The electrocaloric effect (ECE) has attracted significant interest in the development of environmentally friendly solid-state refrigerators. To maximize the magnitude of the ECE at ambient temperature, several approaches have been applied, including the optimization of composition, microstructure, and field-protocol. In this paper we show that defect-related aging can effectively tune the ECE in lead-free 0.95(Na0.5Bi0.5)TiO3-0.05BaTiO3 ceramics at room temperature. The ECE is measured both directly and indirectly at different stages of aging. It reaches a maximum near the transition temperature between the relaxor and field-induced ferroelectric states. This temperature was shown to decrease upon aging, leading to a gradual shift in the ECE maximum toward room temperature. We have attributed the aging phenomena in the material under study primarily to the formation of dipolar defects consisting of vacancies in the A- and oxygen sublattices, and their alignment within the ferroelectric domains.

The purpose of this study was to investigate the effect of cation substitutions on the formation of Aurivillius phases at various annealing temperatures using solid-state synthesis. The phase formation of lanthanide bismuth ferrotitanates with the Aurivillius phase structure Ln(2)Bi(3)FeTi(3)O(15), where Ln = Tb, Ho, Er, Yb, has been studied. The obtained samples were characterized by X-ray diffraction, infrared spectroscopy, differential thermal and thermogravimetric analysis, and their elemental composition was studied. The predominant formation of a phase with a pyrochlore structure was revealed in the entire series studied in the chosen synthesis conditions. The exception was the sample containing ytterbium(III) cations, in which phases of the pyrochlore type and layered perovskite with the Aurivillius structure coexist. The main pyrochlore-type phase in all samples crystallizes in the cubic syngony. It is shown that in the Ln(2)Bi(3)FeTi(3)O(15) samples, where Ln = Tb, Ho, Er, the crystal lattice parameters decrease due to a decrease in the cationic radius of the Ln(III) ions. However, in the last sample of the studied series Yb2Bi3FeTi3O15, this pattern is violated because of Yb(III) ions distribution between the perovskite and pyrochlore phases. It has been established that the thermal effects observed in the samples Ln(2)Bi(3)FeTi(3)O(15), where Ln = Tb, Ho, Er, can be attributed to an order-disorder phase transition in the pyrochlore structure. When studying the temperature behavior of a two-phase sample of Yb2Bi3FeTi3O15, two reversible phase transitions were revealed: a low-intensity thermal effect characterizes changes in the pyrochlore-type structure, and a more intense thermal effect can be attributed to a ferroelectric phase transition in the structure of a layered perovskite of the Aurivillius family.

Ceramic samples of new compositions (1-x)(0.9NaNbO3 - 0.1BaTiO3) - xBaZrO3 (x = 0 divided by 0.05) modified by SiO2 oxide additive were prepared; their phase composition, structure parameters, microstructure, dielectric and ferroelectric properties were studied. Formation of a phase with a perovskite structure with a tetragonal unit cell was established, which volume increased due to partial replacement of A and B cations by Ba2+ and Zr4+ ones with large ionic radii. Decrease in the temperature of ferroelectric phase transitions and corresponding increase in polarization and electrocaloric effect indicates a positive effect of doping on the functional properties of sodium niobate-based ceramics.

The number of types of nanostructures 3–80 nm in size, found in ternary copolymers—fluorocopolymers Viton GFLT 600S and Viton GFLT 200S with a change in the their history, exceeds (according to the large-angle X-ray diffraction data) that found in double copolymers—SKF-26 and SKF-32. This process is affected more strongly by the chemical structure of spacings in macromolecules rather than the molecular weight. The complex and different character of change in the dynamic viscosity of fluorocopolymers with increasing temperature is caused by multiple phase transitions. It is shown that the fluoroelastomer SKF-32, in contrast to the three fluorocopolymers, does not pass to the viscous flow state up to 190°С due to the presence of nanostructures of intermolecular type, 5 nm in size, whose strength is much higher than that of (3–4) nm nanostructures in the fluorocopolymers. It is found that the rotational mobility of the TEMPO radical is determined to a greater extent by the flexibility of passage chains, connecting ordered formations into a unified system, rather than the intermolecular distances in the disordered part of copolymers.

The electrocaloric effect (ECE) refers to the isothermal entropy or adiabatic temperature change of polar crystals when an electric field is applied or removed. This effect is a promising approach for the development of compact solid state coolers. Among the promising materials are environmentally friendly lead-free relaxors of the (Na0.5Bi0.5)TiO3 -BaTiO3 (NBT-BT) family showing a large ECE in a wide temperature range. Intriguingly, some groups have reported negative ECE in these materials and attributed this to the antiferroelectric features of NBT. Both relaxor and antiferroelectric properties of NBT can be enhanced by doping, e.g., by adding bismuth magnesium titanate (BMT). In this study, the effect of the incorporation of BMT on the structural, electrical, and electrocaloric properties of lead-free ceramics (1-x)[0.95(Na0.5Bi0.5)TiO3-0.05BaTiO3]-(x)Bi(Mg0.5Ti0.5)O3, where x = 0, 0.1, and 0.2, was investigated. The temperature dependences of the dielectric permittivity exhibit behavior characteristic of relaxors. With increasing BMT content, the maximum polarization value decreases from 23 to 8.5 mu C/cm2, and the depolarization temperature shifts from 322 to 254 K, which points out an enhancement in relaxor properties. We have shown that double hysteresis loops observed in the studied materials are not caused by the antiferroelectric nature of the compositions, but by the pinning effect by point defects. Furthermore, the negative ECE estimated from the indirect measurements using Maxwell's relation was not confirmed by the direct measurements using a quasi-adiabatic calorimeter. The interplay between defect dipoles and the role of BMT in stabilization of ergodic relaxor behavior at lower temperatures and electrocaloric effects are discussed. The study also shows that the NBT-5BT-10BMT composition is promising for energy storage applications.

Single phase ceramic samples of new compositions (1 − x)NaNbO3 – хLiNbO3 (x = 0; 0.05; 0.10; 0.15), modified with a LiF additive, are prepared through solid state reactions, and their crystal structure, microstructure and dielectric and nonlinear optical properties are studied. A drop from 108.1 to 42.8 nm of the volume-weighted crystallite size distribution function corresponding to coherent scattering regions is observed. A rise in the temperature of phase transitions and a weakening of nonlinear optical properties are revealed as the concentration of Li cations grows.

Методом твердофазного синтеза получены однофазные керамические образцы новых составов: (1 – x )(K 0.5 Na 0.5 )NbO 3 ⋅ x BaZrO 3 ( x = 0–0.06), в том числе модифицированные добавками оксидов SiO 2 и ZnO, и изучены их кристаллическая структура, микроструктура, диэлектрические и нелинейные оптические свойства. Установлено формирование фазы со структурой перовскита с ромбической элементарной ячейкой во всех синтезированных образцах, зафиксировано увеличение объема элементарной ячейки в результате частичного замещения катионов базового состава на катионы комплексной добавки. Сегнетоэлектрические фазовые переходы подтверждены методами диэлектрической спектроскопии и генерации второй гармоники лазерного излучения. Выявлено понижение температуры фазовых переходов при допировании добавками оксидов SiO 2 и ZnO из сегнетоэлектрической ромбической фазы в сегнетоэлектрическую тетрагональную, затем – в кубическую параэлектрическую фазу.

— Single-phase (1 – x )(K 0.5 Na 0.5 )NbO 3 ⋅ x LiNbO 3 (KNN–LN) perovskite-structure ceramic materials with x = 0–0.10 modified with CuO and KCl additions have been prepared by solid-state synthesis, and their phase composition, structure, microstructure, and dielectric and ferroelectric properties have been studied. Increasing the percentage of lithium niobate has been shown to increase their Curie temperature and lower the temperature of their polymorphic phase transition, which is accompanied by a decrease in perovskite cell parameters, in accord with the ionic radii of the A-site cations. The x = 0.2 material has been found to have an increased room-temperature dielectric permittivity, which correlates with the observed increase in spontaneous polarization, as evidenced by laser radiation second harmonic generation intensity measurements.

Improvement of the properties of KNN ceramics requires tools allowing one to reveal a relation between the structure and composition, on the one hand, and ferroelectric property, on the other. In this work, lead-free (1-x) (K0.5Na0.5)NbO3-xAgNO3 and (1-x) (K0.5Na0.5)NbO3-xBaZrO3 ceramics with different contents of the ferroelectric phase have been studied using piezoresponse force microscopy (PFM), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). A correlation between the PFM image contrast, the SEM phase contrast (BEI) and the elemental composition of the phases (EDS) has been found. Specimens with a low content of the ferroelectric phase contain secondary phase with composition and structure different from those of ABO3 perovskite. XPS has shown the presence of two types of potassium ions with different chemical states, i.e., inside (K1) and outside (K2) the perovskite lattice. The observed correlation between the ferroelectric properties of the ceramics and the K2 concentration allows one to use it as a reliable indicator of ferroelectric properties.

Single-phase ceramic samples with new compositions (1 – x – у )(Na 0.5 Bi 0.5 )TiO 3 – х BaTiO 3 – у (K 0.5 Na 0.5 )NbO 3 ( x = 0.05, у = 0–0.15) modified with ZnO and GeO 2 additions were obtained by solid-phase synthesis. Their crystal structure, microstructure, and dielectric and local piezoelectric properties were studied. A phase with a perovskite structure with a pseudocubic unit cell was found to form in all of the synthesized samples; it was shown that the unit cell volume increased as a result of the partial replacement of base cations by complex additive cations. The ferroelectric phase transitions were confirmed by dielectric spectroscopy. Residual piezoelectric hysteresis loops were obtained for the synthesized samples in the polarization switching spectroscopy mode, which confirmed the switching of ferroelectric polarization.

— Single-phase (1 – x )(K 0.5 Na 0.5 )NbO 3 ⋅ x BaZrO 3 ( x = 0–0.06) ceramics with new compositions, including those modified with SiO 2 and ZnO oxide additions, have been prepared and their crystal structure, microstructure, and dielectric and nonlinear optical properties have been studied. A phase with the perovskite structure and an orthorhombic unit cell has been shown to form in all of the synthesized materials. Partial replacement of cations of the basic composition by cations of the combined additive has been demonstrated to cause an increase in unit-cell volume. The ferroelectric phase transitions in the ceramics have been confirmed by dielectric spectroscopy and laser radiation second harmonic generation measurements. Doping with SiO 2 and ZnO oxide additions has been shown to lower the temperatures of the transitions from the orthorhombic ferroelectric phase to a tetragonal ferroelectric one and then to a cubic paraelectric phase.

Single phase ceramic samples of new compounds (1 − x )(K 0.5 Na 0.5 )NbO 3 − x La(Ag 0.5 Sb 0.5 )O 3 ( x = 0–0.15), modified by metal oxide additives ZnO, CuO, and MnO 2 , are prepared via a solid state reaction. The crystal structure, microstructure, and dielectric and ferroelectric properties of the samples are studied. It is established that a phase with a perovskite structure and an orthorhombic unit cell formed in each sample. Ferroelectric phase transitions are confirmed via dielectric spectroscopy. Generation of the second harmonic is observed, along with a drop in the temperature of transitions from the ferroelectric orthorhombic phase to the ferroelectric tetragonal phase, and then to the cubic paraelectric phase.

Influence of (K0.5Bi0.5)TiO3 (KBT) dopants on crystal and microstructure, ferroelectric and dielectric properties of ceramic composition from morphotropic phase boundary in the system (1-x-y)(Na0.5Bi0.5)TiO3 - xBaTiO(3) - y (K0.5Bi0.5)TiO3 (x = 0.06, y = 0-0.15) additionally modified by ZnO additives was studied using a complex of methods. Changes from 42.8 nm to 62.6 nm of the volume-weighted crystallite size distribution function G(L) corresponding to coherent scattering regions were observed. The microstructure and the second harmonic generation data indicated a change in the relative content of polar nanoregions in tetragonal nonpolar matrix. Decrease in temperatures of phase transitions T (m) and T (d) was observed with KBT concentration increasing.

Single-phase ceramic samples of new compositions (Na 1 – х Sr х ) 0.5 Bi 0.5 TiO 3 ( x = 0–0.5), including those modified by additives of SiO 2 and ZnO oxides, have been obtained by solid-phase synthesis. The crystal structure and microstructure of these samples, as well as their dielectric, nonlinear optical, and local piezoelectric properties, have been studied. The formation of a perovskite-type phase with a pseudocubic unit cell in all synthesized samples and an increase in the cell volume as a result of partial substitution of perovskite structure cations are established. A decrease in the temperature of ferroelectric phase transitions (confirmed by the methods of dielectric spectroscopy and laser second-harmonic generation) to the tetragonal paraelectric phase is revealed. Remanent piezoelectric hysteresis loops are obtained for the synthesized samples in the polarization switching mode; this result confirms the occurrence of ferroelectric polarization switching.

Influence of cation substitution on phase content, crystal structure parameters, microstructure, and dielectric propertiesDielectric Properties of (1-x-y)(Na0.5Bi0.5)TiO3 –xBaTiO3 – y(K0.5Na0.5)NbO3 ceramics with x = 0.05, y = 0–0.15 and additionally modified by 1.5 w.% of ZnO was studied. The samples prepared by the solid-state reaction method are characterized by phase transitions with corresponding peaks of dielectric permittivity at ~550–600 K and anomalies near ~350–425 K. A decrease in the unit cell parameters changes was observed in modified compositions correlating with radii of substituting cations. A decrease in the temperatures of phase transitions Tm and Td was observed as well. Additionally, effects of dielectric relaxation caused by the formation of oxygen vacancies in ZnO-doped samples were observed at temperatures >700 K. An increase in the dielectric permittivity value at the room temperature was observed at y = 0.10 and at y = 0.05 in ZnO modified samples confirming prospect of functional properties improvement.

The phase formation in the series Ln(2)Bi(3)FeTi(3)O(15) (Ln = La, Pr, Nd, Sm, Gd) was studied. It was found that the layered perovskite structure was formed after annealing at 1000 degrees C. Further increase in the synthesis temperature to 1100 degrees C did not affect the phase composition, excluding Gd-containing sample, where perovskite structure was partially destroyed to form the pyrochlore phase based on Bi2Ti2O7. Reversible anomalies obtained by differential thermal analysis indicate the phase transitions with a change in the crystal lattice symmetry in case of a perovskite-like phase and a transition of the order-disorder type in case of a pyrochlore structure.

Influence of dopants on structure, microstructure, dielectric and ferroelectric properties of ferroelectric-relaxor (Na0.5Bi0.5)TiO3 ceramics modified by Ba2+ cations and overstoichiometric additives (SiO2 and Na2O) was studied. Changes in structure, microstructure, and dielectric parameters were observed depending on solid solutions compositions.

n Erratum to this paper has been published: https://doi.org/10.1134/S0020168521120165