B. Fraygola

Structural and Dielectrics Properties of (1-x)Pb(Fe2/3W1/3)O3-xPbTiO3 Ceramics

Electric and dielectric properties of (1-x)Pb(Fe2/3W1/3)O3-xPbTiO3 solid solu¬tions were investigated as a function of the frequency and temperature. Dielectric measurements over the temperature showed the presence three contributions, which were related to the ferro-paraelectric phase transition (around 180K) and to conductive contributions (in the range 200K- 700 k). The FE phase transition of Pb(Fe2/3W1/3)O3 (PFW) is shifted to higher temperatures by PbTiO3 (PT) additions. Conductive contributions were associated to an electronic conductivity with activation energy around 0.45 – 0.55 eV.

Local structure around Fe ions on multiferroic Pb(Fe1/2Nb1/2)O3 ceramics probed by x-ray absorption spectroscopy

Local structure around Fe ions on Pb(Fe1/2Nb1/2)O3 ceramics was probed by x-ray absorption spectroscopy in order to settle the controversies about its structure. It is observed that the shell structure around Fe atoms exhibits a monoclinic local symmetry at 130 and 230 K, tetragonal local symmetry at room temperature, and cubic local symmetry at 410 K. Independently of the coordination, temperature, or symmetry, Fe-O mean bond-length does not vary significantly.

Anelastic and Dielectric Characterization of Pb(Fe0.50Nb0.50)O3 Multiferroic

Magnetoelectric coupling in multiferroics materials remains as an interesting subject from fundamental as well as technological point of view, and in many cases is mediated via lattice strain. Therefore, anelastic measurements can be a powerful procedure to better understand this phenomenon. In this work, as fired Pb(Fe 0.50 Nb 0.50 )O 3 ceramics were investigated through dielectric and ultrasonic techniques in temperature range from 15 K to 450 K. The anelastic experimental results present anomalies (additional to those related to the ferroelectric (T C ) and antiferromagnetic (T N ) ordering), which could be associated with not reported ferroelectric-ferroelectric (T∼ 315 K) and an antiferromagnetic-ferromagnetic phase transitions (110 K), respectively.

On the capacitance versus voltage response and tunability of ferroelectrics: A microscopic model

The dielectric permittivity is one of the most important properties of ferroelectrics and is strongly dependent upon the measuring conditions (electric field strength and frequency, external stress, among others). The electric field dependence of the dielectric permittivity is modeled considering ferroelectrics in which domain walls act as a stretched membrane under a homogeneous external electric field E(t)=E0+E1 sin ωt. Considering that the applied field is uniaxial and that the deformed membrane remains plane, it is possible to formulate the membrane vibration problem as a linear boundary value problem, which can be solved analytically. Real and imaginary dependence of the permittivity as a function of the frequency are derived from the analytic solution. By choosing an appropriate relationship between the membrane tension and the applied field, it is possible to describe the observed nonlinear hysteretic dependence of the permittivity under a bias electric field (CV response or tunability). The model ...

Low-temperature elastic anomalies in CaTiO3: dynamical characterization.

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.

Fe K-edge X-ray absorption spectroscopy study of Pb(Fe2/3W1/3)O3-PbTiO3 multiferroic ceramics

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.

Magnetic and Electric Orderings and Intrinsic Magnetoelectric Coupling in Pb(Fe,W)O3 Based Ceramics

In this work structural, electric, dielectric, and anelastic properties of single phase high density (> 97%) (1-x)PFW-xPT ceramics, with 0 ≤ x ≤ 0.30, are investigated. The experimental results show that the temperatures, in which magnetic (TN ) or electric (TFE ) ordering occur, can be interchanged by PbTiO3 addition. Correlations between the experimental results allow to estimate intrinsic ferromagnetoelectroelastic coefficients at the temperatures, where the multiferroic state exist. The intrinsic magnetoelectric coefficient is maximum for compositions, in which the temperatures for magnetic and ferroelectric ordering coincide, no matter in what sequence the orderings (FE or AFM) occur.

Anelastic Characterization of Pb(Fe2/3W1/3)O3 Multiferroic Ceramics

Multiferroic relaxor ceramics Pb(Fe2/3W1/3)O3 (PFW) has been synthesized by a modified B-site precursor method and characterized by anelastic measurements. Anomalies in the temperature dependence of the elastic constant have been observed at TN and TC, indicating that the establishment of ferroelectric and antiferromagnetic orders occurs via linear coupling with lattice strain. In both transitions, FE and AFM, the coupling with the elastic part of the system is linear-quadratic, ie F (S, M) = gSL2 (L = M2−M1) and F (S, P) = β SP2, justifying the presence of the term in ηL2P2 free energy expansion.

Room Temperature Multiferroic Behavior in Pb(Fe1/2Nb1/2)O3 Ceramics

The co-existence of ferroelectric and ferromagnetic properties at room temperature is very rarely observed. In multiferroics, microscopic coupling interaction between the ferroelectric and magnetic order results in the macroscopic correlation between the dielectric and magnetic properties, which is defined as magnetodielectric effect. In this paper, the magnetic behavior of the Pb(Fe1/2Nb1/2)O3 ceramics at high temperature has been extensively studied, showing that PFN exhibits ferroelectric and ferromagnetic ordering at room temperature, converting ferroelectric PFN into a multiferroic material at this temperature.

W LIII-edge XANES and EXAFS studies of Pb(Fe2/3W1/3)O3-PbTiO3 multiferroic ceramics

The local structure of Pb(Fe2/3W1/3)O3 (PFW) and Pb(Fe2/3W1/3)O3-PbTiO3 (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.