A first-principles study of phase transitions in ultrathin films of BaTiO3

We determine the effects of film thickness, epitaxial strain and the nature of electrodes on ferroelectric phase transitions in ultrathin films of BaTiO₃ using a first-principles effective Hamiltonian in classical molecular dynamics simulations. We present results for polarization and dielectric properties as a function of temperature and epitaxial strain, leading to size-dependent temperature-strain phase diagram for the films sandwiched between ‘perfect’ electrodes. In the presence of non-vanishing depolarization fields when non-ideal electrodes are used, we show that a stable stripe-domain phase is obtained at low temperatures. The electrostatic images in the presence of electrodes and their interaction with local dipoles in the film explain these observed phenomena.      

Double hysteresis loops induced by Mn doping in Pb0.99Nb0.02(Zr0.95Ti0.05)0.98O3 ferroelectric ceramics

Pb_0.99Nb_0.02(Zr_0.95Ti_0.05)_0.98O₃ (PNZT95/5) ceramics with 1 mol% and without Mn doping were prepared via conventional solid state reaction process. X-ray diffraction patterns show that the PNZT95/5 and Mn-doped PNZT95/5 (PNZTM95/5) ceramics, with composition near the boundary of the ferroelectric phase (FE)/antiferroelectric phase (AFE), have a rhombohedral perovskite structure. The ferroelectric behavior of PNZT95/5 ceramics is strongly affected by Mn doping. Without any aging process the PNZTM95/5 ceramics possess double hysteresis loops (P–E loops), whereas the PNZT95/5 ceramics possess normal single hysteresis loops. Due to the defect dipoles formed by effectively negatively charged Mn³⁺ dopants and positively charged O²⁻ vacancies, the PNZTM95/5 ceramics exhibit the double P–E loops. The defect dipole effect has been proved by investigating the P–E loops under different external fields. As a result, the PNZTM95/5 ceramics become “hardened”, exhibiting a high mechanical quality factor (1300).     

Two-dimensional electron gas at the interface of Ba<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>TiO<Subscript>3</Subscript> ferroelectric and LaMnO<Subscript>3</Subscript> antiferomagnet

The temperature dependence of the electrical resistance has been studied for heterostructures formed by antiferromagnetic LaMnO₃ single crystals of different orientations with epitaxial films of ferroelectric Ba_0.8Sr_0.2TiO₃ deposited onto them. The measured electrical resistance is compared to that exhibited by LaMnO₃ single crystals without the films. It is found that, in the samples with the film, for which the axis of polarization in the ferroelectric is directed along the perpendicular to the surface of the single crystal, the electrical resistance decreases significantly with temperature, exhibiting metallic behavior below 160 K. The numerical simulations of the structural and electronic characteristics of the BaTiO₃/LaMnO₃ ferroelectric?antiferromagnet heterostructure has been performed. The transition to the state with two-dimensional electron gas at the interface is demonstrated.     

Ferroelectric phase transitions in ultrathin films of BaTiO3

We present molecular dynamics simulations of a realistic model of an ultrathin film of BaTiO3 sandwiched between short-circuited electrodes to determine and understand effects of film thickness, epitaxial strain, and the nature of electrodes on its ferroelectric phase transitions as a function of temperature. We determine a full epitaxial strain-temperature phase diagram in the presence of perfect electrodes. Even with the vanishing depolarization field, we find that ferroelectric phase transitions to states with in-plane and out-of-plane components of polarization exhibit dependence on thickness; it arises from the interactions of local dipoles with their electrostatic images in the presence of electrodes. Secondly, in the presence of relatively bad metal electrodes which only partly compensate the surface charges and depolarization field, a qualitatively different phase with stripelike domains is stabilized at low temperature.     

外延铁电薄膜相变温度的尺寸效应

考虑外延钙钛矿型铁电薄膜内的等效应力、表面晶格变化和表面电荷引起的退极化效应等机电耦合边界条件,利用铁电薄膜系统的动态金茨堡-朗道方程(DGL),系统分析和讨论了外延铁电薄膜相变温度与临界相变厚度的尺寸效应.结果表明,铁电薄膜相变温度与临界相变厚度完全依赖于各种与薄膜厚度相关的力电耦合边界条件.也给出了BaTiO₃外延铁电薄膜相变温度在各种边界条件下随厚度的变化,从结果看出,本文的分析与结论更符合实验数据. 关键词： 尺寸效应 外延铁电薄膜 相变温度 力电耦合边界     

Polarization fatigue resistance of Ca-doped Pb(Zr<Subscript>0.52</Subscript>Ti<Subscript>0.48</Subscript>)O<Subscript>3</Subscript> thin films prepared by the sol–gel method

The ferroelectric and polarization fatigue characteristics of Pb_1-xCa_x(Zr_0.52Ti_0.48)O₃ (PCZT) thin films prepared using the sol–gel method were studied. The Ca-doping slightly suppresses the ferroelectricity of Pb(Zr_0.52Ti_0.48)O₃ (PZT) because of the quantum paraelectric behavior of CaTiO₃. Compared with PZT thin films, the PCZT (x=0.2) thin films show enhanced fatigue resistance at room temperature, further emphasized by the almost fatigue-free behavior at 100 K. The temperature-dependent dc-conductivity suggests a decrease of the oxygen vacancy density by almost 20 times and a slightly declined activation energy U for oxygen vacancies, upon increasing of the Ca-doping content from 0.0 to 0.2. It is argued that the improved fatigue endurance is ascribed to the decreasing density of oxygen vacancies due to the Ca-doping, although the lowered activation energy of oxygen vacancies is unfavorable. PACS 77.84.Dy; 66.30.-h; 68.35.Fx     

First principles study on the ferroelectricity of the perovskite ABO3 ferroelectrics

In order to understand well the different ferroelectric behaviour of quantum paraelectrics and ferroelectrics and the origin of the ferroelectricity of the solid solution KTa0.5Nb0.5O3(KTN),we calculated the electronic structure of CaTiO3,BaTiO3 and KTN by first principles calculation.From total energy analysis,it is shown that,with increasing cell volume,the crystals (CaTiO3,SrTiO3) will have a ferroelectric instability.For BaTiO3,the ferroelectricity will disappear as the cell volume is decreased.From the density of states analysis,it is shown that the hybridization between B d and O p is very important for the ferroelectric stability of ABO3 perovskite ferroelectrics.This is consistent with the analysis of band structure.     

Electronic structure and phase separation in La<Subscript>x</Subscript>MnO<Subscript>3</Subscript> films (0.83≤<Emphasis Type="Italic">x</Emphasis>≤1.10): Optical and magnetooptical data

The variations in the electronic structure and the evolution of phase separation as a result of creation of vacancies or excess of lanthanum in lanthanum manganites are studied on the basis of analysis of optical and magnetic properties of La_xMnO₃ epitaxial films (0.83≤x≤1.10) in the fundamental absorption range. The Kerr effect, the temperature dependences of resistivity, optical density, and magnetoabsorption of light indicate the charge and magnetic phase separation in the films. The fine structure observed in the spectrum is attributed to spectral overlapping of electron transitions with charge transfer and geometrical resonances reflecting an inhomogeneous nanoscopic structure of the films, which strongly depends on stoichiometry and stresses emerging during film deposition. It is shown that, in contrast to bulk polycrystals, the gradient of stresses over the film thickness significantly affects the phase separation in the films.     

Phase transitions in barium–strontium titanate films on MgO substrates with various orientations

A comparative study of the lattice dynamic upon phase transitions in a polycrystalline Ba_0.8Sr_0.2TiO₃ (BST) film on a Pt substrate and in epitaxial BST films grown on various sections of an MgO substrate has been performed by Raman spectroscopy. It has been found that different sequences of phase transitions take place in these films. The BST/Pt films demonstrate the same sequence of phase transitions that is observed in the bulk ceramics. The hardening of a soft mode in BST/(001)MgO and BST/Pt films shows that the transition from the tetragonal ferroelectric phase to the paraelectric phase has features of the displacement-type phase transition and also the order–disorder phase transition. When approaching the ferroelectric transition temperature, the soft mode in the BST/(111)MgO film is softened, following the Cockran law, which indicates the displacement-type phase transition.     

Film thickness dependence of electro-optic effect in epitaxial BaTiO3 thin films

Based on the phenomenological Landau-Devonshire theory, we investigate the film thickness dependence of ferroelectric and electro-optic properties of epitaxial BaTiO₃ thin films grown on SrTiO₃ 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 BaTiO₃ thin films can be explained. Moreover, a large quadratic electro-optic effect was obtained in the BaTiO₃ thin films, which is in good agreement with the experimental result of BaTiO₃ thin films on the MgO substrate.     

Phase transition in thin epitaxial ferroelectric films as derived from thermal measurements

The temperature dependence of the heat capacity of thin epitaxial films BaTiO₃/MgO is studied by the dynamic 3ω method in the thickness range 50–500 nm. It is revealed that the heat capacity exhibits diffuse anomalies due to phase transitions. The temperature of the ferroelectric phase transition T _C increases with decreasing film thickness. The reasons for the strong diffuseness of the transition and the nonlinear dependence of the transition temperature on the film thickness are discussed.     

Na0.25K0.25Bi0.5TiO3无铅压电陶瓷的介电特性研究

用固相反应法制备了Na_0.25K_0.25Bi_0.5TiO₃ (NKBT50)陶瓷，研究了该陶瓷在室温至400℃温度范围内的介电性能.发现该陶瓷的介电温谱与烧结气氛、极化状态有关.在空气中烧结的未极化样品在70℃附近存在介电和损耗峰，而极化后及在氧气氛中烧结的样品并不存在该介电、损耗峰.分析认为70℃的介电和损耗峰与氧空位形成的缺陷偶极子的极化弛豫有关.热激电流显示，陶瓷的去极化温度为225℃，与此相对应的介电、损耗峰也 关键词： 介电性能 氧空位 极化弛豫 钛酸铋钠钾     

Ground state and properties of ferroelectric superlattices based on crystals of the perovskite family

The crystal structure of the ground state of ten free-standing ferroelectric superlattices based on crystals with the perovskite structure (BaTiO₃/SrTiO₃, PbTiO₃/SrTiO₃, PbTiO₃/PbZrO₃, SrZrO₃/SrTiO₃, PbZrO₃/BaZrO₃, BaTiO₃/BaZrO₃, PbTiO₃/BaTiO₃, BaTiO₃/CaTiO₃, KNbO₃/KTaO₃, and KNbO₃/NaNbO₃) was calculated from first principles within the density functional theory taking into account criteria for stability of the structures with respect to acoustic and optical distortions. It was shown that the ground state in all the considered superlattices corresponds to the ferroelectric phase. It was found that the polarization vector has a tendency toward a tilt to the plane of the superlattice layers, which makes it possible to decrease the electrostatic and elastic energy in the superlattices consisting of materials with different ferroelectric properties. The importance of the inclusion of structural distortions due to unstable phonons at the Brillouin zone boundary, which, in a number of cases, lead to significant changes in ferroelectric and dielectric properties of the superlattices, was demonstrated.     

Ferroelectricity in SrTiO3 nanocrystalline disks

SrTiO₃ and CaTiO₃ conventional bulk materials are incipient ferroelectrics. In this note, we report for the first time that ferroelectricity could occur in SrTiO₃ nanocrystalline disks even at room temperature. The peak in the temperature dependence of permittivity for a CaTiO₃ nanocrystalline disk at a low temperature is also observed. The observed ferroelectricity (or permittivity peak) in SrTiO₃ (or CaTiO₃) nanocrystalline disks could be attributed to the strain effect.     

Dynamic analysis of non-linear wake-up behavior in Hf0.7Zr0.3O2 thin film

We report on the nonlinear wake-up behavior against the external electric field cycling in the ferroelectric Hf_0.7Zr_0.3O₂ thin film. Two distinct scaling regimes during the increase of the remnant polarization with different activation energies were observed in TiN/Hf_0.7Zr_0.3O₂/TiN cells. The transmission electron microscopy revealed the structural phase transition from the monoclinic structure to the orthorhombic structure of the Hf_0.7Zr_0.3O₂ film after the wake-up behavior. During the phase change, as the remnant polarization enhanced, the dielectric constant of the Hf_0.7Zr_0.3O₂ film increased with the external field cycling. The temperature dependence of the wake-up behavior revealed that each estimated activation energies for the early and later enhancement of the remnant polarization are 1.12 eV and 0.73 eV, respectively. First principle calculations show that the oxygen vacancies can reduce the activation energy barrier for the structural phase transition.     

Raman spectroscopy study of lattice dynamics of macro-, micro-, and nanostructured barium titanates

The temperature dependences of the components A ₁(2TO) and E(1TO) of the soft ferroelectric mode during phase transitions in single crystals, ceramics, polycrystalline and epitaxial thin films of barium titanate, as well as a superlattice consisting of alternating layers of barium and strontium titanates, have been studied using the Raman spectroscopy method. Abrupt changes in soft mode frequencies have been observed in the single crystal during phase transitions between tetragonal, orthorhombic, and rhombohedral phases. Smoothing of the temperature dependences of soft modes and the coexistence of phases have been observed in ceramics and polycrystalline films. In the epitaxial film, the sequence of structural transformations fundamentally differs from that observed in the single crystal; in the superlattice, the ferroelectric phase is stable to 550 K.     

Effect of electromechanical boundary conditions on the properties of epitaxial ferroelectric thin films

The effects of internal stresses and depolarization fields on the properties of epitaxial ferroelectric perovskite thin films are discussed by employing the dynamic Ginzburg-Landau equation (DGLE). The numerical solution for BaTiO₃ film shows that internal stress and the depolarization field have the most effects on ferroelectric properties such as polarization, Curie temperature and susceptibility. With the increase of the thickness of the film, the polarization of epitaxial ferroelectric thin film is enhanced rapidly under high internal compressively stress. With the thickness exceeding the critical thickness for dislocation formation, the polarization increases slowly and even weakens due to relaxed internal stresses and a weak electrical boundary condition. This indicates that the effects of mechanical and electrical boundary conditions both diminish for ferroelectric thick films. Consequently, our thermodynamic method is a full scale model that can predict the properties of ferroelectric perovskite films in a wide range of film thickness.     

The under-pressure behaviour of mechanical,electronic and optical properties of calcium titanate and its ground state thermoelectric response

Abstract

In this study, the elastic, electronic, optical and thermoelectric properties of CaTiO₃ perovskite oxide have been investigated using first-principles calculations. The generalised gradient approximation (GGA) has been employed for evaluating structural and elastic properties, while the modified Becke Johnson functional is used for studying the optical response of this compound. In addition to ground state physical properties, we also investigate the effects of pressure (0, 30, 60, 90 and 120 GPa) on the electronic structure of CaTiO₃. The application of pressure from 0 to 90 GPa shows that the indirect band gap (Γ-M) of CaTiO₃ increases with increasing pressure and at 120 GPa it spontaneously decreases transforming cubic CaTiO₃ to a direct (Γ-Γ) band gap material. The complex dielectric function and some optical parameters are also investigated under the application of pressures. All the calculated optical properties have been found to exhibit a shift to the higher energies with the increase of applied pressure suggesting potential optoelectronic device applications of CaTiO₃. The thermoelectric properties of CaTiO₃ have been computed at 0 GPa in terms of electrical conductivity, thermal conductivity and Seebeck coefficient.     

Ab Initio Study of the Polarization,Electronic, Magnetic,and Optical Properties of Perovskite SrMO<Subscript>3</Subscript> (M = Fe,Mn) Crystals and Thin Films Containing Magnetic Ions

The magnetic, electronic, and polarization properties of the SrFeO₃ and SrMnO₃ compounds with a perovskite structure are calculated using the density functional theory in the bulk and thin-film states. A ferroelectric instability is found to be absent in the bulk state, and the polar mode is softened in the thin-film state of SrMnO₃ in the presence of tensile stresses in the substrate. As a result, a polar phase with a polarization of 23 μC/cm² appears, which agrees with experimental data. The study of the magnetic and electronic properties demonstrates the existence of G-type antiferromagnetic ordering in SrMnO₃ and the appearance of a dielectric gap of about 1.5 eV in its thin film. A ferromagnetic phase with metallic conduction in both the bulk and thin-film states is detected in SrFeO₃.     

Anomalous behavior of the dielectric response of quantum paraelectric CaTiO3 with iron impurities

We report on the studies of lattice dynamics and crystal structure of natural perovskite with the general formula CaTiO₃:0.01Fe by dielectric spectroscopy and X-ray diffraction in a wide temperature range. In contrast to the “chemically pure” synthetic perovskite, the lattice dynamics of the Fe-containing natural sample exhibits a structural instability in the vicinity of 240 K. Our studies of the behavior of magnetic moment of CaTiO₃:0.01Fe demonstrate that in the temperature region from 4.3 to 300 K this material is paramagnetic. No changes in the crystal symmetry have been observed in the structural studies, which can be explained by the small degree of lattice distortions, as has been observed previously for isotopically substituted SrTiO ₃ ¹⁸ . Conductivity measurements and analysis of the dielectric response dispersion of CaTiO₃:0.01Fe have shown that the dispersion is due to the Maxwell–Wagner mechanism and can be attributed to oxygen vacancies present in the mineral.