PbFe1/2Nb1/2O3 Research Articles

Magnetic properties of the multiferroic double perovskite lead iron niobate: Role of disorder

The peculiarities of the magnetism of one of the first known and most studied multiferroic compounds remain a challenge for solid-state theory. The antiferromagnetic ordering of PbFe1/2Nb1/2O3 (PFN) occurs at TN≈150 K. On the one hand, this value is much larger than the TN value for most of the other double perovskites. On the other hand, it is significantly lower than theoretical estimates. The temperature dependence of magnetic susceptibility is substantially different from that one expected for conventional antiferromagnets. In order to find the solution of these puzzles, several density functional theory calculations of the magnetic interactions in PFN are reported. The interactions between first-, second-, third- and fourth-neighbor Fe3+ ions (S=5/2) are calculated. The magnetic response of Fe spins is shown to depend both on the interaction value and the mutual spin arrangement. The magnetic susceptibility of various spin lattices is calculated using the tenth-order high-temperature expansion method and compared with the experiment. A substantial role of disorder for the understanding of the magnetic properties is discussed.

Magnetic phase transitions in solid solutions of Fe-containing perovskite multiferroics

Concentration dependences of magnetic phase transitions temperatures TM for several solid solutions of perovskite multiferroics BiFeO3, PbFe1/2Nb1/2O3 (PFN), and PbFe0.5Sb0.5O3 (PFS) have been studied using Mössbauer spectroscopy. In all the solid solutions studied, the values of TM were found to depend not only on the concentration of the Fe3+ ions in the lattice, but also on the short-range and/or long-range ordering of Fe3+ and B5+ ions, as well as on the mean lattice parameter values and on the nontrivial magnetic superexchange involving the empty p-orbitals of Pb2+ and Bi3+ cations.

Ferroelectromagnetic Properties of PbFe1/2Nb1/2O₃ (PFN) Material Synthesized by Chemical-Wet Technology.

In this work, PbFe1/2Nb1/2O3 (PFN) ceramic samples synthesized by chemically wet method (precipitation from the solution) were obtained. Due to the tendency to form powder agglomerates, the synthesized powder was subjected to ultrasound. The sintering was carried out under various technological conditions, mainly through controlling the sintering temperature. -X-ray powder-diffraction (XRD), scanning electron microscope (SEM) microstructure analysis, as well as the examinations of dielectric, ferroelectric, and magnetic properties of the PFN ceramics were carried out. Studies have shown that hard ceramic agglomerates can be partially minimized by ultrasound. Due to this treatment, closed porosity decreases, and the ceramic samples have a higher density. Optimization and improvement of the technological process of the PFN material extends the possibility of its use for the preparation of multiferroic composites or multicomponent solid solutions based on PFN. Such materials with functional properties find applications in microelectronic applications, e.g., in systems integrating ferroelectric and magnetic properties in one device. The optimal synthesis conditions of PFN ceramics were determined to be 1050 °C/2 h.

Misfit strain-induced changes in the Fe-sublattice of multiferroic Pb(Fe1/2Nb1/2)O3 epitaxial nanofilm seen via Raman spectroscopy

Magnetic phase transition temperature of the PbFe1/2Nb1/2O3 (PFN) nanofilm on the SrTiO3 substrate was reported to be about 50 K higher than that in the bulk samples. We carried out comparative Raman studies of a 90 nm-thick PFN epitaxial film on SrRuO3-buffered (001) SrTiO3 substrate and the bulk samples of ceramic PFN and a single crystal. It was found out that the bands at ≈250 cm−1 and ≈700 cm−1 in the spectrum of the film, corresponding to FeO vibrational modes, are shifted by about 20 cm−1 to higher frequencies as compared to the bulk samples. This result is in line with the predictions of the recent first-principles calculations that compressive misfit strain stimulates the disorder in the Fe and Nb distribution in the lattice, leading to an increase of the magnetic phase transition temperature. Such disordering might cause the changes in the Fe-sublattice of PFN seen in the Raman spectrum.

Dielectric Properties of Undoped and Li-doped Pb(Fe1/2Nb1/2)O3 Ceramics Sintered from Mechanochemically Synthesized Powders

Optimal sintering temperature of PbFe1/2Nb1/2O3 (PFN) ceramics is 1120–1150°C, but ceramics sintered in this temperature range have low resistivity and high losses. Mechanochemical synthesis allows one to lower the optimal sintering temperature of PFN ceramics down to 1000°C, however dielectric losses remain high and the permittivity exhibits large frequency dispersion. In the present work it has been found that similar to the case of the routine solid state synthesis, Li doping enables one to reduce dramatically the losses of the PFN ceramics obtained by sintering the mechanochemically synthesized powders.

X-ray photoelectron study and first principle calculations of the electronic structure of PbFe1/2Nb1/2O3 single crystal in the ferroelectric and paraelectric phases

The present study is the first in which the theoretical prediction of the changes in the density of states of single crystal of PbFe1/2Nb1/2O3 (PFN) multiferroic depending on a temperature at the ferroelectric – paraelectric transition is experimentally confirmed by the X-ray photoelectron spectroscopy. The valence band XPS spectra of PFN single-crystal at three temperatures T1 = 297 K (monoclinic phase), T2 = 368 K (tetragonal) and T3 = 403 K (cubic) were obtained by X-ray photoelectron study using ESCALAB 250. It is found out that the experimental valence band XPS spectra of PFN single-crystals in ferroelectric and paraelectric phases consist of two main peaks with a difference in relative intensity and energy position of the second peak. The density of states of PFN was calculated for tetragonal ferroelectric (368 K) and cubic paraelectric (403 K) phases. The first main peak corresponds to strongly hybridized Fe3d, Nb3d and O2p states while the second peak is mainly composed of Pb6s states. According to the experimental data, the position of the second main peak of the total density of states of PFN in the ferroelectric monoclinic phase is shifted toward the first peak relatively to its position in the paraelectric phase by 0.28 eV and for the tetragonal phase by 0.15 eV, the latter value coincides well with the calculated value equal to 0.17 eV.

Relaxation dynamics, phase pattern in the vicinity of the Curie temperature, Fe valent state and the Mössbauer effect in PFN ceramics

High-density lead ferroniobate, PbFe1/2Nb1/2O3 (PFN), was prepared by the conventional ceramic technology. Its dielectric properties, phase pattern in the vicinity of transition into the polar phase, and the x-ray electron and Mössbauer spectra were studied. The relaxation dynamics discovered at the temperature exceeding the Curie temperature at the frequencies of 3×10−2–105Hz is described in detail. Near TC, the following sequence of phase transitions was established: rhombohedral (T<368K)→pseudocubic (368K<T<387K)→cubic (T>387K). It is shown that in both the ferroelectric and paraelectric phases the Fe ions are, mainly, in a high-spin valent state Fe3+ in the octahedral environment.

Invar effect in PbFe1/2Nb1/2O3 ceramics

High-density lead ferroniobate PbFe1/2Nb1/2O3 (PFN) is prepared by conventional ceramic technology. Its structural properties are studied in a wide temperature range (293 ≤ T ≤ 973 K). The following chain of phase transitions is established in the vicinity of the transition to the polar phase: Rh (rhombohedral phase) (T 387 K). The paraelectric range contains five ranges of constant unit-cell volume (invar effect): I (387 ≤ T ≤ 413 K), II (433 ≤ T ≤ 463 K), III (553 ≤ T ≤ 613 K), IV (743 ≤ T ≤ 773 K), and V (798 ≤ T ≤ 823 K). It is shown that the anomalous behavior of the PFN dielectric characteristics above the Curie temperature, which was revealed previously, is associated with the specific features of its real (defect) structure, which is caused by the crystal-chemical specificity of the main structure-forming agents: α-Fe2O3 and αht-Nb2O5.

The relaxation dynamics, iron valence state, and Mössbauer effect in PFN ceramics

The dielectric properties, X-ray emission spectra, and Mossbauer effect in ceramics made of PbFe1/2Nb1/2O3 (PFN) compound were studied. The relaxation dynamics revealed above Curie temperature TC at a frequency of 3 × 10−2–105 Hz is described in detail. Analysis of the X-ray emission and Mossbauer spectra showed that at room temperature (T = 300 K), the iron ions in PFN are mainly in the high-spin valence state Fe3+. The Mossbauer spectral parameters obtained at T = (300, 353, and 393 K) indicate an octahedral environment for Fe3+ in both the ferroelectric and paraelectric phases.

Dielectric, magnetoelectric, structure, and dissipative properties and the Mössbauer effect in PbFe1/2Nb1/2O3 ceramics in wide frequency and temperature ranges

High-quality fine-grained ceramic samples of classical multiferroics PbFe1/2Nb1/2O3 (PFN) were synthesized. Their dielectric, magnetoelectric, and magnetic characteristics, including the Mossbauer effect, were measured over wide ranges of temperatures (10–1000 K) and field frequencies (from 25 Hz to 1 MHz). The temperature dependence of the dielectric loss exhibits a maximum between 150 and 170 K, likely due to magnetic ordering. The dependence of ɛ on the magnetic field displays an anomalous increase near the Curie temperature (370 K) that rises to 40% upon heating.

Influence of the sintering conditions on the physical proprieties of the ceramic PFN multiferroics

An influence of compacting conditions on applied properties of the multiferroic PbFe1/2Nb1/2O3 (PFN) ceramics synthesized by the columbite method and powder calcination technique is presented in this work. The obtained specimens were subjected to X-ray and chemical composition homogeneity analyses, magnetic tests, electric direct current conductivity and electromechanical tests. Differences in the degree of diffuseness, densities and a grain size were observed depending on the sintering method. The dielectric and magnetic parameters as a function of temperature were measured. The tests confirmed that PFN powder compacting by a hot uniaxial pressing method had an advantageous influence on the crystalline structure ordering and the ceramics microstructure, what improves the applied properties of the material.

Ionic displacements and structure of disordered PbFe1/2Nb1/2O3

A sequence of structural models of unit cells of complex oxides with perovskite-type structure has been constructed to refine the average structure of PbFe1/2Nb1/2O3 (PFN) at temperatures above the ferroelectric Curie point (T C ∼ 110°C). Owing to the analysis of the probability theory considerations, each model is characterized by only two positional fitting parameters. With the use of the intensities of 95 symmetrically independent X-ray reflections from a PFN single crystal at 160°C, a model with the R factor below 3% (K = 2.44%) was chosen. The ravine method has been used to verify the existence of a single minimum for the R factor with respect to the fitting parameter.

Diffuse ferroelectric phase transitions in Pb-substitutedPbFe1∕2Nb1∕2O3

We investigate effects of cation (K, Ca, Sr, Ba, La, Bi, Tl, Ag) substitution at Pb-site in PbFe1/2∕Nb1/2O3 (PFN) on its diffuse ferroelectric phase transition (DPT) through measurements and modeling of the temperature dependence of its dielectric constant. We find that the chemical trends in experimentally determined rates of change of transition temperature and diffuseness with substituent concentration are governed by two parameters, nominal and Born effective charges calculated within first-principles density functional theory. We introduce “ferroactivity” of an atom using these charges and use it in a generalized Weiss molecular field theory that accounts for the disorder to explain chemical trends in the DPT found experimentally, suggesting guidelines to tune properties of ferroelectrics.