[show abstract][hide abstract] ABSTRACT: Temperature-dependent and frequency-dependent dielectric investigations have been performed in TbMnO3 polycrystals sintered in either oxidative or reductive atmospheres. The results revealed the occurrence of two dielectric anomalies above 100 K, which are caused by the thermal activation of charge carriers and their motion in grain cores and grain boundaries. The temperature dependence of the bulk dc conductivity was also analysed and indicates that charge carriers move between inequivalent sites according to a variable-range-hopping mechanism. Also, a strong correlation between dielectric properties and crystalline structure was observed. Furthermore, a low-temperature dielectric relaxation, commonly reported in rare-earth manganite crystals, was observed in both samples. This relaxation follows the empirical Cole-Cole model and was attributed to small-polaron tunnelling. Polaron motion was observed to be affected by the magnetic transitions, structural properties and intrinsic anisotropies in TbMnO3. It is also worth mentioning that the dielectric anomaly due to motion of charge carriers in grain boundaries is the only one of extrinsic origin, while the anomalies related to carrier motion in grain cores and small-polaron tunnelling are intrinsic to TbMnO3.
[show abstract][hide abstract] ABSTRACT: Structural, ferroelectric, and magnetic arrangements, and electron density in the vicinity of cations, were modeled from high-resolution X-ray and neutron powder diffraction data in La modified BiFeO3-PbTiO3 compounds. Important features for controlling the intrinsic mechanism for the magnetoelectric coupling in these materials, as prototypes for perovskite structured magnetoelectric multiferroics, are pointed out and discussed. It is shown that the magnetoelectric coupling angle is governed by covalent-like forces, which also affect the structural and ferroelectric distortions in the unit cell.
[show abstract][hide abstract] ABSTRACT: Discussed is the influence of the fuel and water employed in combustion synthesis of single-phase (perovskite) PZN-10PT nanopowder with an x = 0.10 composition. Pb(NO3)2, Zn(NO3)2 · 6H2O, (NH4NbO(C2O4)2), and C8H20O4Ti were used as cation precursors while urea, glycine, glycine/urea (50/50 ratio), and tetraformal triazine (TFTA), as fuels. Two sets of precursors (denoted as set-1 and set-2) were used with each of these fuels, and four different fuels: without and with the addition of 250 mL of water. The results indicated that the highest percentage of perovskite phase in the PZN-10PT nanopowders was obtained using an urea/glycine mixture as a fuel. When the urea/glycine mixture was added to the solution containing cations precursors, the two fuels form a gel in aqueous solution, this gel contributes not only to obtain homogeneously mixed in the starting material but also aids explosive combustion leading to a high-temperature reaction within a shorter period of time, which is a condition that favors the formation of metastable PZN-10PT nanopowders.
International Journal of Self-Propagating High-Temperature Synthesis 08/2013; 21(1).
[show abstract][hide abstract] ABSTRACT: Structurally tuned multiferroic state is demonstrated for BiFeO3-based compounds. The electric and magnetic orders are strongly affected by the coexistence of R3c and Cm phases, i.e., by structural softness through monoclinicity, which leads the multiferroism to be driven by the same cation. The Cm phase enhances the ferroelectric and magnetic responses through Bi/Ba–O and Fe/Ti–O bonds by influencing structural distortions and ion valence. We also show the strong correlations between ferroic orders, structural arrangements, and tuning of the ion valence in the perovskite B site.
Applied Physics A 08/2013; 111(2). · 1.55 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this work, templated grain growth (TGG) and reactive templated grain growth (RTGG) texture techniques combined with uniaxial hot pressing were used for the first time to produce high dense monolithic textured 0.6PMN–0.4PT ceramics. Microstructural analysis of the textured ceramics showed that both TGG and RTGG texture methods are efficient to promote anisotropic grain nucleation around the SrTiO3 single crystal templates. The structural data remarkably revealed that although there was no previous reaction of the powder to form the lead magnesium niobate–lead titanate (PMN–PT) compound in the RTGG samples and the ceramic phase formation was 100% perovskite indicating that RTGG texture technique is more attractive than TGG in the case of SrTiO3 templated PMN–PT. Rather, dielectric characterizations performed in the RTGG samples parallel and perpendicular to the template axis revealed a high anisotropy in the electrical permittivity for RTGG samples (1.48) that were comparable to an estimated value for 0.62PMN–0.38PT single crystals. Piezoelectric characterization of RTGG samples resulted in strain levels up to 0.34% and the highest d33 coefficient was 1100 pC/N, which showed significant increase compared to random ceramic.
Journal of the European Ceramic Society 07/2013; 27(6):2463-2469. · 2.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper, structural and dielectric properties of BaTiO3 ceramics obtained under extreme conditions were investigated. The temperature dependent dielectric investigations revealed that the phase transition temperatures of the BaTiO3 ceramics were raised as a function of residual strains associated to the nanostructuration, while structural characterizations showed a tetragonal arrangement at room temperature. From the frequency dependence analyses of the imaginary parts of dielectric permittivity, impedance and modulus function, three relaxation processes were identified. Two of them exhibit activation energies of 0.45 and 0.63 eV, and were attributed to single and double-ionization of oxygen vacancies. The whole set of results also indicated that the electrons resulting from the ionization of oxygen vacancies are trapped and do not contribute to the electrical conductivity, while the physical properties of the analyzed samples were enhanced by retaining a strained microstructure.
Materials Research Bulletin 05/2013; 48(5):1772-1777. · 1.91 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper, the microscopic mechanism for structural, ferroelectric, and magnetic stabilities in displacive perovskite structured multiferroics is investigated by studying BiFeO3-PbTiO3 solid solutions. The results revealed that thermal stability and ferroic orders are intimately connected with the changes in A and B perovskite sites. The breakdown mechanism of the cycloidal magnetic network, which permits to observe the resulting macroscopic magnetization is reveled, compared, and presented for various compositions of the BiFeO3-PbTiO3 system. The delicate energetic balance that stabilizes the electrically polar structures in displacive perovskite multiferroics is proposed and it was showed that it is fully connected to the configuration (atomic occupancy and formal ionic charge) of each individual site of the perovskite unit cell.
Journal of Applied Physics 03/2013; 113(11). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Dielectric and magnetic properties were investigated in Pb(Fe2/3W1/3)O3–PbTiO3 ceramics. The dielectric constant in these materials exhibits changes at the magnetic ordering temperature, which presents a close connection with the magnetoelectric intrinsic effect (ME). An approximation of the ferroelectromagnetoelastic coefficient was determined from the dielectric susceptibility response. The analysis of magnetic and dielectric susceptibilities based on the Landau-Devonshire thermodynamic formalisms indicates that the ME effects is larger (in absolute value) when the FE and AFM transitions are close together.
Solid State Communications 03/2013; 158:54–57. · 1.53 Impact Factor
[show abstract][hide abstract] ABSTRACT: Magnetoelectric coupling can be observed in multiferroic materials in the regions where magnetic and ferroelectric ordering coexist and in many cases is mediated via lattice strain. Pb(Fe1/2Nb1/2)O3 (PFN) is a well-known multiferroic, presenting ferroelectric ordering (below TC = 380 K) and a Néel temperature (antiferromagnetic ordering) TN = 143 K. However, recent experimental results suggest that PFN can display ferromagnetic ordering at higher temperatures and a still unclear sequence of phase transitions. In this work, high-density PFN ceramics were prepared by a conventional ceramic method. Dielectric, magnetic and anelastic responses were measured as a function of temperature. The experimental results present complementary evidence of anomalies (additional to those which can be directly associated with the reported transitions at TC and TN) which cold be associated with a structural phase transition (T ∼ 315 K), an antiferromagnetic–ferromagnetic phase transition (110 K) and magnetoelectroelastic instabilities around 250 K.
[show abstract][hide abstract] ABSTRACT: The potentialities of BiFeO3-PbTiO3 multiferroic compounds for magnetic-field sensing applications is investigated by characterizing the dynamical magnetoelectric response of La doped (0.6)BiFeO3-(0.4)PbTiO3 piezoceramics. The results are compared with those obtained in Nb doped (1 wt. %) PbZr0.53Ti0.47O3 (PZT-5A) piezoceramics and the observed non-linearity of the oscillating voltage response of (0.6)BiFeO3-(0.4)PbTiO3 piezoceramics is attributed to the overlapping of the intrinsic and extrinsic magnetoelectric effects that were successfully decoupled by analyzing the in-phase and out-of-phase voltage components relative to applied oscillating magnetic field. For an alternating cosine-like magnetic field of 0.32 Oe, at the piezoelectric resonance frequency (184.2 kHz), the alternating voltage response decreases from 7.77 mV, at −10 kOe, to 2.71 mV, at 1.8 kOe, to subsequently increases until 5.97 mV at 10 kOe. This non-linear voltage response, which can be easily converted into a linear signal by using the phase difference, can be used for static magnetic field sensing in a wide range of magnetic fields.
Journal of Applied Physics 01/2013; 113(3). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: The local structure of Pb(Fe2/3W1/3)O3
(PFW100xPT, x=0.10 and x=0.20) multiferroic ceramics was probed by X-ray
absorption spectroscopy at the W LIII-edge. The analysis of
XANES spectra did not show significative modifications in the intensity
or energy of transitions as a function of temperature or
PbTiO3 content. The fitting of EXAFS spectra of PFW samples
revealed a local rhombohedral symmetry of W atoms around 120 K, which is
consistent with the electrical properties of this compound. Concerning
the local structure of W atoms in PFW100xPT samples, in all cases, at
290 K, a local tetragonal symmetry was determined although by X-ray
diffraction a cubic local symmetry was expected.
Journal of Physics Conference Series 01/2013; 430:012111.
[show abstract][hide abstract] ABSTRACT: The present paper is a comprehensive study concerning Fe K-edge X-ray absorption spectroscopy (XAS) measurements, which were performed to characterize the local structure of (1 − x)Pb(Fe2/3W1/3)O3–xPbTiO3 samples as a function of temperature and PbTiO3 content. Results obtained by the fits of extended X-ray absorption fine structure consist with rhombohedral symmetry for Pb(Fe2/3W1/3)O3 composition at temperatures lower than room temperature. This result is in apparent disagreement with X-ray and neutron diffraction characterization which have been reported. This apparent disagreement is related to the fact that XAS probes the short-range order, whereas X-ray diffraction provides structural information about the average structure. Moreover, as the PbTiO3 content increases, a disorder has been detected at local structure of the FeO6 octahedron. Analysis of X-ray absorption near edge structure spectra did not show modifications in intensity nor energy of transitions.
Journal of Applied Physics 01/2013; 113:114104. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper structural, electric, magnetic, and Mössbauer spectroscopy studies were conducted in (x)BiFeO3–(1-x)BaTiO3, 0.9 ≥ x ≥ 0.3, solid solutions. X-ray diffraction and Rietveld refinement studies indicated the formation of single-phased materials crystallized in a distorted perovskite structure with the coexistence of rhombohedral and monoclinic symmetries. Room temperature ferroelectric hysteresis loops showed that the electric polarization increases with the increase of the BaTiO3 content due to the singular structural evolution of the studied solid solutions. All samples presented weak ferromagnetic ordering, which indicates that the BaTiO3 substitution in the BiFeO3 matrix released the latent magnetization. Mössbauer studies revealed a magnetic spectral signature corresponding to ordered Fe3+ ions, and a decrease of the magnetic hyperfine magnetic fields with the increase of the BaTiO3 content. The composition 0.3BiFeO3–0.7BaTiO3 presented a spectral signature corresponding to a paramagnetic behavior, which strongly suggests that the observed magnetization in this sample is due to the Ti3+ ions.
Journal of Applied Physics 11/2012; 112(10). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Pulse-echo ultrasonic measurements of elastic coefficients of CaTiO(3) show anomalous behavior around 200 K, with a notable rise in the attenuation coefficient. Molecular dynamics simulation is used to simulate the elastic response of a mono-domain (MDm) and a poly-domain (PDm) configuration of CaTiO(3) using the Vashishta-Raman interatomic potential. The PDm is obtained by cooling the melt from 3600 to 300 K at a rate of 0.5 K ps(-1), so that it recrystallizes to the PDm orthorhombic configuration. The elastic behavior is simulated in the temperature range from 300 to 20 K. In the MDm, it is observed that the bulk modulus varies linearly with temperature, while in the PDm an anomalous hardening is seen around 210 K. The bulk modulus of the PDm fluctuates strongly and is lower than that of the MDm. Neither the pair correlation function nor the Ti-Ti-O bonding angle indicate a true structural phase transition in this range of temperatures. Given the absence of any apparent change in the structure, a possible explanation for this phenomenon is the emergence of a certain class of dynamical instability associated with domain wall motion. Curiously, the pressure fluctuations in both the MDm and PDm configurations follow a power law distribution f ∼ P(-α), with the exponent independent of applied strain and temperature. Time series for pressure are used to analyze the dynamics by time-delay reconstruction techniques. The calculus of embedding and correlation dimension indicates that in the polycrystalline configuration, low-dimension dynamics (<26) appears, which tend to disappear at higher temperatures.
[show abstract][hide abstract] ABSTRACT: The origin of enhanced ferroism in La-modified BiFeO3-PbTiO3 compounds is investigated by examining chemical bonding in high-resolution structural measurements. It is shown that structural modifications in the perovskite A-site, such as absent or inactive lone-pair ions, lead to reduced covalence in A-O bonds and increased covalence in B-O bonds for the tetragonal phase. This change in bond character results in a structural accommodation that favors the rhombohedral over the tetragonal phase, which also enhances the ferroic properties. These observations highlight the influence of modifiers on BiFeO3-PbTiO3 bonds and phase stabilization mechanisms.
[show abstract][hide abstract] ABSTRACT: Dielectric and magnetic properties were investigated in Pb(Fe2/3W1/3)0.83Ti0.17O3 ceramics. The dielectric constant in these samples exhibits colossal changes at the magnetic ordering temperature under the presence of bias external electric fields, which presents a close connection with magnetoelectrics effects (ME), confirming the possibility to control magnetic proprieties with electric fields. The ferroelectromagnetoelastic coefficient was determined from the dielectric response as a function of the electric field. The analysis of magnetic and dielectric susceptibilities based on the Landau-Devonshire thermodynamic formalisms indicates that the ME effects is a contribution of intrinsic ME coupling and a field dependent term.
[show abstract][hide abstract] ABSTRACT: This work presents a study of the domain wall dynamics in Pb(Zr1−xTix)O3 (PZT)-based piezoceramics by means of the temperature dependence non-linear dielectric response and hysteresis loop measurements. In soft PZT, non-linear response gradually increases as the temperature is raised. A similar response is displayed by hard PZT at low temperatures. However, rather more complex behavior is detected at temperatures above 200 K. The anomalous response, which is very marked at room temperature, becomes even greater when the electric field is increased. The non-linear dielectric response is analyzed in the framework of the Rayleigh model. The results suggest a clear change in the domain wall dynamics in hard PZT, which is not observed in soft PZT. Observation of the hysteresis loops confirms that a strong effect of domain wall pinning emerges near room temperature. The change in domain wall dynamics appears as the main cause of the dielectric response difference between both kinds of materials at room temperature.
Journal of Applied Physics 07/2012; 112(1). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: Changes in dielectric response features of lead lanthanum zirconate titanate (PLZT) ceramics by manganese addition are reported in the present work. Both the PLZT relaxor behaviour and the frequency dependence of the room temperature dielectric constant are shown to be suppressed by the addition of manganese. Similar results are obtained by applying a bias external field or by chemical reduction of PLZT samples, thus indicating the presence of an interrelationship between oxygen vacancies and the random ‘bias’ electric field. Consequently, the defects created by oxygen vacancies appear as the most important factor causing the disappearance of relaxor behaviour in Mn-doped PLZT ceramics. No modifications in the diffuse phase transition with the manganese content are found, which may be due to the persistence of compositional disorder. Additionally, it was verified that the spin glass model cannot be used to describe these observed effects.
Journal of the European Ceramic Society 07/2012; 32(8):1659–1665. · 2.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper, the relationship between ferroic states and the physicochemical mechanism which governs the (Bi/Pb/La)–O bonds in polycrystalline La doped (0.6)BiFeO3–(0.4)PbTiO3 compounds were investigated. An abrupt change in the symmetries of perovskite-structured samples, from tetragonal (P4mm) to rhombohedral (R3c), occurs with the increase of the La concentration, highlighting the diminishment of the hybridization of the (Bi/Pb)–O bonds when La is added into the solid solution, and the emergence of a characteristic morphotropic phase boundary tuned by doping. A maximum remnant polarization (10 μC/cm2) was observed for low La concentrations (where R3c and P4mm phases coexist), while the maximum remnant magnetization (∼75 × 10−3 emu/g) was observed for highly La concentrated samples. These results show that ferroelectric and weak-ferromagnetic orders are strongly correlated to the structural arrangement, which in turn are directly related to the chemical A–O and B–O bonds (long-range and short-range forces) and with the chemical characteristics of the elements that fulfills the A site of the perovskite structure.
Journal of Applied Physics 06/2012; 111(11). · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: High density nanostructured BaTiO3 ceramics were synthesized by an innovative processing route, which involves high-energy ball milling and spark plasma sintering. Structural analyses showed only tetragonal (P4mm) single-phase samples. Ferroelectric measurements revealed a typical remnant polarization (3.7 μC cm−2) of nanostructured BaTiO3 samples and a saturation polarization (20 μC cm−2) similar to that of BaTiO3 single crystals. The potential application of nanostructured BaTiO3 ceramics as multilayer capacitors was demonstrated by dielectric permittivity (ε′ = 3000, at 300 K and 1 kHz) measurements.