M. A. Malitskaya

High dielectric permittivity in AFe1/2B1/2O3 nonferroelectric perovskite ceramics (A=Ba, Sr, Ca; B=Nb, Ta, Sb)

AFe1/2B1/2O3(A=Ba, Sr, Ca; B=Nb, Ta, Sb) ceramics were synthesized and temperature dependences of dielectric permittivity were measured at different frequencies. The experimental data obtained show very high values of dielectric permittivity in a wide temperature interval that is inherent to so-called high-k materials. Analyses of these data establish a Maxwell–Wagner mechanism as a main source for the phenomenon observed.

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.

Dielectric and Mossbauer studies of ferroelectric and magnetic phase transitions in a-site and b-site substituted multiferroic PbFe 0.5 Nb 0.5 O 3

In the present study, we diluted either A- or B- sublattice of perovskite multiferroic PbFe0.5Nb0.5O3 (PFN) and studied the changes of the magnetic and ferroelectric phase transition temperatures and dielectric properties caused by such dilution. Dielectric studies of PFN single crystals show that, in contrast to the commonly adopted view, the ferroelectric phase transition in PFN is non-diffused and the relaxor-like behavior usually observed in ceramic samples has an extrinsic nature. A-site substitutions (Ba, Ca) lead to the smearing of the permittivity-temperature maximum, lowering its maximum temperature, Tm and inducing relaxor behavior. B-site diluting of PFN by Ti increases Tm and only slightly affects the permittivity maximum diffuseness. Both A-site and B-site substitutions in PFN lead to lowering of its Neel temperature, TN. However, above a certain compositional threshold, fast lowering of TN stops and a new magnetic state with comparatively high (~50K) transition temperature becomes stable in a rather wide compositional range.

NaNbO3-Based Relaxor

Binary NaNbO3-ABO3 perovskite solid solutions belonging to the group II according to N.N. Krainik classification (Izv. Akad. Nauk SSSR, Ser. Phys. 28, 643 (1964)) exhibit a relaxor-type behavior when the second component concentration exceeds a threshold value x0, corresponding to the abrupt decrease of the dielectric permittivity ϵ maximum temperature Tm. The Tm values of all the known binary NaNbO3-ABO3 relaxor compositions are well below the room temperature and the maximal ϵ values ϵm are much lower than in the case of Pb-containing relaxors. However both Tm and ϵm values may be increased substantially by Li or K-doping leading to the formation of ternary NaNbO3-ABO3-LiNbO3 (KNbO3) solid solutions.

Dielectric and piezoelectric properties of PbFe 1/2 Nb 1/2 O 3 -PbTiO 3 ceramics from the morphotropic phase boundary compositional range

Dielectric, X-ray, and piezoelectric studies of highly-resistive Li-doped (1-x)PbFe_1/2Nb_1/2O₃-(x)PbTiO₃ (PFN-xPT) ceramics from the 0 ≤ x ≤ 0.2 range fabricated by solid-state synthesis and usual sintering have been carried out. Distinct anomalies of dielectric and piezoelectric parameters, corresponding to the transition between rhombohedral (monoclinic) and tetragonal ferroelectric phases, have been observed in pure PFN and PFN-xPT compositions with PbTiO₃ content up to 8 mol.%. The x,T-phase diagram of the PFN-xPT solid solution system has been constructed using these data.

Growth and study of PbFe1/2Ta1/2O3 single crystals

Single crystals of the composition PbFe1/2Ta1/2O3 are grown by the method of mass crystallization from flux. It is established that, unlike the PbFe1/2Ta1/2O3 ceramic, the synthesized single crystals possess pronounced relaxor properties: the maximum of the dielectric constant is diffuse and its temperature, Tm, increases by more than 70 K with an increase in the frequency from 102 to 106 Hz. It is assumed that the unusual properties of the PbFe1/2Ta1/2O3 crystals are caused by mesoscopically inhomogeneous compositional ordering and comparatively high conductivity providing favorable conditions for the appearance of the volume-charge and thermal electron polarization.

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.

Bias Field Effect on the Dielectric and Pyroelectric Response of Pb(Fe0.5Ta0.5)O3 Relaxor Multiferroic Ceramics

Dielectric and pyroelectric studies of PbFe0.5Ta0.5O3 ceramics have been carried out under different bias field values. E,T-phase diagram has been constructed using these data. It has been found out that PbFe0.5Ta0.5O3 combines the properties of both relaxors and usual ferroelectrics. The results obtained show that the origin of the relaxor-like behavior in PbFe0.5Ta0.5O3 seems to be somewhat different as compared to canonical relaxors.

The Effect of PbO-Nonstoichiometry on Dielectric and Semiconducting Properties of PbFe0.5Nb0.5O3-Based Ceramics

Dielectric, X-ray and electric resistivity studies of nonstoichiometric PbFe0.5Nb0.5O3-based ceramics obtained by solid-state reaction route with PbO/(Fe0.5Nb0.5O2) molar ratio differing from 0.6 to 1.2 have been carried out. Deviation from stoichiometric PbFe0.5Nb0.5O3 composition leads to formation of diphase mixtures containing ferroelectric perovskite PbFe0.5Nb0.5O3 phase and nonferroelectric PbO or pyrochlore Pb3Nb4O13 phases and to decreasing of the dielectric permittivity values. The electric resistivity of PbFe0.5Nb0.5O3 ceramics decreases dramatically when the sintering temperature increases. The formation of the pyrochlore phase decreases the room temperature resistivity values while the anomaly of the positive temperature coefficient of resistivity above the ferroelectric Curie temperature gradually diminishes with the increase of the pyrochlore content.