperature [26,27]. But actually the dependence of polarization and Curie temperature on defects in FE thin films is not so intensively studied theoretically.…”

“…A first attempt to find out the influence of defects on the polarization of FE thin films are elucidated by Alpay et al [26,27]. A thermodynamic analysis has been carried out to investigate the role of dislocations in FE materials.…”

Electrothermal properties of perovskite ferroelectric films

“…After some rearrangement, the following expression obtains [65]: (25) We limit our discussion here only to the phase transformation from a cubic non-polar to a tetragonal ferroelectric "c-domain" phase. As shown theoretically for BaTiO 3 and PbTiO 3 , other tetragonal variants and non-tetragonal ferroelectric phases may also form, depending on the sign and magnitude of the misfit strain [75].…”

“…(9) is primarily controlled by the nature of the ferroelectric transition and plays an important role in determining the electrothermal coupling properties of ferroelectric materials. In thin film structures, the order of the ferroelectric transition may be changed by the chemical composition, as in solid solution ferroelectrics, and/or by a change in the mechanical boundary conditions arising from heteroepitaxial and thermal expansion mismatch with the substrate [41,[63][64][65][66]. In general, it is expected that adjusting conditions to produce a transition lying close to a tricritical point (i.e., a crossover from first-order to second-order behavior) will always produce the largest equilibrium electrocaloric response, since the derivative (dP/dT) → ∞ at this point [67].…”

Understanding order in compositionally graded ferroelectrics: Flexoelectricity, gradient, and depolarization field effects

“…GLD theory has been successfully applied in homogeneous bulk [16][17][18][19] and thin-film [20,21] ferroelectrics to explore possible crystal and domain structures, electric-field, thermal, stress, and other susceptibilities, as well as chemical, strain, and other perturbations to these materials. Particularly fruitful has been the combination of advanced GLD models with recent developments in the growth of ferroelectric thin films which has enabled unprecedented study of model versions and understanding of the fundamental physics of these materials [22][23][24][25].…”