Probing the ferroelectric phase transition in sol–gel‐derived polycrystalline bismuth ferrite thin films

Polycrystalline BiFeO₃ (BFO) thin films were successfully grown on Pt/Ti/SiO₂/Si(100) and SrTiO₃ (STO) (100) substrates using the chemical solution deposition (CSD) technique. X‐ray diffraction (XRD) patterns indicate the polycrystalline nature of the films with rhombohedrally distorted perovskite crystal structure. Differential thermal analysis (DTA) was performed on the sol–gel‐derived powder to countercheck the crystal structure, ferroelectric (FE) to paraelectric (PE) phase transition, and melting point of bismuth ferrite. We observed a significant exothermic peak at 840 °C in DTA graphs, which corresponds to an FE–PE phase transition. Raman spectroscopy studies were carried out on BFO thin films prepared on both the substrates over a wide range of temperature. The room‐temperature unpolarized Raman spectra of BFO thin films indicate the presence of 13 Raman active modes, of which five strong modes were in the low‐wavenumber region and eight weak Raman active modes above 250 cm⁻¹. We observed slight shifts in the lower wavenumbers towards lower values with increase in temperature. The temperature‐dependent Raman spectra indicate a complete disappearance of all Raman active modes at 840 °C corresponding to the FE–PE phase transitions. There is no evidence of soft mode phonons. Copyright © 2008 John Wiley & Sons, Ltd.     

Landau-type pressure dependence of the order parameter: application to the ferroelectric-paraelectric pressure-induced transition in KNbO3

Following the Landau model, the pressure–temperature dependence of the order parameter is derived. Using the Lyddane–Sachs–Teller (LST) relationship, the model is applied to ferroelectricity to deduce the pressure behaviour of the soft mode driving the transition. Comparison with experiment is made using recent data obtained on KNbO₃ under pressure over a large temperature range. The results indicate that the ferroelectric–paraelectric (FE–PE) transition observed in KNbO₃ at high pressure from ～4 to ～25?GPa is of the second-order type.     

A shell model for the H-bonded ferroelectric KH2PO4

A shell model for KH₂PO₄ (KDP), the prototype compound of the family of H-bonded ferroelectric materials, has been constructed by adjusting the interaction parameters to first-principles calculations. Structural properties, energy barriers, phonons, and the relative stability between the ferroelectric (FE) phase and a relevant antiferroelectric metastable structure associated to domain walls, compare very favorably to available first-principles and experimental data. Molecular dynamics simulations show that the model behaves satisfactorily within the FE phase. This model will be used to study the elusive structure of the paraelectric (PE) phase and the nature of the FE–PE phase transition.     

Electron-polarization coupling in superconductor-ferroelectric superlattices

We present a phenomenological model of periodic ferroelectric-superconductor (FE-S) heterostructures containing two alternating ferroelectric and superconducting layers. The interaction at the FE-S contacts is described as a coupling of the local carrier density of the superconductor with the spontaneous ferroelectric polarization near the FE-S interface. We obtain a stable symmetric domain-type phase, exhibiting a contact-induced polarization and a ferroelectric domain structure at temperatures above the bulk ferroelectric transition temperature. With increasing coupling energy, we find the ferroelectric phase coexisting with the suppressed superconductivity in the S-film. The system is analyzed for different thicknesses of the FE- and S-films, demonstrating the dramatic change in the topology of the phase diagrams with a variation of the layers thickness. The results are expected to shed light on processes occurring in high-temperature superconducting films grown on perovskite alloy-substrates exhibiting ferroelectric properties at lower temperatures. PACS 74.81.-g; 74.78.Fk; 77.80.-e     

The effect of a strain induced field on the dielectric property of a ferroelectric bilayer system

The Transverse Ising Model (TIM) based on the effective-field theory has been developed to study the physical properties of the ferroelectric bilayer system BaTiO₃/SrTiO₃ (BTO/STO), based on the differential operator technique. The effect of strain on the interfacial layers between two different slabs (A and B) can be described by the effective built-in field E₂. The ferroelectric behavior of a bilayer system is strongly influenced by strain and associated with slab thickness. The phase transition temperature shifts toward a higher value on increasing the slab thickness. The susceptibility strongly depends on both the strength of strain and the slab thickness. The height of the peak from the plot of susceptibility against temperature decreases on increasing the slab thickness. The pyroelectric coefficient changes into a round peak at the transition temperature that is different from the sharp peak in the absence of external and strain-induced fields.     

Anisotropic dielectric properties of PbYb1/2Ta1/2O3 single crystals

PbYb_1/2Ta_1/2O₃ single crystals were obtained for the first time. They were grown by the flux method. The PbOPbF₂B₂O₃ system was used as a solvent. Dielectric investigations were carried out in 1 0 0_c, 1 1 0_c and 1 1 1_c pseudocubic directions. These studies pointed to anisotropy of dielectric properties. Frequency-independent ε′(T) and ε″(T) maxima related to the antiferroelectric–paraelectric (AFE—PE) phase transition are observed for all directions at 562 K. The frequency-dependent ε′(T) and ε″(T) maxima near 400 K related to the ferroelectric (FE)–AFE phase transition are observed only in 1 1 1_c direction. The hysteresis loops were observed in this direction only. These results point that ferroelectric relaxor properties appear only in 1 1 1_c direction. We propose to consider the ferroelectric phase as ferrielectric one.     

Piezoelectricity in La0.85Ce0.15MnO3 layer of BiFeO3/ La0.85Ce0.15MnO3 based ferroelectric/semiconductor oxide superlattice

Superlattice comprising of ferroelectric (FE) and non-ferroelectric (non-FE) material has been developed to induce a ferroelectric response in the non-ferroelectric layer. In this study, a superlattice thin film comprising of BiFeO₃ (BFO) as FE oxide and La_0.85Ce_0.15MnO₃ (LCMO) as semiconducting non-FE oxide is fabricated and investigated the FE and piezoelectric responses in LCMO. We observed the piezoelectric response of individual layers using time-resolved X-ray microdiffraction experiment under an external electric field. Piezoelectric response of individual layers was resolved by comparing it with the values obtained from kinematic diffraction calculation. Piezoelectric coefficient (d₃₃) of LCMO monolayer is found to be approximately 27 pm/V, which is a similar value as that of the BFO layer.     

Observation of the ferroelectric phase transition in Pb5Ge3O11 by the chemical potential changes

We observed the transition from the ferroelectric (FE) to paraelectric (PE) phase in the semiconducting, ferroelectric Pb₅Ge₃O₁₁ single crystal with the use of the contact electrode method. To this purpose a thin, metallic layer was placed onto the Pb₅Ge₃O₁₁ crystal surface, forming the contact electrode. At opposite ends of the contact electrode, silver wires were glued and a voltage was applied to the contact electrode in such a way that the electric current could flow only through the attached electrode. The electric resistance R(T) of the electrode was measured as a function of temperature. Two series of measurements were performed. In one of them the ferroelectric c-axis of the investigated crystal was perpendicular to the contact electrode. In the second one the c-axis was parallel to the attached electrode. We used gold as the contact electrode material. The anomaly in the R(T) in a form of a kink at T _kink?=?452?K was found for both c-axis orientations. The measured value of T _kink, appearing in the temperature dependence of contact electrode resistance, corresponds exactly to the phase transition temperature T _C from the FE to PE phase of the investigated Pb₅Ge₃O₁₁ material. This result demonstrates that the contact electrode method, primarily proposed exclusively to find critical temperatures of metallic samples, also works well in the case of ferroelectric and semiconducting materials like Pb₅Ge₃O₁₁. We ascribe the effect of the resistance kink in the temperature dependence of the contact electrode R(T) to thermal excitations of the electrons with different rates below and above T _C due to different electronic activation energies in the FE and PE phases of the investigated Pb₅Ge₃O₁₁ crystal. It, however, means that the phase transition in the electronic subsystem of the Pb₅Ge₃O₁₁ transfers into the electron gas of the contact electrode via the chemical potential relation µ _sample?=?µ _electrode due to the contact between the sample and the electrode. The magnitude of the kink, observed in the R(T) dependence, was higher on heating than on cooling. The additional measurement of the thermally stimulated current (TSC) was carried out on the non-polarised Pb₅Ge₃O₁₁ sample. In this series of measurements, the sample was covered with gold layers sputtered on the two opposite surfaces of the crystal. The TSC anomaly occurred, related to the residual pyroelectric effect, several degrees below the Curie temperature, T _C, and does not disturb the detection of the critical point with the use of the contact electrode method.     

Low‐temperature Raman spectroscopic studies in NaNbO3

Raman spectroscopic measurements were carried out in the temperature range 10–300 K to understand the low‐temperature antiferroelectric (AFE)–ferroelectric (FE) phase transition in NaNbO₃. Several modes in the low wavenumber range were found to disappear, while some new modes appeared across the transition. The temperature dependence of mode wavenumbers suggests that, during cooling, the AFE–FE phase transition begins to occur at 180 K, while the reverse transition starts at 260 K during heating. During cooling, the two phases were found to coexist in the temperature range of 220–160 K. Upon heating, the FE phase is retained up to 240 K and both FE and AFE phases coexist in the temperature range 240–300 K. In contrast to the earlier reports, the present results suggest a different coexistence region and the reverse transition temperature. The reported relaxor‐type FE behaviour over a broad temperature is consistent with the observed coexistence of phases during cooling and heating cycles. Copyright © 2010 John Wiley & Sons, Ltd.     

Electrocaloric effect in ferroelectric polymers

The electrocaloric effect (ECE) of poly (vinyledene fluoride?Ctrifluoroethylene) (P(VDF?CTrFE)) 55/45?mol% copolymers was directly measured, which confirms the results deduced from Maxwell relation. The adiabatic temperature change ??T under a given electric field peaks at the ferroelectric?Cparaelectric (FE?CPE) transition. Away from it, ECE becomes small. ??T versus applied electric field can be described well by a modified Belov?CGoryaga equation. The ECE in ferroelectric polymers, especially near FE?CPE transition where larger ECE is observed, are analyzed under different boundary conditions employing phenomenological theory and constitutive equations. The secondary pyroelectricity is found to play a significant role which enhances ECE in ferroelectric polymers.     

Field-induced reentrant novel phase and a ferroelectric-magnetic order coupling in HoMnO3

A reentrant novel phase is observed in the hexagonal ferroelectric HoMnO3 in the presence of magnetic fields in the temperature range defined by a plateau of the dielectric constant anomaly. The plateau evolves with fields from a narrow dielectric peak at the Mn-spin rotation transition at 32.8 K in zero field. The anomaly appears both as a function of temperature and as a function of magnetic field without detectable hysteresis. This is attributed to the indirect coupling between the ferroelectric (FE) and antiferromagnetic (AFM) orders, arising from an FE-AFM domain wall effect.     

Size-induced appearance of ferroelectricity in thin antiferroelectric films

In the paper we consider size effects on phase transitions and polar properties of thin antiferroelectric films. We extend the phenomenological approach proposed by Kittel for thin films allowing for gradient (correlation) energy and depolarization field energy. Surface piezoelectric effect as well as misfit strain appear due to lattice constants mismatch between the film and its substrate. Direct variational method is used to derive the free energy with renormalized coefficients depending on the film thickness. Obtained free energy expression allows the calculation of phase diagrams and all electro-physical properties by a conventional minimization procedure. Approximate analytical expressions for the paraelectric–antiferroelectric–ferroelectric transition temperature dependences on film thickness, polarization gradient coefficient, and extrapolation lengths were obtained. The thickness dependence of the electric field critical value that causes antiferroelectric–ferroelectric phase transition was calculated. Under favorable conditions the antiferroelectric phase at first transforms into ferroelectric one and then into paraelectric phase with the decrease of the film thickness. Proposed theoretical consideration explains the experimental results obtained in antiferroelectric PbZrO₃ thin films.     

Collinear to spiral spin transformation without changing the modulation wavelength upon ferroelectric transition in Tb1-xDyxMnO3

Lattice modulation and magnetic structures in magnetoelectric compounds Tb1-xDyxMnO3 have been studied around the ferroelectric (FE) Curie temperature TC by x-ray and neutron diffraction. Temperature-independent modulation vectors through TC are observed for the compounds with 0.50 相似文献   

High-resolution calorimetric study of pb(mg_{1/3}nb_{2/3})o_{3} single crystal

Motivated by the long-standing unresolved enigma of the relaxor ferroelectric ground state, we performed a high-resolution heat capacity and polarization study of the field-induced phase transition in the relaxor ferroelectric single crystal Pb(Mg_{1/3}Nb_{2/3})O_{3} (PMN) oriented along the [110] direction. We show that the discontinuous evolution of polarization as a function of the electric field or temperature is a consequence of a true first order transition from a glassy to ferroelectric state, which is accompanied by an excess heat capacity anomaly and released latent heat. We also find that in a zero field there is no ferroelectric phase transition in bulk PMN at any temperature, indicating that the nonergodic dipolar glass phase persists down to the lowest temperatures.     

The ferroelectric phase transition in tridymite type BaAL2O4 studied by electron microscopy

The paraelectric-ferroelectric (PE-FE) phase transition in stuffed tridymite BaAl₂O₄ was studied in situ by transmission electron microscopy. Electron diffraction revealed that the PE and FE phases have hexagonal symmetry. The PE-FE phase transition is accompanied by a doubling of the cell dimensions in the a-b plane. The transition is reversible, takes place over a wide temperature range (400–670 K.) and the interfaces related to the transition have a fluctuating character. The crystal structure of the high temperature PE phase was determined by high resolution electron microscopy. The structures of the PE phase (space group P6₃22, a≈ 5.22 Å, c ≈ 8.8 Å) and of the FE phase (space group P6₃, a= 10.4469(1)Å, c = 8.7927(1)Å) differ mainly by the configuration of the Al-O strings oriented along the c-axis. In the PE phase all the strings are equivalent whereas straight and corrugated strings alternate in an ordered manner in the FE phase resulting in doubling of the a and b cell parameters. Translation and orientation domains due to the decrease of the translation and point symmetry were frequently observed.     

Ferroelectric phase stabilization, phase transformations and?thermal properties in (1? x )PbZrO3– x Pb(Co1/3Nb2/3)O3 solid solution

The solid solution between the antiferroelectric PbZrO₃ (PZ) and relaxor ferroelectric Pb(Co_1/3Nb_2/3)O₃ (PCoN) was synthesized by the columbite method. The phase structure and thermal properties of (1−x)PZ–xPCoN, where x=0.0–0.3, were investigated. With these data, the ferroelectric phase diagram between PZ and PCoN has been established. The crystal structure data obtained from XRD indicates that the solid solution PZ–PCoN, where x=0.0–0.3, successively transforms from orthorhombic to rhombohedral symmetry with an increase in PCoN concentration. The AFE→FE phase transition was found in the compositions of 0.0≤x≤0.10. The AFE→FE phase transition shift to lower temperatures with higher compositions of x. The width of the temperature range of FE phase was increased with increasing amount of PCoN. It is apparent that the replacement of the Zr⁴⁺ ion by (Co_1/3Nb_2/3)⁴⁺ ions would decrease the driving force for antiparallel shift of Pb²⁺ ions, because they interrupt the translational symmetry. This interruption caused the appearance of a rhombohedral ferroelectric phase when the amount of PCoN was more than 10 mol%.     

钛酸盐ATiO3钙钛矿的铁电和反铁畸变

钛酸盐钙钛矿氧化物体系的研究对理解具有铁电极化的功能材料具有非常重要的科学意义.本文对ATiO3(A=Ca,Sr,Ba,Pb,Cd)体系的低温相、结构相变、及其对应的位移模式进行综述.我们着重比较了该体系存在的两类不稳定声子模式,即铁电极化模式(Ferroelectric,FE)和反铁畸变模式(Antiferrodistortive,AFD),在不同体系中所起到的作用.我们举例阐述了晶格失配应变、人工构造超晶格和化学掺杂等方法对这两种模式的调控思路.最后我们给出本文的总结讨论及研究展望.     

Coexistence of ferroelectric and antiferroelectric microregions in the paraelectric phase of NH4H2PO4 (ADP)

Ammonium dihydrogen phosphate NH₄H₂PO₄ (ADP) is an antiferroelectric (AFE) compound belonging to the KDP-type family of hydrogen-bonded ferroelectrics. Recent ab initio results have shown that the optimization of the N–H–O bridges in ADP leads to the stabilization of the AFE state over a FE one. However, electron spin probe measurements have suggested that microregions of both phases may coexist above the critical antiferroelectric–paraelectric transition temperature. We have performed first principles studies of the energetics and relative stability of different AFE and FE defects embedded in a paraelectric (PE) matrix of ADP. Our analysis indicates that FE and AFE clusters are stable and may coexist in the PE phase, thus confirming the above suggestion.     

Phase coexistence in proton glass

Abstract

Proton glasses are crystals of composition M_1?x(NW₄)_xW₂A0₄, where M = K,Rb,Cs, W = H,D, A = P, As. For x = 0 there is a ferroelectric (FE) transition, while for x-1 there is an antiferroelectric (AFE) transition. In both cases, the transition is from a paraelectric (PE) state of tetragonal structure with dynamically disordered hydrogen bonds to an ordered state of orthorhombic structure. For an intermediate x range there is no transition, but the hydrogen rearrangements slow down, and eventually display nonergodic behavior characteristic of glasses. We and others have shown from spontaneous polarization, dielectric permittivity, nuclear magnetic resonance, and neutron diffraction experiments that for smaller x there is coexistence of ferroelectric and paraelectric phases, and for larger × there is coexistence of antiferroelectric and paraelectric phases. We present a method for analytically describing this coexistence, and the degree to which this coexistence is spatial and/or temporal. We discuss also the experimental determination of these coexistence parameters.     

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