Dielectric properties of SrTiO3/PbTiO3/SrTiO3 epitaxial structures

Within the phenomenological Landau–Ginzburg–Devonshire theory, we discuss the paraelectric-ferrolectric transition in superstructures consisting of ferroelectric and paraelectric layers of equal thickness. The polar axis of the ferroelectric is perpendicular to the layer plane as expected in fully strained BaTiO3/SrTiO3 superstructures on SrTiO3 substrates with pseudomorphic electrodes. We concentrate on the electrostatic effects and do not take into account the boundary conditions other than the electrostatic ones. We find that when the ferroelectric phase transition in the superstructures is into a multidomain state, both its temperature and its character, i. e., the profile of the polarization appearing at the phase transition is strongly influenced by the nature of the near-electrode region. This is also the case for the layer thickness separating the single-and multidomain regimes of the transition. Such a finding makes us question the idea that these superstructures can be thought of as infinite systems, i.e., periodic superstructures similar to a crystal. The irrelevance of this idea in certain conditions is demonstrated by comparing the phase transitions in two different superstructures consisting of ferroelectric and paraelectric layers of the same thickness. In one of them, the ferroelectric layer is in immediate contact with an ideal metallic electrode, whereas at the other boundary, it is the paraelectric layer that is in contact with the electrode. In another superstructure, one paraelectric layer is split in two equal parts which are placed as the first and last layer between the electrodes and the ferroelectric layers which are closest to the electrodes. We show (with some formal reservations) that the phase transition temperature in the first superstructure can be over 100 °C more than in the second one if the material parameters of BaTiO3/SrTiO3 are used for the estimations. Moreover, the profile of the polarization arising at the phase transition is inhomogeneous along the superstructure and has the maximum amplitude in the ferroelectric layer contacting the electrode. We argue that this situation is general and results in smearing of the phase transition anomalies for the layer thicknesses corresponding to multidomain transitions. The work is mainly analyical but numerical methods have been used to support some statements that have been put forward as hypotheses.

Human LDL undergoes a reversible thermal order-disorder phase transition associated with the cholesterol ester packing in the lipid core. Structural changes associated with this phase transition have been shown to affect the resistance of LDL to oxidation in vitro studies. Previous electron cryo-microscopy studies have provided image evidence that the cholesterol ester is packed in three flat layers in the core at temperatures below the phase transition. To study changes in lipid packing, overall structure and particle morphology in three dimensions (3D) subsequent to the phase transition, we cryo-preserved human LDL at a temperature above phase transition (53°C) and examined the sample by electron microscopy and image reconstruction. The LDL frozen from 53°C adopted a different morphology. The central density layer was disrupted and the outer two layers formed a "disrupted shell"-shaped density, located concentrically underneath the surface density of the LDL particle. Simulation of the small angle X-ray scattering curves and comparison with published data suggested that this disrupted shell organization represents an intermediate state in the transition from isotropic to layered packing of the lipid. Thus, the results revealed, with 3D images, the lipid packing in the dynamic process of the LDL lipid-core phase transition.

Inorganic Anions Regulate the Phase Transition in Two Organic Cation Salts Containing [(4-Nitroanilinium)(18-crown-6)]+ Supramolecules

In this work, the temperature-dependent Raman spectra and electrical properties of the [001]-oriented 0.5 mol. % Mn-doped 0.35Pb(In1/2Nb1/2)O3-0.35Pb(Mg1/3Nb2/3)O3-0.30PbTiO3-Mn (PIMNT-Mn) single crystals were investigated. All the unpoled and poled PIMNT-Mn single crystals experience a ferroelectric tetragonal phase to paraelectric cubic phase transition (FET-PC) around 183 °C (TC), which exhibits a second-order transition behavior. Whereas, the poled PIMNT-Mn single crystals exhibit another two dielectric anomalies around 130 °C (TRM) and 148 °C (TMT), in which the ferroelectric rhombohedral phase to ferroelectric monoclinic phase (FER-FEM) and the ferroelectric monoclinic phase to ferroelectric tetragonal phase (FEM-FET) transitions take place, respectively. Both the two ferroelectric phase transitions exhibit a first-order transition behavior. The discontinuous change of the phase degree (θ) and frequencies (fr and fa) around TRM suggest the occurrence of the FER-FEM phase transition in the poled PIMNT-Mn single crystals. The narrowing of the 510 cm−1 and 582 cm−1 Raman modes around the TRM, TMT, and TC temperatures shown in the temperature-dependent Raman spectra suggests their increased ordering of the local structure. The intensity ratio of I272 cm−1/I801 cm−1 increases obviously around the phase transition temperatures (TRM, TMT, and TC), indicating the reduction of the long-range order. The anomalous broadening of the 272 cm−1 Raman mode around the TRM, TMT, and TC temperatures indicates the occurrence of the successive ferroelectric phase transitions (FER-FEM, FEM-FET, and FET-PC) with increasing temperature in the poled PIMNT-Mn single crystals.

Simultaneous control of the magnetic and electric properties of materials is crucial for their application in next-generation memory and sensor devices. Herein, we report a single-crystal Co(II) co...

Structural, electric and magnetic properties of (BaFe11.9Al0.1O19)1-x - (BaTiO3)x composites

A method based on resonant excitation of waveguide modes with a prism coupler is proposed for measuring the thickness and refractive index of thin-film layers in multilayer dielectric structures. The peculiarities of reflection of TE- and TM-polarised light beams from a structure comprising eleven alternating layers of zinc sulfide (ZnS) and magnesium barium fluoride , whose thicknesses are much less than the wavelength of light, are investigated. Using the mathematical model developed, we have calculated the coefficients of reflection of collimated TE and TM light beams from a multilayer structure and determined the optical constants and thicknesses of the structure layers. The refractive indices of the layers, obtained for TE and TM polarisation of incident light, are in good agreement. The thicknesses of ZnS and layers, found for different polarisations, coincide with an accuracy of . Thus, we have demonstrated for the first time that the prism-coupling technique allows one to determine the optical properties of thin-film structures when the number of layers in the structure exceeds ten layers.

Body: Vanadium dioxide (VO2) is well-known for its temperature-driven metal-to-insulator transition between the monoclinic (M1) phase and the rutile (R) phase, which is accompanied with abrupt changes in electrical and optical properties. These intriguing changes have attracted great interest and promises a lot of potential applications. However, the phase transition of bulk VO2 usually occurs around 68 °C with significant thermal hysteresis induced by the latent heat of the first-order transition. The relatively high phase transition temperature Tc and broad hysteresis width ΔTc are adverse to its practical application. Therefore, the modulation of its phase transition behavior is quite necessary. Accumulating evidence suggests that the strain state plays an important role in the phase transition behavior of VO2 and it has been confirmed that the specific strain between VO2 and TiO2 can significantly reduce Tc[1, 2]. Based on this, in order to finely modulate the phase transition behavior and further investigate its relationship with strain state, we inserted TiO2 epitaxial film between VO2 film and the (100) α-Al2O3 substrate as the buffer layer. By controlling the thickness of both VO2 film and TiO2 buffer layer, the variational strain state can be introduced and further lead to the change of lattice parameters and phase transition behavior of VO2 film. Specifically speaking, when both VO2 and TiO2 layers are thin (~10 nm), these two layers are highly strained to the Al2O3 substrate. Due to inconsistent lattice mismatch between [100] and [010] direction, the lattice parameter a and b of VO2 will be stretched and compressed respectively, leading to the suppression of phase transition. If increasing the thickness of TiO2 layer to 200 nm and maintaining the VO2 thickness of 10 nm, due to the lattice relaxation, TiO2 will be relaxed to the rutile phase. As the result, the structure of VO2 layer which is still strained to TiO2 will change synchronously and exhibit the sharper phase transition with reduced Tc of ~34 °C. This phenomenon is similar to the VO2 film epitaxially grown on TiO2 substrate. Lastly, by further increasing the VO2 thickness from 5.5 to 50 nm, the VO2 layer will also tend to be relaxed to bulk M1 phase and the Tc can be continuously adjusted in a wide range from 34 to 58 °C. More interestingly, when the VO2 is thicker than 30 nm, the ΔTc is extremely narrowed to ~0.4 °C and the lattice parameter c exceeds the bulk value correspondingly. We consider the strain-induced lattice parameter change can modify both of the Tc and ΔTc. And by controlling the strain state, the phase transition behavior of VO2 can be finely designed. These results may be useful for the design of VO2-based functional material and device. [1] Y. Muraoka, Z. Hiroi, Appl. Phys. Lett. 80, 583-585 (2002) [2] N. B. Aetukuri et al., Nat. Phys. 9.10, 661-666 (2013)

Leakage current, ferroelectric and structural properties in Pb 1−xBa xTiO 3 thin films prepared by chemical route

Bismuth titanate Bi4Ti3O12 (BTO) is a member of the layered Aurivillius phase perovskite ferroelectrics and has a structural phase transition at Tc of 675 C. Below Tc, BTO shows ferroelectricity with a large spontaneous polarization (30 C/cm) parallel to the layered structure. BTO attracted much attention as candidate material for ferroelectric memories because of the fatigue-free nature of the BTO-based mixed crystal Bi4 xLaxTi3O12 (BLT-x). Although the space group of BTO at room temperature is determined to be B2cb ð1⁄4 Aba2 1⁄4 C2cbÞ (Z 1⁄4 4) only from the X-ray and neutron diffraction patterns, its space group has finally been concluded to be Pc (Z 1⁄4 2) with monoclinic symmetry, taking into account the existence of the small spontaneous polarization (4 C/cm) perpendicular to the layered structure. Here, Z is the number of the chemical formula in the conventional unit cell. Various physical properties of BTO, such as a soft mode behavior and domain wall structures, have been intensively investigated. The Landau theory of phase transition in BTO was presented by assuming the point group 4=mmm in the hightemperature phase, where the phase transition of BTO from the tetragonal (4=mmm) phase to the monoclinic (m) phase was named the triggered phase transition. The splitting of the triggered phase transition, that is the existence of the intermediate orthorhombic phase (B2cb), was observed in the mixed crystal systems in Bi4 xRxTi3O12 (R = La, Nd, Sm, and Gd). It was reported that the prototype structure of BTO shows the space group I4=mmm (Z 1⁄4 4). It is conjectured, however, that neither the orthorhombic (B2cb) nor the monoclinic (Pc) phase can be directly induced from the tetragonal (I4=mmm) phase, on the basis of the group theoretical consideration. Even the primitive unit cell size in I4=mmm and B2cb phases is different. Note that the proper ferroelectric phase transition from I4=mmm to B2cb is impossible, because the latter is not any subgroup of the former at the -point. On the other hand, it was reported, on the basis of experimental results, that the Curie–Weiss behavior of the dielectric constant in the high-temperature phase is observed in BTO, indicating the proper ferroelectric phase transition. This seems to be in contradiction. In order to solve the above contradiction, Rae et al. and Perez-Mato et al. assumed the Fmmm symmetry to be the parent phase, where Fmmm is a subgroup of I4=mmm. If the high-temperature parent phase above 675 C is indeed orthorhombic as they claim, however, the Landau theory so far proposed of the triggered phase transition in BTO, which is based on the assumption that it is tetragonal, may fail. Thus, the symmetry of the high-temperature parent phase seems to be of crucial importance. Under these circumstances, to clarify the structural phase transition at 675 C and the symmetry of the parent phase of BTO, we investigate the domain wall structure in the high-temperature phase and discuss its symmetry on the basis of the group theory. Single crystals of BTO were grown by the flux method from a Bi2O3–TiO2 system. For domain wall observation, some c-plate crystals with an area of about 5mm and thicknesses of about 10–50 m were selected among asgrown crystals as samples. The temperature dependence of the domain wall structure in the c-plate of the BTO crystal was observed using a polarizing microscope with a hightemperature sample stage (Linkam TS-1500), where the possible temperature range of this stage is from room temperature to a maximum of 1500 C. Figures 1(a) and 1(b) show photographs of the c-plate crystal in BTO under crossed-Nicols below and above Tc, respectively, where the sensitive plate was used in Fig. 1(b). In Fig. 1(a), the 90 domain wall structure is observed, while no domain wall structure appears in Fig. 1(b), indicating that the c-plate sample of BTO is optically isotropic. Thus, it was confirmed that the symmetry of the high-temperature phase is tetragonal. In our experiment, the phase front was also observed near Tc, indicating the first-order phase transition. 200 μm A P

The chemical composition and structure variation law of the oxidation product layer, which formed on the surface of carrollite during bioleaching, were studied by the means of SEM, XPS and EDS in this paper. It was demonstrated that an oxidation product layer formed on the mineral surface when the oxidation product adhered gradually increased. Initially, the layer mainly was composed by jarosite and sulfite, and the layer thickness was less than 2 μm. Additionally, the structure of layer was loose and pocket, and covered the partial mineral surface. In the middle stage, the layer was composed of elemental sulfur, jarosite and sulfite. Moreover, the structure of the layer became compact, and the surface of carrollite was completely coated by the layer. Lastly, the element sulfur containing in the layer was totally oxidized, thus the layer was mainly composed of jarosite. Meanwhile, the layer thickness had a tendency to continue to increase, and the layer thicknesses can reach over 6 μm.

Grazing-incidence small-angle X-ray scattering and reflectivity on nanostructured oxide films

The doping-induced ferroelectric phase transition in Ca-doped ${\text{SrTiO}}_{3}$ is investigated by observing the birefringence and coherent phonons. The structural phase-transition temperature is determined by the birefringence measurement. Coherent phonons of the soft modes are studied by using ultrafast polarization spectroscopy. Two phonon modes are observed to be softened toward the ferroelectric phase-transition point at 28 K and other two phonon modes are observed to be softened toward the structural phase-transition point at 180 K. Another structural deformation at 25 K is found below the ferroelectric phase-transition temperature. From the temperature dependence of phonon frequencies under an ultraviolet (UV) illumination, a shift of the ferroelectric phase-transition point toward the lower temperature side is found. A decrease in phonon frequencies after the UV illumination is also found.

We have studied the phase transition behavior of Pb0.76Ca0.24TiO3 thin films using Raman scattering and dielectric measurement techniques. We also have studied the leakage current conduction mechanism as a function of temperature for these thin films on platinized silicon substrates. A Pb0.76Ca0.24TiO3 thin film was prepared using a soft chemical process, called the polymeric precursor method. The results showed that the dependence of the dielectric constant upon the frequency does not reveal any relaxor behavior. However, a diffuse character-type phase transition was observed upon transformation from a cubic paraelectric phase to a tetragonal ferroelectric phase. The temperature dependency of Raman scattering spectra was investigated through the ferroelectric phase transition. The soft mode showed a marked dependence on temperature and its disappearance at about 598 K. On the other hand, Raman modes persist above the tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive above the phase transition temperature. The origin of these modes must be interpreted in terms of a local breakdown of cubic symmetry by some kind of disorder. The lack of a well-defined transition temperature suggested a diffuse-type phase transition. This result corroborate the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in the thin film. The leakage current density of the PCT24 thin film was studied at elevated temperatures, and the data were well fitted by the Schottky emission model. The Schottky barrier height of the PCT24 thin film was estimated to be 1.49 eV.

Advances in strain engineering on oxide heteroepitaxy