The local and average structure of Ba(Ti, Ce)O3 perovskite solid solution: effect of cerium concentration and particle size.

The amazing properties of ferroelectric perovskite BaTiO3 (BT) and its solid solutions make them indispensable for many technological applications (e.g. multilayer capacitors). Unfortunately, the so-called `size effect' limits their use. Indeed, under a certain critical particle size, these materials show a suppression of the spontaneous polarization and thus of the ferroelectric properties. In pure nanometric BaTiO3, this is related to a certain local structural disorder. However, only a few studies have explored BT solid solutions, where the doping effect, coupled to the reduced particle size, can play an important role. Therefore, in this work, the structure of BaCexTi1-xO3 (x = 0.02-0.20) was explored by traditional Rietveld method and Pair Distribution Function. Samples present a particle size from 80-160 nm to 400-1000 nm depending on increasing x. The carbox approach was applied, investigating the evolution of the local structure, its modifications and the structural coherent correlation length, as a function of cerium amount. Results demonstrate a cooperative effect of composition and reduced size in the ferroelectricity loss. The two, in fact, contribute to intensify the local structural disorder, decreasing the structural coherent correlation length. The local structural disorder is thus confirmed to be a relevant factor in the ferroelectric properties degradation.

Revealing the Role of the Intermediates during the Synthesis of BaTi5O11.

BaTi5O11 has been extensively studied because of their microwave dielectrics properties. Traditionally, it is difficult to achieve this material as single-phase. Here, we report an effective method to obtain BaTi5O11 powder with nanometer-scale crystals, by solid-state reaction at moderate temperatures and using as precursors nanostructured particles consisting of BaTiO3 and TiO2. The main advantage is the intimate contact between the BaTiO3 and TiO2 that ensure, when the solid-state reaction takes place, the formation of complex solid compounds from three or more constituents. The formation mechanism of BaTi5O11 has been studied as a function of both the thermal treatment and the time reaction. The reaction was monitored by Raman spectroscopy combined with Confocal microscopy, the aim of this characterization technique is to provide the description of the general strategy and design principles to obtain BaTi5O11 powder. Consequently, this work is a challenging task for the compositional and structural study of complex inorganic nanoparticles.

Structural features of Sm- and Gd-doped ceria studied by synchrotron X-ray diffraction and µ-Raman spectroscopy.

A structural study of Sm- and Gd-doped ceria was performed with the aim to clarify some unexplained structural features. (Ce1-xREx)O2-x/2 samples (RE ≡ Sm, Gd; x = 0, 0.1, ..., 1) were prepared by coprecipitation of mixed oxalates and subsequent thermal treatment at 1473, 1173, or 1073 K in air; they were then analyzed at room temperature both by synchrotron X-ray diffraction and µ-Raman spectroscopy. Two structural models were adopted to fit the experimental data, namely, a fluoritic one, resembling the CeO2 structure at low RE content, and a hybrid one at higher RE content, intermediate between the CeO2 and the RE2O3 structures. Two main transitions were detected along the compositional range: (a) an RE-dependent transition at the boundary between the fluoritic and the hybrid regions, of a chemical nature; (b) an RE-independent transition within the hybrid region at ∼0.5, having a purely geometrical nature. The presence of two finely interlaced F- and C-based structures within the hybrid region was confirmed, and hints of their composition were obtained by µ-Raman spectroscopy. The obtained results indicate a possible explanation for the non-Vegard behavioral trend of the cell parameters.

Role of Microstructures in the Dielectric Properties of PVDF-Based Nanocomposites Containing High-Permittivity Fillers for Energy Storage.

Polymer-based nanocomposites containing inorganic ferroelectric inclusions, typically ABO3 perovskites, have emerged as innovative dielectric materials for energy storage and electric insulation, potentially coupling the high breakdown strength (BDS) and easy processing of polymers with the enhancement of dielectric constant provided by the ferroelectric phase. In this paper, experimental data and three-dimensional finite element method (3D FEM) simulations were combined to shed some light on the effect of microstructures on the dielectric properties of poly(vinylidene fluoride) (PVDF)-BaTiO3 composites. The existence of particle aggregates or touching particles has a strong effect on the effective dielectric constant and determines an increase of the local field in the neck region of the ferroelectric phase with a detrimental effect on the BDS. The distribution of the field and the effective permittivity are very sensitive to the specific microstructure considered. The degradation of the BDS can be overcome by coating the ferroelectric particles with a thin shell of an insulating oxide with a low dielectric constant, such as SiO2 (εr = 4). The local field is highly concentrated on the shell, while the field in the ferroelectric phase is reduced almost to zero and that on the matrix is close to the applied one. The electric field in the matrix becomes less homogeneous with increasing the dielectric constant of the shell material, as happens with TiO2 (εr = 30). These results provide a solid background to explain the enhanced dielectric properties and the superior BDS of composites containing core-shell inclusions.

Role of Density and Grain Size on the Electrocaloric Effect in Ba0.90Ca0.10TiO3 Ceramics.

Pure perovskite Ba0.90Ca0.10TiO3 ceramics, with a relative density of between 79 and 98% and grain sizes larger than 1 µm, were prepared by solid-state reaction. The dielectric and electrocaloric properties were investigated and discussed considering the density and grain size of the samples. Room temperature impedance measurements show good dielectric properties for all ceramics with relative permittivity between 800 and 1100 and losses of <5%. Polarization vs. E loops indicates regular variation with increasing sintering temperature (grain size and density), an increase in loop area, and remanent and saturation polarization (from Psat = 7.2 µC/cm2 to Psat = 16 µC/cm2). The largest electrocaloric effect was 1.67 K for ceramic with GS = 3 µm at 363 K and electrocaloric responsivity (ζ) was 0.56 K mm/kV. These values are larger than in the case of other similar materials; thus, Ba0.90Ca0.10TiO3 ceramics with a density larger than 90% and grain sizes of a few µms are suitable materials for electrocaloric devices.

Ferroelectric order driven Eu3+ photoluminescence in BaZrxTi1-xO3 perovskite.

The ability to tune and enhance the properties of luminescent materials is essential for enlarging their application potential. Recently, the modulation of the photoluminescence emission of lanthanide-doped ferroelectric perovskites by applying an electric field has been reported. Herein, we show that the ferroelectric order and, more generally the polar order, has a direct effect on the photoluminescence of Eu3+ in the model BaZrxTi1-xO3 perovskite even in the absence of an external field. The dipole arrangement evolves with increasing x from long-range ferroelectric order to short-range order typical of relaxors until the non-polar paraelectric BaZrO3 is achieved. The cooperative polar interactions existing in the lattice (x < 1) promote the off-center displacement of the Eu3+ ion determining a change of the lanthanide site symmetry and, consequently, an abrupt variation of the photoluminescence emission with temperature. Each type of polar order is characterized by a distinct photoluminescence behaviour.

Second-harmonic generation of single BaTiO3 nanoparticles down to 22 nm diameter.

We investigate the second-harmonic generation (SHG) signal from single BaTiO3 nanoparticles of diameters varying from 70 nm down to 22 nm with a far-field optical microscope coupled to an infrared femtosecond laser. An atomic force microscope is first used to localize the individual particles and to accurately determine their sizes. Power and polarization-dependent measurements on the individual nanoparticles reveal a diameter range between 30 and 20 nm, where deviations from bulk nonlinear optical properties occur. For 22 nm diameter particles, the tetragonal crystal structure is not applicable anymore and competing effects due to the surface to volume ratio or crystallographic modifications are taking place. The demonstration of SHG from such small nanoparticles opens up the possibilities of using them as bright coherent biomarkers. Moreover, our work shows that measuring the SHG of individual nanoparticles reveals critical material properties, opening up new possibilities to investigate ferroelectricity at the nanoscale.

Ferroelectric hollow particles obtained by solid-state reaction.

Hollow particles of barium titanate were obtained by a two-step process combining colloidal chemistry and solid-state reaction. BaCO(3) crystals (size ≈1 µm) suspended in a peroxy-Ti(IV) aqueous solution were coated with an amorphous TiO(2) shell using a precipitation process. Calcination of the BaCO(3)@TiO(2) core-shell particles at 700 °C resulted in the formation of BaTiO(3) hollow particles (shell thickness of ≈70 nm) which retain the morphology of the BaCO(3) crystals. Formation of the cavity occurs because out-diffusion of the core phase is much faster than in-diffusion of the shell material. X-ray diffraction (XRD) and Raman spectroscopy indicate that the hollow particles possess a tetragonal ferroelectric structure with axial ratio c/a = 1.005. Piezoresponse force microscopy has shown strong piezoactivity and 180° ferroelectric domains. The process described provides a general route to fabricate hollow ferroelectric structures of several compounds.