Toroidal ferroelectricity in PbTiO3 nanoparticles

We report from first-principles-based atomistic simulations that ferroelectricity can be sustained in PbTiO(3) nanoparticles of only a few lattice constants in size as a result of a toroidal ordering. We find that size-induced topological transformations lead to the stabilization of a ferroelectric bubble by the alignment of vortex cores along a closed path. These transformations, which are driven by the aspect ratio of the nanostructure, change the topology of the polarization field, producing a rich variety of polar configurations. For sufficiently flat nanostructures, a multibubble state bridges the gap between 0D nanodots and 2D ultrathin films. The thermal properties of the ferroelectric bubbles indicate that this state is suitable for the development of nanometric devices.     

Size effect in isometric PbTiO3 crystals

The phase transitions Pm3m⇄P4mm and domain-structure formation in 5–500 μm isometric PbTiO₃ crystals were investigated. The phase transition is characterized by a high rate and by the formation of a single flat interphase boundary {023}. A size effect was observed: In crystals smaller than a critical size (about 20 μm), formation of 90° domains stops and, in agreement with a phenomenological theory, the temperature hysteresis of the phase transition doubles. Fiz. Tverd. Tela (St. Petersburg) 40, 1546–1547 (August 1998)     

Dynamical study of phonons in PbTiO3

Lattice dynamical formalism, using rigid ion model, was applied to the crystal of PbTiO₃ in the tetragonal phase. The model includes long range Coulomb forces and short range axially symectric forces. The radial part of the short range force constants was assumed to be of Born-Meyer type. The parameters of the model were determined through a least squares analysis of zone center and a few zone boundary phonons. Dispersion curves for [100] and [001] directions are presented.     

Formation of the ferroelectric domains in epitaxial PbTiO3 thin films grown by metalorganic chemical vapor deposition

3 thin films have been prepared by metalorganic chemical vapor deposition under reduced pressure. The formation of ferroelectric domains in films grown on SrTiO₃ and LaAlO₃ substrates was investigated by synchrotron radiation and Rutherford backscattering spectroscopy. Single-domain (3000-Å thick) and multi-domain (4500-Å thick) PbTiO₃ films were produced on SrTiO₃. For multi-domain PbTiO₃ film, the c-domain presented epitaxial structure with its c-axis perpendicular to the substrate surface, while a-domains aligned four-fold symmetrically with c-domains by 2.79^° off the c-axis of c-domains. In the film, the measured lattice constants (a, b and c) of the a- and c-domains were different from each other, indicating that the films suffered a modulated strain during domain formation. In contrast, both the a and c domains of films on LaAlO₃ were alternatively aligned on substrate with the a-axis of the a-domain and the c-axis of c-domains perpendicular to the substrate surface. Two-dimensional distribution of these domains is proposed and the formation of these kinds of domains is discussed. The surface morphology and phase transition process of single and multi domain PbTiO₃ film on SrTiO₃ were studied by atomic force microscope (AFM) and high temperature X-ray diffraction, respectively. Received: 15 August 1996/Accepted: 21 January 1997     

Ferroelectricity and tetragonality in ultrathin PbTiO3 films

The evolution of tetragonality with thickness has been probed in epitaxial c-axis oriented PbTiO3 films with thicknesses ranging from 500 down to 24 A. High resolution x ray pointed out a systematic decrease of the c-axis lattice parameter with decreasing film thickness below 200 A. Using a first-principles model Hamiltonian approach, the decrease in tetragonality is related to a reduction of the polarization attributed to the presence of a residual unscreened depolarizing field. It is shown that films below 50 A display a significantly reduced polarization but still remain ferroelectric.     

Ferroelectrics

A diatomic linear chain model is used to describe the dynamical properties of displacive type ferroelectric compounds. For these materials the non-bondingp-orbitals of the chalcogen ions play an essential role, which is taken into account by assuming a nonlinear fourth-order polarizability at the chalcogen-ion lattice site. Within the self-consistent phonon approximation soft modes, phonon dispersion curves, phonon anomalies and related quantities can be calculated for all temperatures. Going beyond the self-consistent phonon approximation, the model yields, in the continuum limit, interesting new solutions such as periodic non-linear waves, kinks, which describe the statics and dynamics of ferroelectric domain walls, and pulse solutions.Dedicated to Professor Harry Thomas on the occasion of his 60th birthday     

Dielectric characteristics of La-modified PbTiO3 nanoceramics

The nanoceramics of lanthanum (La³⁺)-modified lead titanate (PbTiO₃) with a general formula Pb_1?yLa_yTi_(1?y/4)O₃ (y = 0.00, 0.06, 0.08, 0.10, 0.12 Lead Lanthanum Titanate (PLT)) were synthesized by a mechanothermal method. Thermal analysis of the PLT samples exhibits their nature of the reaction and thermal stability. The incorporation of La at the Pb site of lead titanate (PT) not only changes the ratios of its unit cell parameters but also reduces the grain and particle size to nanoscale. The dielectric constant of PLT is dependent on La concentration. The La concentration-dependent phase transition temperature of PbTiO₃ is one of the main characteristics of the material.     

Stabilization of monodomain polarization in ultrathin PbTiO3 films

Using in situ high-resolution synchrotron x-ray scattering, the Curie temperature TC has been determined for ultrathin c-axis epitaxial PbTiO3 films on conducting substrates (SrRuO3 on SrTiO3), with surfaces exposed to a controlled vapor environment. The suppression of TC was relatively small, even for the thinnest film (1.2 nm). We observe that 180 degrees stripe domains do not form, indicating that the depolarizing field is compensated by free charge at both interfaces. This is confirmed by ab initio calculations that find polar ground states in the presence of ionic adsorbates.     

Summary data on ferroelectric PbTiO3 structure

The original data and the data available in literature on PbTiO₃ structure are considered and discussed. From the results of an X-ray diffraction study carried out at room temperature it is concluded that the displacive model is more relevant than the order-disorder one. At the same time it is stressed that the Debye-Waller factors of the atoms are very unusual in the region of the phase transition: B(Pb) > B(O) > B(Ti). This may be evidence of order—disorder phenomena taking place in this temperature region.     

Isofrequency Opalescence in Ferroelectrics

Isofrequency dependences of the spectral intensities of inelastic scattering in LiTaO₃, Pb₅Ge₃O₁₁, BaTiO₃, and KNbO₃ crystals are registered at a fixed frequency of the spectrometer close to that of the exciting radiation. Results from soft mode analysis are presented.     

PbTiO3/PbZr0.3Ti0.7O3/PbTiO3夹心结构铁电薄膜子晶层的优化

用Sol-Gel法制备了Pb(1 x)TiO3/PbZr0.3Ti0.7O3/Pb(1 x)TiO3(PT/PZT/PT)夹心结构及PZT铁电薄膜,为了获得高质量的PT/PZT/PT夹心结构铁电薄膜,使用不同过量Pb配比(x)的PbTiO3 (PT)层进行制备,以获得优化的PT子晶层.X射线衍射和原子力显微镜分析结果表明PT层中过量Pb配比(x)对薄膜的微结构影响很大,只有PT层中Pb过量配比x＝0.10-0.15的薄膜为表面晶粒大小均匀致密的纯钙钛矿结构.X射线电子能谱对薄膜微区进行元素成分分析表明,对x=0.00的薄膜,在表面和界面处Pb明显的缺乏;而x=0.20时的薄膜,Pb则明显的过量.薄膜的铁电性能、疲劳特性和漏电流特性等电学性能与PT层中过量Pb配比(x)没有明显的变化趋势,但与薄膜的结晶性能密切相关.结晶性能较好的薄膜,其电学性能也较好.说明PT层中过量Pb配比(x)是通过影响PT子晶层自身的结晶,而影响整个薄膜的结晶行为,并进一步影响到整个薄膜的电学性能.因此,在其他工艺参数都相同时,PT层中合适的过量Pb配比应为x=0.10-0.15.优化的子晶层不仅能获得结晶性能较好的薄膜,而且薄膜的电学性能也好.     

Ferroelectrics in Plasma Chemistry

The qualitative considerations are presented concerning the use of plasma, generated at the surface of ferroelectric samples, for the investigation of chemical processes. The physical principles of the phenomena, its possibilities and limitations are described. Also, preliminary conclusions resulting from the investigations carried out up to the present, are drawn up.     

Changes in the structure of PbTiO3 with temperature variation

Methodics of tests on structural changes with temperature of crystalline PbTiO₃ are described and results are presented. In the vicinity of 100 °C a certain anomaly was found, that is a specific structural transition related to the parameters of electric conduction and of a relaxation nature.     

Dispersion Curve of PbTiO3 in Tetragonal and Cubic Phases

The dispersion curves of PbTiO₃ in both tetragonal (ferroelectric) and cubic (paraelectric) phases are calculated using the rigid-ion model with a small set of parameters. The soft modes in two phases are discussed.     

Phase Transitions of KIO3 Ferroelectrics in Al2O3-Based Nanoporous Matrices

Physics of the Solid State - Temperature dependences of the linear permittivity ε' and the third harmonic amplitude γ3ω of composites prepared by introducing ferroelectrics KIO3...     

Unusual behavior of the ferroelectric polarization in PbTiO3/SrTiO3 superlattices

Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO3 to SrTiO3 was changed. For PbTiO3 layer thicknesses larger than the 3-unit cell SrTiO3 thickness used in the structure, the polarization is found to be reduced as the thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO3 to SrTiO3 of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.     

Pulsed High-Frequency EPR Investigation of Gadolinium-Doped PbZrO3 and PbTiO3

Using a resonator-free setup, pulsed high-frequency (240 GHz) electron paramagnetic resonance (EPR) experiments on gadolinium-doped PbTiO₃ and PbZrO₃ samples have been performed. It could be demonstrated that echo-detected EPR spectra can be recorded routinely from these materials. These compounds are highly absorptive at microwave frequencies, thus preventing the use of microwave resonators at very high frequencies. As echo-detected EPR allows us to record the EPR absorption directly, the effect of relative suppression of broad unstructured spectral components in conventional field-derivative EPR is avoided. The analysis of the high-frequency EPR spectra indicated that Gd³⁺ ions are additionally also positioned at highly distorted sites. This might indicate that charge compensation leads to the formation of closely correlated Gd³⁺-V _Pb′′-Gd³⁺ defects under high doping conditions in addition to Gd³⁺ inserted substitutionally at Pb²⁺ sites with undistorted oxygen and lead neighboring shells. For the orthorhombic low-temperature phase of PbZrO₃ two crystallographically inequivalent Pb²⁺ sites of equal abundance are present. The contribution of Gd³⁺ inserted substitutionally at these sites could be confirmed.     

Orientation dependence of the electrocaloric effect in PbTiO3/SrTiO3 multilayers

A phenomenological thermodynamic model is presented to investigate the effects of the orientation and the relative thickness of SrTiO₃ layer on the electrocaloric effect in PbTiO₃/SrTiO₃ multilayers. Theoretical calculations show that complete polarization suppression and a large electrocaloric effect take place at the critical relative thickness of SrTiO₃ layer of 46%, 33%, and 30% for the (001), (110), and (111) oriented PbTiO₃/SrTiO₃ multilayers respectively, not occuring at the critical relative thickness of 63%, 50%, and 46% reported by Wu et al. [H.P. Wu, A.P. Liu, L.Z. Wu, S.Y. Du, Appl. Phys. Lett. 93 (2008) 242909]. Moreover, (111) oriented multilayers have the largest electrocaloric effect which is different from that of BaTiO₃/SrTiO₃ multilayers where (110) oriented multilayers have the largest electrocaloric effect. Consequently, the electrocaloric effect of PbTiO₃/SrTiO₃ multilayers can be adjusted by the orientation and the relative thickness of SrTiO₃ layer.     

Photoluminescence of Copper-Doped Lithium Niobate Crystals

The photoluminescence (PL) of copper-doped lithium niobate single crystals is studied using different UV–Vis light-emitting diodes and a pulse-periodic laser with a wavelength of 266 nm as excitation radiation sources. With the resonance excitation from a 527-nm light-emitting diode, the intensity of PL increases sharply (by two orders of magnitude). When using a 467-nm light-emitting diode for excitation, the PL spectrum is characterized by the presence of multiphonon lines in the range of 520–620 nm.     

X-ray photoelectron diffraction study of ultrathin PbTiO3 films

Full hemispherical X-ray photoelectron diffraction (XPD) experiments have been performed to investigate at the atomic level ultrathin epitaxial c-axis oriented PbTiO₃ (PTO) films grown on Nb-doped SrTiO₃ substrates. Comparison between experiment and theory allows us to identify a preferential ferroelectric polarization state in a 60 ? -thick PTO film. Multiple scattering theory based on a cluster-model [ Phys. Rev. B , 075404 (2001)] is used to simulate the experiments.