Phenomenological Landau-Devonshire Theory Research Articles

Effect of misfit strain on the electrocaloric effect of P(VDF-TrFE) copolymer thin films

Based on the phenomenological Landau-Devonshire theory, we investigate the effect of misfit strain on the electrocaloric effect of P(VDF-TrFE) copolymer thin films. Theoretical analysis indicates that the compressive misfit strain reduces the working temperature to a great extent where the electrocaloric effect is maximized, which is different from the result of the conventional ferroelectric thin films, such as BaTiO3. Although the compressive or tensile misfit strain does not change the maximum of the electrocaloric coefficient, the compressive misfit strain decreases the maximum of the adiabatic temperature change and the tensile misfit strain results in the opposite effect. Consequently, control of the misfit strain provides potential means to vary the working temperature for use in cooling systems.

Film thickness dependence of electro-optic effect in epitaxial BaTiO 3 thin films

Based on the phenomenological Landau–Devonshire theory, we investigate the film thickness dependence of ferroelectric and electro-optic properties of epitaxial BaTiO 3 thin films grown on SrTiO 3 and MgO substrates. By using the effective substrate lattice parameter concept, the film thickness dependence of misfit strain is incorporated into the theory. Therefore, the film thickness dependence of ferroelectric and electro-optic properties in epitaxial BaTiO 3 thin films can be explained. Moreover, a large quadratic electro-optic effect was obtained in the BaTiO 3 thin films, which is in good agreement with the experimental result of BaTiO 3 thin films on the MgO substrate.

The polarization and dielectric susceptibility of a ferroelectric bilayer film

A ferroelectric bilayer film consisting of two different ferroelectric constituent films with a transition layer within each constituent film and interfacial coupling between two materials is investigated based on the Ginzburg–Landau–Devonshire phenomenological theory. A parameter α, which describing the differences between physical properties of two constituent films is first introduced in our paper, and reflects a more realistic situation compared to the previous treatments assuming the same two constituent films. We study the polarization and dielectric susceptibility properties of the ferroelectric bilayer film with two different constituent films. The results present some interesting phenomena due to the introduction of parameter α.

Strain-driven phase transitions and associated dielectric/piezoelectric anomalies in BiFeO3 thin films

Strain-driven phase transitions and related intrinsic polarization, dielectric, and piezoelectric properties for single-domain films were studied for BiFeO3 using phenomenological Landau–Devonshire theory. A stable and mixed structure between tetragonal and rhombohedral-like (monoclinic) phases is predicted at a compressive misfit strain of um=−0.0382 without an energy barrier. For a tensile misfit strain of um=0.0272, another phase transition between the monoclinic and orthorhombic phases was predicted with sharply high dielectric and piezoelectric responses.

Tuning the remanent polarization of epitaxial ferroelectric thin films with strain

The effect of biaxial strain on the remanent polarization of epitaxial thin films of various ferroelectric materials is studied using phenomenological Landau–Devonshire theory. It is shown that the strain dependences of the remanent polarizations are strongly dependent on crystal symmetries and film orientations. For (001)p-oriented ferroelectric films with (distorted) rhombohedral symmetry, strain-induced polarization rotation leads to stronger strain dependences than in ferroelectric films with tetragonal or orthorhombic symmetries. For (111)p-oriented ferroelectric films with rhombohedral symmetry, however, the remanent polarization is less sensitive to the biaxial strain.

Effect of misfit strain on the electrocaloric effect in epitaxial SrTiO3 thin films

Based on a phenomenological Landau-Devonshire theory, the effect of misfit strain on the electrocaloric effect in epitaxial SrTiO 3 thin films is investigated. Theoretical analysis indicates that both the electrocaloric coefficient and the adiabatic temperature change strongly depend on the misfit strain. Moreover, the maximum of electrocaloric coefficient almost does not change with increasing the compressive or tensile misfit strain. However, the enhancement of misfit strain enlarges both the adiabatic temperature change and the temperature at which it is maximized. Most importantly, the largest room-temperature electrocaloric effect can be attained at a critical misfit strain. Therefore, it may open more opportunities for practical application in refrigeration devices.

Theoretical study on the mechanism of abnormal dielectric susceptibility behaviors of ferroelectric bilayer films

Based on the modified Ginzburg–Landau–Devonshire phenomenological theory, a ferroelectric bilayer film with the transition layer within each constituent film and an interfacial coupling between two materials has been studied. The dielectric susceptibility of a bilayer film composed of two equally thick ferroelectric constituent films is discussed. The results show that there is a transition point, which is an equilibrium point of two contrary actions of the transition layer and interfacial coupling. The anomalous dielectric susceptibility behaviors of a ferroelectric bilayer film are ascribed to the competition of the transition layer and interfacial coupling deviated from this transition point.

Combined effect of the transition layer and interfacial coupling on the properties of ferroelectric bilayer film

Within the framework of modified Ginzburg–Landau–Devonshire phenomenological theory, a ferroelectric bilayer film with a transition layer within each constituent film and an interfacial coupling between two materials has been studied. Properties including the Curie temperature and the spontaneous polarization of a bilayer film composed of two equally thick ferroelectric constituent films are discussed. The results show that the combined effect of the transition layer and the interfacial coupling plays an important role in explaining the interesting behaviour of ferroelectric multilayer structures consisting of two ferroelectric materials.

Critical Point of Relaxor K1−x Li x TaO3 Under an Electric Field

Optical second harmonic (SH) intensities and dielectric constants of Li doped KTaO 3 are measured under an external electric field. Applying the Landau-Devonshire phenomenological theory to the temperature dependence of SH intensities, we find that the order of ferroelectric phase transition changes from 2nd to 1st with the increase of Li concentration. The dielectric measurement reveals that a critical behaviour similar to Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 is observed. Additionally, we find that the critical electric field depends on the frequency of electric field and the effect becomes conspicuous in lower frequencies in Li doped KTaO 3 .

Effects of anisotropic in-plane strains on the phase diagram of BaxSr1−xTiO3 thin film

Based on the Landau-Devonshire phenomenological theory, phase diagram of epitaxial BST50∕50 thin films on anisotropic in-plane strains is investigated. Different from BaTiO3 thin films, the paraelectric phase appears under the anisotropic misfit strains on barium strontium titanate (BST)50∕50 thin films at room temperature. The pyroelectric property of the BST films is also calculated. We find that the position of pyroelectric peak greatly depends on anisotropic misfit strains.

Effect of thickness of epitaxial PbZr0.4Ti0.6O3 film on the physical properties

The thickness dependence of the spontaneous polarization and the hysteresis loop of epitaxial PbZr0.4Ti0.6O3 thin films deposited on (001) SrTiO3 substrate was investigated via the Landau-Devonshire's phenomenological theory considering the coupling of the stress field of the edge dislocation and the polarization. The results show that the critical thickness for the formation of misfit dislocation is ～1.27nm, and there is a drastic variation in the polarization near the dislocation in films with thickness greater than the critical value, which results in the formation of the dead layer that severely degrades ferroelectric properties. With decreasing the film thickness, both the dislocation spacing and the ratio of the dead layer to film total thickness increase. The thickness dependence of the hysteresis loop indicates that the remnant polarization decreases as the thickness decreases.

Thermodynamic study ofc-axis-oriented epitaxialPb(Zr,Ti)O3thin films

Thermodynamic characteristics of single crystalline $\mathrm{Pb}({\mathrm{Zr}}_{0.52}{\mathrm{Ti}}_{0.48}){\mathrm{O}}_{3}$ (PZT) were investigated using c-axis oriented PZT films. The PZT films were epitaxially grown on Pt/MgO substrate and dielectric and ferroelectric properties were measured as a function of one-dimensional stress. The stress dependence of dielectric and ferroelectric properties was examined on the basis of the Landau-Devonshire's phenomenological theory and the free energy coefficients of single crystalline PZT films were obtained. The dielectric stiffness coefficients and electrostrictive coefficient of epitaxial PZT films were obtained to be ${\ensuremath{\alpha}}_{1}=\ensuremath{-}1.30\ifmmode\times\else\texttimes\fi{}{10}^{8}(\mathrm{m}/\mathrm{F}),$ ${\ensuremath{\alpha}}_{11}=3.07\ifmmode\times\else\texttimes\fi{}{10}^{8}({\mathrm{m}}^{5}/{\mathrm{C}}^{2}\mathrm{F}),$ ${\ensuremath{\alpha}}_{111}=\ensuremath{-}3.11\ifmmode\times\else\texttimes\fi{}{10}^{7}({\mathrm{m}}^{9}/{\mathrm{C}}^{2}\mathrm{F}),$ and ${Q}_{12}=\ensuremath{-}5.70\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}({\mathrm{m}}^{4}/{\mathrm{C}}^{2}),$ which are different from the values derived from the analysis of polycrystalline PZT. The temperature dependence of dielectric constant of the PZT films showed clear Curie-Weiss law and the dielectric stiffness coefficient ${\ensuremath{\alpha}}_{1}$ derived from this measurement was almost same value from the analysis of stress dependence of the dielectricity of the epitaxial PZT films.

Two-dimensional thermodynamic theory of epitaxialPb(Zr,Ti)O3thin films

Effects of film stresses on the phase stability and various ferroelectric properties of the epitaxially oriented $\mathrm{Pb}(\mathrm{Zr},\mathrm{Ti}){\mathrm{O}}_{3}$ thin film were theoretically investigated. For this purpose, a two-dimensional thermodynamic model was developed using the Landau-Devonshire phenomenological theory. The theoretical computation predicts an increase in the para-ferroelectric transition temperature and an enhanced thermodynamic stability of the tetragonal-phase field, irrespective of the sign of stress (i.e., both tensile and compressive). In particular, it is predicted that tensile stress makes a new ferroelectric orthorhombic phase stable. We have also shown that dielectric properties usually exhibit their maximum values at a newly established Zr-rich morphotropic phase boundary, except for those parallel to the direction of the spontaneous polarization.

Monoclinic phase ofPbZr0.52Ti0.48O3ceramics: Raman and phenomenological thermodynamic studies

This work reports Raman detection of the tetragonal $\ensuremath{\rightarrow}$ monoclinic phase transition in ${\mathrm{PbZr}}_{0.52}{\mathrm{Ti}}_{0.48}{\mathrm{O}}_{3}$ ceramics near the morphotropic phase boundary at low temperatures. The transition is characterized by changes in the frequency of lattice modes with temperature. The results presented here confirm the previous one recently reported by Noheda et al. [Appl. Phys. Lett. $74,$ 2059 (1999)] using high-resolution synchrotron x-ray powder diffraction technique and dielectric measurements. The stability of the new phase is discussed within the framework of phenomenological Landau-Devonshire theory.

Theoretical investigation of the influence of space charges on ferroelectric properties of PbZrTiO3 thin film capacitor

During ion etching (IE) processes used for making a Pt electrode in the fabrication of ferroelectric capacitors, the film is exposed to radiation and energetic ion bombardment. The influence of such processes on polarization-electric field characteristics of lead zirconate titanate thin film capacitor has been reported by E. G. Lee [Appl. Phys. Lett. 69, 1223 (1996)]. A large field shift and a constriction in the hysteresis loop are experimentally observed. For these authors, this behavior is probably due to the existence of space charges trapped near electrodes. For a better understanding of these phenomena, we have developed a model of ferroelectric capacitor based on the phenomenological Landau–Devonshire theory. A two-dimensional lattice of dipoles is assumed to describe the film. By solving Poisson’s equation, the electric-field distribution inside the film is calculated, which allows us to locally determine the evolution of the polarization by minimizing free energy. IE effects and probably the existence of space charges in the film are introduced by means of a doping layer in the film near the electrode. The influence of important parameters such as doping level, and thickness of the doped layer on the hysteresis loop are investigated. Main experimental thin film electric behaviors are well reproduced and explained considering dipoles switched inside the lattice. Results are interpreted by splitting the hysteresis loop deformation into two effects with different origins: a constriction effect related to the number of impurities inside the film and a shift effect related to the asymmetrical impurities distribution.

Ferroelectric phase transitions and three-dimensional phase diagrams of a Pb(Zr, Ti)O3 system under a hydrostatic pressure

Effects of hydrostatic pressure on various ferroelectric phase transitions in Pb(Zr, Ti)O3 (PZT) have been investigated using the thermodynamic formalism based on the Landau–Devonshire phenomenological theory. For these purposes, the rotostrictive coefficient related to the coupling between the tilting of oxygen octahedron and the stress was evaluated first. We have then simulated three-dimensional phase diagrams of the PZT system using composition, stress and temperature as the three independent thermodynamic variables. It has been shown that, with increasing tensile hydrostatic pressure, the para–ferro transition temperature increases but the transition temperature between the two ferroelectric rhombohedral phases (FR(HT)–FR(LT)) decreases, and the morphotropic phase boundary moves slightly toward the tetragonal-phase field. Contrary to these, opposite tendencies are predicted under a compressive stress. The thermodynamic computation of ferroelectric properties in the vicinity of the FR(HT)–FR(LT) phase transition further suggests that a first-order phase transition gradually becomes a continuous second-order transition with increasing tensile stress.

Thermodynamic Analysis of the FR(LT)‐FR(HT) Phase Transition in the Pb(Zr,Ti)O3 System under a Hydrostatic Pressure

Effects of hydrostatic pressure on the ferroelectric phase transition between the two rhombohedral phases in the Zr‐rich Pb(Zr,Ti)O3 (PZT) system have been investigated using the Landau‐Devonshire phenomenological theory. For this purpose, the rotostrictive coefficients related to the coupling between the oxygen octahedron and the stress were evaluated first. It was shown that the transition temperature between the two rhombohedral phases FR(LT) and FR(HT) decreased with increasing tensile hydrostatic pressure. Contrary to this, an opposite trend was predicted under the condition of compressive hydrostatic pressure.

Size driven phase transition in ferroelectric particles

The phenomenological Landau-Devonshire theory is used for finite size ferroelectric particles. The spatial distribution and size dependence of polarization are calculated. The latter reveals a size driven phase transition from ferroelectric to paraelectric. The ferroelectric critical size predicted is compared with that of PbTiO 3 and BaTiO 3 particles.

Measurement of specific heat at constant electric flux density CD of ferroelectrics by AC method

The measurement of cD and cE was carried out on ferroelectric TGSe and TGS with an ac calorimetry technique. It was revealed that cDs of TGSe and TGS do not show any anomaly at the transition, and coincide within the experimental error with the lattice background in agreement with the Landau-Devonshire phenomenological theory.