D. A. Sarychev

Dielectric and Mössbauer Studies of High-Permittivity BaFe1/2Nb1/2O3 Ceramics with Cubic and Monoclinic Perovskite Structures

Ceramic samples of monoclinic and cubic modifications of ternary perovskite BaFe 1/2 Nb 1/2 O 3 have been obtained by solid-state reaction route. Mossbauer 57 Fe spectra and Neel temperature values appeared to be very similar for both modifications. The values and temperature dependences of the dielectric permittivity for BaFe 1/2 Nb 1/2 O 3 samples were found to depend primarily on their apparent resistivity values and not on their structure. Thus high permittivity values of BaFe 1/2 Nb 1/2 O 3 are due not to the ferroelectric relaxor behavior but rather to the relaxation of the Maxwell-Wagner type and/or relaxation contributed by the oxygen vacancies.

Studies of Ferroelectric and Magnetic Phase Transitions in Pb1−xAxFe1/2Nb1/2O3 (A-Ca, Ba) Solid Solutions

Dielectric and Mossbauer studies of Pb1-xAxFe1/2Nb1/2O3 (A- Ba, Ca) ceramic compositions show that the Néel temperature in these solid solutions drops down at x ≈ 0.15–0.20. At the same threshold the polarization also loses its long-range order and compositions with x > 0.1 exhibit relaxor behavior. The data obtained suggests that there exists a Pb-mediated superexchange controlling the long-range antiferromagnetic order. Thus, studied multifunctional solid solutions demonstrate the possibility of controlling the magnetic and ferroelectric properties simultaneously by changing the concentration of nonmagnetic lead cations only.

Dielectric and Mossbauer Studies of Pb(Fe1/2Ta1/2)O3 Multiferroic Ceramics Sintered from Mechanoactivated Powders

The effect of high-energy mechanical activation on dielectric and magnetic properties of PbFe1/2Ta1/2O3 (PFT) multiferroic was studied. Mössbauer studies have shown that the Neel temperature TN of the antiferromagnetic phase transition in PFT powders can be changed substantially by mechanical activation and subsequent annealing. The increase of TN by more than 70 K, as compared to PFT single crystal, was achieved in the mechanically activated PFT powder annealed at 700°C. Dielectric studies have shown that in contrast to ceramics obtained by usual sintering, PFT ceramics fabricated by sintering of the mechanoactivated powder does not show the relaxor-like behavior.

Studies of Ferroelectric and Magnetic Phase Transitions in Multiferroic PbFe0.5Ta0.5O3

X-ray, magnetic, and Mossbauer studies of a PbFe0.5Ta0.5O3 powder have been carried out. For the first time temperature dependence of the unit cell parameters was obtained. Antiferromagnetic Neel temperature TN ≈180 K was determined using the results of Mossbauer studies. In the vicinity of TN an anomaly in the temperature dependence of the monoclinic angle β was revealed in PFT. This anomaly seems to be due to the magnetoelectric coupling.

Comparative Studies of Ferroelectric and Magnetic Phase Transitions in Pb(Fe1/2Nb1/2)O3-PbMO3 (M-Ti, Zr) Multiferroic Solid Solutions

Some peculiar features of magnetic x,T-phase diagram of (1-x)PbFe1/2Nb1/2O3-xPbTiO3 system at x ≈ 0.1 have been ascribed previously to the change of the lattice symmetry from rhombohedral to tetragonal or to the decrease of the lattice parameter with x. We carried out structural, dielectric, magnetization, and Mossbauer studies of (1-x)PbFe1/2Nb1/2O3-xPbZrO3 system, where the symmetry remains rhombohedral, while the lattice parameter increases with x. However, the magnetic x,T-phase diagram of (1-x)PbFe1/2Nb1/2O3-xPbZrO3 appeared to be very similar to that of (1-x)PbFe1/2Nb1/2O3-xPbTiO3. Anomalies of composition dependence of magnetic phase transition temperatures at x ≈ 0.1 are supposed to be due to the percolation phase transition.

Dielectric and Mossbauer Studies of B-Cation Order-Disorder Effect on the Properties of Pb(Fe1/2Ta1/2)O3 Relaxor Ferroelectric

In Pb(Fe1/2Ta1/2)O3 ceramics with Li2CO3 addition the mean temperature of the antiferromagnetic phase transition (Neel temperature), determined from the Mossbauer spectra studies was found to depend on the sintering temperature. Dielectric studies show that lowering of Neel temperature is accompanied by increasing of the permittivity maximum temperature and diminishing of the permittivity frequency dispersion. The obtained results are explained by changes of the degree of Fe3+ and Ta5+ compositional ordering.

Electron microscopy, X-Ray diffraction and Mossbauer studies of PbFe0.5Nb0.5O3, PbFe0.5Ta0.5O3 and BaFe0.5Nb0.5O3 ceramics sintered from mechanoactivated nanopowders

ABSTRACT The effect of high-energy mechanical activation on magnetic phase transition temperature TM of PbFe0.5Nb0.5O3, PbFe0.5Ta0.5O3, and BaFe0.5Nb0.5O3 perovskites has been studied. Mossbauer studies have shown that TM values in all the perovskites studied can be changed substantially by mechanical activation and subsequent annealing. The increase of TM by 40-50 K, as compared to corresponding single crystals and ceramics sintered from the non-activated powders, was achieved in the mechanoactivated powders of both lead-containing and lead-free perovskites studied.

Structural, dielectric and Mossbauer studies of PbFe0.5Sb0.5O3 ceramics with differing degree of compositional ordering

ABSTRACT Ceramic samples of a new perovskite multiferroic PbFe0.5Sb0.5O3 (PFS) with a differing degree S of long-range compositional ordering of the Fe3+ and Sb5+ cations were fabricated by a variation of the high-pressure-synthesis conditions. The S values estimated from the XRD data varied from 0.93 down to 0.17. The unit cell parameter was found to decrease with increasing S. In line with these results the fraction of the singlet component in the Mössbauer spectra was found to decrease with decreasing S. However, the temperatures and diffusion of dielectric permittivity maxima as well as the temperature of magnetic phase transition determined from Mössbauer data do not change substantially with S. These results imply that PFS remains highly ordered at the nanoscale.

Electron microscopy, XRD, Mossbauer and dielectric studies of Pb(Fe0.5Nb0.5)1-xSnxO3 multiferroic ceramics

ABSTRACT Multiferroic Pb(Fe0.5Nb0.5)1-xSnxO3 (x = 0.025, 0.05, 0.075, 0.10) ceramic compositions were prepared by solid state synthesis and usual sintering. XRD studies have shown a formation of the perovskite structure in all the compositions studied. However, a small amount of pyrochlore phase was detected in Sn-doped compositions with x ≥ 0.05. Scanning electron microscopy studies have shown that at the boundaries of the large grains there exist a lot of small submicron grains enriched by Sn. Concentration dependencies of the unit-cell parameter and ferroelectric phase transition temperatures also imply that solubility of Sn in PFN is limited to ≈ 5 at.%.

The Effect of Mechanical Activation on the Synthesis and Properties of Multiferroic Lead Iron Niobate

The present work studies the effect of high-energy mechanical activation using the planetary-centrifugal ball mill AGO-2 and subsequent annealing on the synthesis and magnetic properties of PbFe0.5Nb0.5O3 (PFN). This technique enables one to perform the mechanically activated synthesis of PFN at much shorter time. The results of X-ray phase analysis, electron microscopic studies and transmission Mossbauer 57Fe spectra measurement are presented and discussed. Mossbauer studies show that the temperature of magnetic phase transition in PbFe0.5Nb0.5O3 powders can be changed by mechanical activation and subsequent annealing.