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.     

SEM,SAED, and TEM investigations of domain structure in PZT ceramics at morphotropic phase boundary

SEM investigations of ferroelectric domain structure in PbZr_0.53Ti_0.47O₃ are consistent with a model of spatial domain configuration for the piezoelectric ceramics previously proposed for BaTiO₃. TEM and SAED results revealed not only the twinning relation of adjacent tetragonal 90° domains but also the simultaneous presence of the ferroelectric rhombohedral phase. A succession model of ferroelectric domains T₁RT₂RT₁... which needs a smaller energy for the rotation of the polarization vector due to the coexistence of a R domain between the two T 90° domains is proposed. This model is also confirmed by the estimated value of elastically stored energy in the mixed wall and by the dependence upon the sintering temperature of T and R unit cell distortions previously measured by x-ray diffraction.     

Investigation of specific features of the lattice dynamics and the ferroelectric transition in perovskite crystals

The specific features of diffuse X-ray scattering in BaTiO₃, KNbO₃, and PbTiP₃ perovskite crystals have been investigated. The former two perovskite compounds in cubic, tetragonal, and orthorhombic phases exhibit anomalous sheets due to diffuse X-ray scattering, whereas no similar sheets are observed in the case of diffuse X-ray scattering in PbTiO₃. For these compounds, the phonon spectra are calculated in the quasi-harmonic approximation within the polarizable-shell model, and the mechanism of stabilization of the soft mode above the temperature of the phase transition to the ferroelectric state is considered. It is demonstrated that, in the cubic phase of BaTiO₃ and KNbO₃ crystals, there exist quasi-one-dimensional “soft” modes of vibrations of ions in M-O-M-O- chains, where M = Ti or Nb. In PbTiO₃, this feature of the soft mode has not been revealed. The pair correlation functions of simultaneous atomic displacements in BaTiO₃, KNbO₃, and PbTiO₃ are determined and used to calculate the intensity of diffuse X-ray scattering. The results obtained are in good agreement with experimental data. This is a strong argument in support of the hypothesis that the specific features of diffuse scattering are associated with the existence of quasi-one-dimensional correlations of atomic displacements in the soft optical mode and that the ferroelectric transition in perovskites is a displacive ferroelectric phase transition. The possible influence of the specific features revealed in the phonon spectra of the perovskite crystals on the processes of nuclear magnetic resonance and X-ray absorption (extended X-ray absorption fine structure spectra) is briefly discussed.     

The interactions between dislocations and ferroelectric domains in KNbO3 crystals

It is found that it is rather easy to introduce dislocations into KNbO₃. With transmission electron microscopy we have determined the slip plane to be (110) and the Burgers vector of dislocations to be [110]. Using hot stage in a JEM-200CX electron microscope, we have made in situ observations during phase transitions at 435°C (cubic-tetragonal) and 225°C (tetragonal-orthorhombic). We have found evidence indicating the interactions between dislocations and ferroelectric domains. Especially during phase transitions, the new ferroelectric phase first appear in the vicinity of dislocations showing that the stress field of dislocations may raise Curie point of the crystals.     

The channeling of light ions in the cubic and tetragonal (ferroelectrically polarized) phases of BaTiO3

Abstract

The channeling properties of thin BaTiO₃ crystals at temperatures both above and below the ferroelectric Curie point (T _c ? 120°C)have been measured with 3.8 MeV protons. In the cubic phase (T > T _c ), values for critical angles and minimum yields, have been measured for the major crystal axes and planes by detecting backscattered and transmitted protons, and characteristic X-rays. These values are compared with those predicted by current theories of ion channeling. In the tetragonal phase (T < T _c ), measurements were made with single ferroelectric domains. The channeling characteristics of the (100) planes in BaTiO₃ are found to be strongly dependent on the relative orientations of the electric polarization vector P, the (100) planes, and the beam direction. For example, when P lies in the channeling plane, strong channeling occurs; but when P is normal to the plane, the incident beam is rapidly dechanneled. These effects are attributed to the relatively large ionic displacements, and the strong internal electric fields in the ferroelectrically polarized state.     

Switching of partially switched KNbO3 single crystals containing cooperatively ordered impurity dipoles

Switching studies have been carried out in partially switched KNbO₃ single crystals by observing switching transients and hysteresis loops. The crystals used contained ordered impurity dipoles that are active in nucleating domains around them. Partial initial switching was obtained by applying known compressive stress to the crystal by means of a spring. The partially switched nature was determined by recording the photograph of the crystal surface. The changed domain structure on the surface gave a clear idea of the extent of partial switching. As the compressive stress was gradually increased, the crystal showed increased initial mechanical switching through the mechanism of evaporation of domain walls associated with ordered impurity dipoles. The dipoles then switch systematically converting 90° domains with polar axes in the plane of plate into 60° domains with polar axes in the perpendicular pseudocubic {001} planes. The initial switching condition changes the switching characteristics as determined by hysteresis loops and switching transients. The results are interpreted in terms of domains in the crystal. If the dipole density is quite high, the effect of the dipoles becomes negligible, and the switching behaviour approximates that of a normal ferroelectric. The switching transients and the hysteresis loops in the crystals containing cooperatively ordered dipoles are basically different from the ones observed in normal ferroelectrics. The anomalous behaviour is detrimental to the use of material in device applications. Hence, it is shown that the switching transients and hysteresis loops provide a ready means of detecting the presence of these ordered impurity dipoles.     

Strain-dependent magnetic configurations in manganite-titanate heterostructures probed with soft X-ray techniques

We present a detailed study on the strain-induced magnetic domain structure of a (La,Sr)MnO₃ thin film epitaxially grown on a BaTiO₃ substrate through the use of polarization-dependent X-ray photoemission electron microscopy and X-ray absorption spectroscopy. Angular-dependent measurements allow us to detect vector magnetization on a single-domain scale, and we relate the strain-induced changes in magnetic anisotropy of the ferromagnetic film to the ferroelectric domain structure of the underlying substrate using X-ray magnetic circular and linear dichroism spectro-microscopy. Comparisons to measurements on a nearly strain free film of (La,Sr)MnO₃ grown on a (La,Sr)(Al,Ta)O₃ substrate illustrate that the BaTiO₃ ferroelectric domain structure imprints specific domain sizes and wall orientations in the (La,Sr)MnO₃/BaTiO₃ artificial multiferroic heterostructure. Furthermore, a change of the BaTiO₃ ferroelectric domain structure either with temperature or with applied electric field results in a corresponding change in the (La,Sr)MnO₃ ferromagnetic domain structure, thus showing a possible route to obtain room-temperature electric field control of magnetic anisotropy at the nanoscale.     

Growth and characterization of ferroelectric SrBi2Ta2O9 single crystals via high-temperature self-flux solution method

SrBi₂Ta₂O₉ (SBT) single crystals were produced by the high-temperature self-flux solution method using a Bi₂O₃ flux modified with B₂O₃. The processing conditions were optimized to obtain large and translucent SBT crystals with a layered habit and typical dimensions of approximately 7 × 5 × 0.2 mm. X-ray diffraction and x-ray topography measurements revealed that the major faces of the crystals with natural rectangular platelet morphology are perfectly (001)-oriented with edges directed along the [110] directions. The high quality of the crystals was confirmed by rocking curves (half-width of 0.04° for the (0018) reflection) and by ferroelectric measurements. The anisotropy in the dielectric and ferroelectric properties was investigated both along the [110] (ab plane) and the [001] (c axis) directions. The growth mechanism, morphology, and dielectric anisotropy of the SBT crystal platelets are discussed based on its crystallographic structure. This article was submitted by the authors in English     

High-resolution identification of ½⟨110⟩ stacking faults in epitaxial Ba0.3Sr0.7TiO3 thin films

The near-interface region of an epitaxial Ba_0.3Sr_0.7TiO₃ thin film grown on LaAlO₃ (001) was found to consist of a high density of ½?110? stacking faults bounded by partial dislocations. The stacking faults can extend over large distances (greater than 50 nm). Various possible atomic configurations of the faults were considered. The atomic structures of the faults were identified using high-resolution electron microscopy and simulation as well as energy-filtered imaging. The ½[101] and faults (where [001] is normal to the film plane) were found to lie predominately on the {100} and {110} planes. The ½lsqb;101] faults on (010), (110) or (1&1tilde;0) have never been observed before in perovskites. The stacking faults on [100] have a structure consisting of a double layer of edge-sharing TiO₆ octahedra. The excess of Ti was detected by energy-filtered imaging. The formation of the extended stacking faults is probably related to a small amount of excess Ti during the film deposition, which may originate from the non-stoichiometry of the ceramic targets BaTiO₃ and SrTiO₃. It is also enhanced by the misfit-induced compressive strain in the early stages of the film growth.     

Unusual domain configurations and local piezoresponse in the [720]‐cut BaTiO3 single crystal

The domain configurations and local piezoresponse property of the [720]‐cut BaTiO₃ single crystal and [001]‐oriented BaTiO₃ crystal were investigated by high‐resolution piezoresponse force microscopy. Large differences in their surface topography features, domain configurations and local piezoresponse were found between [720]‐cut and [001]‐oriented BaTiO₃ crystals. The large surface bending angle due to ferroelastic domain walls leads to a high strain energy appearing in the [720]‐cut BaTiO₃, further resulting in unique band‐like topographic features, needle domains for stress compensation and locally ultrahigh piezoelectric response as well. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Asymmetrical twin boundaries and highly dense antiphase domains in BaNb0.3Ti0.7O3 thin films

Asymmetrical twin boundaries and highly dense antiphase domains were identified by means of transmission electron microscopy in a perovskite-based BaNb_0.3Ti_0.7O₃ thin film grown by laser molecular beam epitaxy on a SrTiO₃ (001) substrate. The microstructural characteristics of the BaNb_0.3Ti_0.7O₃ film were clarified in terms of lamellar {111} twins and antiphase domains, the domain boundaries of which are 1/2?110? stacking faults. It is proposed that the intersections of (111) twinning with the antiphase domain boundaries result in the asymmetrical twin boundaries.     

The growth and domain structure of YBa2Cu3Ox films on neodymium gallate substrates with a deviation of the normal to the surface from the [110] direction in NdGaO3

The growth, crystal structure, and electrophysical properties of YBa₂Cu₃O_x (YBCO) epitaxial films grown with and without a CeO₂ epitaxial sublayer on NdGaO₃ (NGO) substrates with the normal to the surface deviating from the [110] axis by 5° to 26.6° around the [001] axis are investigated. It is shown that the orientation of YBCO epitaxial films grown on such substrates is determined by the existence of symmetry-equivalent directions in the substrate and in the CeO₂ layer, as well as by the rate of film deposition. For a high deposition rate, YBCO films grow on the CeO₂ sublayer in the [001] orientation irrespective of the orientation of the substrate and the sublayer. It was found that when the angle of deviation of the substrate plane is from the (110) NGO plane, twinning of one or both twinning complexes in YBCO may be suppressed.     

Modelling of the phase transitions sequence in KNbO3 and BaTiO3

Interatomic potentials are determined in the framework of the nonlinear oxygen polarizability model to describe the structural behavior of KNbO₃ and BaTiO₃ as a function of temperature. To this purpose, the adiabatic potential is evaluated for different ferroelectric distortions and the model potential parameters are improved by comparing with total energies from full-potential LMTO calculations. Phonon dispersion curves are computed to test if the model reproduces the pronounced two-dimensional character instabilities found by first-principles lattice dynamics using a linear response approach. Finally, the phase diagram for KNbO₃ is obtained through a constant-pressure molecular dynamics simulation.     

Mechanical behaviour and fracture mechanism of flux grown holmium orthoferrite single crystals

Results of indentation induced microhardness and fracture studies on (110) and (001) planes of flux grown HoFeO₃ single crystals using a Vicker's hardness tester, in the load range of 10-100 g are presented. The variation of microhardness with load is found to be non-linear and is explained by using Hays and Kendall's law. The hardness results and the indentation-induced cracking yields the values of the yield strength, fracture toughness and brittleness index for (110) and (001) planes of HoFeO₃ crystals. Transition from Palmqvist to median crack systems takes place at higher loads for (110) plane of the crystal whereas (001) plane is characterized by Palmqvist cracks only.     

Orientation dependence of the electrocaloric effect of ferroelectric bilayer thin films

An analytical thermodynamic theory is applied to investigate the electrocaloric effect of ferroelectric BaTiO₃/SrTiO₃ bilayer thin films with different orientations at room temperature. Theoretical analysis indicates that the strong electrostatic coupling between the layers results in the suppression of ferroelectricity at a critical relative thickness which occurs approximately at 50%, 23%, and 12% of SrTiO₃ fraction in the (001), (110), and (111) bilayer thin films, respectively. The ferroelectric bilayer thin films are respected to have the largest electrocaloric effect at this critical relative thickness. Moreover, the electrocaloric effect strongly depends on the orientation and the (110) oriented bilayer thin films have the largest electrocaloric effect. Consequently, control of the orientation and the relative thickness of SrTiO₃ layer can be used to adjust the electrocaloric effect of ferroelectric bilayer thin films, which may provide the potential for practical application in refrigeration devices.     

Special misorientations in localized deformation regions in Fe-3%Si alloy single crystals

Extended regions located at an angle of 20° to the rolling plane are observed inside deformation bands in a (110)[001] Fe-3%Si alloy single crystal at a high strain (～60%). These regions were interpreted earlier as shear bands. The lattice orientation in these bands is close to (110)[001], and their habit plane is parallel to the {112} planes of the deformed {111}〈112〉 matrix. The misorientations between the bands and the matrix group around special misorientations Σ9, Σ19a, Σ27a, and Σ33a, which are characterized by close angles of rotation about axis 〈110〉. During primary recrystallization, the (110)[001] grains growing from the bands retain segments of the corresponding special boundaries with the deformed matrix.     

Properties of BaTiO3/BaZrO3 ferroelectric superlattices with competing instabilities

Properties of (BaTiO₃)₁/(BaZrO₃) _n ferroelectric superlattices (SLs) with n = 1?7 grown in the [001] direction are calculated from first principles within the density functional theory. It is revealed that the quasi-two-dimensional ferroelectricity occurs in these SLs in the barium titanate layers with a thickness of one unit cell; the polarization is oriented in the layer plane and weakly interacts with the polarization in neighboring layers. The ferroelectric ordering energy and the height of the barrier separating different orientational states of polarization in these SLs are sufficiently large to provide the formation of an array of independent polarized planes at 300 K. The effect of the structural instability on the properties of SLs is considered. It is shown that the ground state is a result of simultaneous condensation of the Γ₁₅ polar phonon and phonons at the M point (for SLs with even period) or at the A point (for SLs with odd period); it is a polar structure with out-of-phase rotations of the octahedra in neighboring layers, in which highly polarized layers are spatially separated from the layers with strong rotations. The competition between the ferroelectric and structural instabilities in biaxially compressed SLs manifests itself in that the switching on of the octahedra rotations leads to an abrupt change of the polarization direction and can cause an improper ferroelectric phase transition to occur. It was shown that the experimentally observed z-component of polarization in the SLs can appear only as a result of the mechanical stress relaxation.     

Effect of preferred orientation on magnetoelectric properties of multiferroic La0.7Sr0.3MnO3/BaTiO3 heterostructure

The epitaxial La_0.7Sr_0.3MnO₃/BaTiO₃ bilayer heterostructures were deposited on LaAlO₃ (001) and (110) substrates by pulsed laser deposition. The inherent ferromagnetic, ferroelectric properties and strong magnetoelectric (ME) effect at room temperature were approved, which correlated to the preferred orientation of the films. Both heterostructures showed similar frequency-dependent ME behavior in 0.1 kHz-100 kHz, the ME voltage coefficients were around 140 mV/cm Oe and 104.8 mV/cm Oe at 1 kHz for (001) and (110) oriented bilayers, respectively. This was at least one order of magnitude higher than previously reported results of the related heterostructures, which is mainly ascribed to the lower dielectric constant of BTO film.     

Thermal faceting of the rutile TiO2(001) surface

Temperature dependent faceting of rutile TiO₂ surfaces cut to the (001) plane has been reported [Tait and Kasowski, Phys. Rev. B20 (1979) 5178]. By comparing LEED data to beam positions calculated for various sets of facet planes, the facet planes have been identified. The first ordered structure observed on annealing ion bombarded surfaces is composed of {011} facets with the facet planes in a (2 × 1) reconstruction. The high temperature structure produced on annealing above 1300K is best described as {114} facets; however, there are deviations of the observed LEED pattern from that calculated for {114} facets, possibly because of the presence of related planes. LEED data have now been obtained on the behavior of (110), (100), (011), (114), and (001) surfaces in UHV. The observed stability of TiO₂ surfaces can be related to the Ti ion coordination numbers in the surface plane as derived from stoichiometric terminations of the rutile lattice.