A phenomenological potential and ferroelectric properties of BaTiO3–CaTiO3 solid solution

A modified phenomenological potential was constructed for BaTiO₃-CaTiO₃ solid solution single crystal based on the Landau–Devonshire theory. The Ba_1−xCa_xTiO₃ solid solution phase diagram of temperature vs concentration (T-x) is obtained and shown to perfectly agree with experimental observations. Sustained Curie temperature can be obtained by the increase of Ca concentration x, while the transition temperatures from tetragonal to orthorhombic phase, and further to rhombohedral phase decrease with increasing Ca content. In this work, spontaneous polarization and dielectric constant dependence on concentration are studied and the relationship between hydrostatic pressure and transition temperatures is established to pave the way for the generation of temperature-concentration-pressure phase diagram.     

Investigation on ferromagnetic and ferroelectric properties of (La,K)-doped BiFeO3–BaTiO3 solid solution

Multiferroic materials of BiFeO₃–BaTiO₃ solid solution have been fabricated in order to improve ferromagnetic and ferroelectric properties. The effects of La (1 mol%) and K (varied from 0.5–5 mol%) doped 0.75BiFeO₃–0.25BaTiO₃ on phase formation, ferromagnetic and ferroelectric properties have been investigated and discussed. The rhombohedral perovskite phase of specimens was characterized by XRD technique. Fracture morphology reveals the grain growth characteristics with increasing K content. (La, K)-doped 0.75BiFeO₃–0.25BaTiO₃ with La=1 mol% and K=3 mol% exhibits the highest remnant polarization and remnant magnetization.     

Dielectric,ferroelectric, and depolarization properties of B-site manganese-doped 0.925(Bi0.5Na0.5)TiO3–0.075BaTiO3 solid solutions

(Bi_0.5Na_0.5)_0.925Ba_0.075(Ti_1−xMn_x)O₃ (x=0, 0.2, 1.0, and 2.0 mol%) ceramics were prepared by solid-state-reaction method to study dielectric, ferroelectric, and depolarization properties. The manganese (Mn) doping can suppress dielectric permittivity and increase relaxor behavior. Coercive field (E_c) increases, while remanent polarization (P_r) decreases as the Mn content increases. P_r exhibits discontinuous anomalies as a function of temperature in all compositions, implying a polarization reorganization of local domains. The depolarization temperature (T_d) reaches the highest value (~152 °C) in 0.2%Mn, and decreases as MnO₂ content increases. The increased T_d in 0.2%Mn is due to two-phase coexistence and structural thermal stability induced by Mn ions. This work suggests that the moderate Mn doping can enhance T_d in lead-free piezoceramics for applications at elevated temperatures.     

Dielectric,ferroelectric, and energy storage properties of Ba(Zn1/3Nb2/3)O3-modfied BiFeO3–BaTiO3 Pb-Free relaxor ferroelectric ceramics

Pb-free bulk ceramics (1-x)[0.65BiFeO₃-0.35BaTiO₃]-xBa(Zn_1/3Nb_2/3)O₃ were produced by traditional solid-state reaction route. In this experiment, Ba(Zn_1/3Nb_2/3)O₃ (BZN) was introduced to destroy long-range order domains in order to obtain higher energy storage performance. Impedance and XPS analysis indicate that oxygen vacancies exist and participate in relaxation processes at high temperatures. With the increase of BZN content, the dielectric relaxation behavior is improved, the hysteresis loop becomes thinner, remnant polarization decreases, and the breakdown electric field increases to 180 kV/cm in 15BZN. A maximum W_rec (1.62 J/cm³) is eventually reached in 7BZN with great temperature stability. The highest efficiency is 91% in 15BZN with W_rec of 1.28 J/cm³. Charge-discharge tests show that ceramics have a quick discharge time of t_0.9 < 0.1 μs, which makes BZN-doped ceramics a potential candidate for energy storage devices.     

Synthesis and properties of multiferroic 0.7BiFeO3−0.3BaTiO3 thin films by Mn doping

Multiferroic BiFeO₃?BaTiO₃ thin films that simultaneously exhibit ferroelectricity and ferromagnetism at room temperature were prepared by chemical solution deposition. Perovskite single-phase 0.7BiFeO₃?0.3BaTiO₃ thin films were successfully fabricated in the temperature range 600–700 °C on Pt/TiO_x/SiO₂/Si substrates. As the crystallization temperature was increased, grain growth proceeded, resulting in higher crystallinity at 700 °C. Although the 0.7BiFeO₃?0.3BaTiO₃ thin films exhibited poor polarization (P)?electric field (E) hysteresis loops owing to their low insulating resistance. The leakage current at high applied fields was effectively reduced by Mn doping at the Fe site of the 0.7BiFeO₃?0.3BaTiO₃ thin films, leading to improved ferroelectric properties. The 5 mol% Mn-doped 0.7BiFeO₃?0.3BaTiO₃ thin films simultaneously exhibited ferroelectric polarization and ferromagnetic magnetization hysteresis loops at room temperature.     

Ferroelectromagnetic characteristic of Na-doped 0.75BiFeO3–0.25BaTiO3 multiferroic ceramics

BiFeO₃-based materials are expected to have both ferroelectricity and ferromagnetism simultaneously. In this study, effects of Na-doping (0.5, 1.0, 3.0, and 5.0 mol%) on ferromagnetic and ferroelectric properties of 0.75BiFeO₃–0.25BaTiO₃ ceramics which have been fabricated by the solid state reaction technique are studied. The effects of Na-doped 0.75BiFeO₃–0.25BaTiO₃ ceramics on the crystal structure, and magnetic and electrical properties were investigated and discussed. Rhombohedrally distorted 0.75BiFeO₃–0.25BaTiO₃ showed weak ferromagnetic and ferroelectric properties. In addition, ferroelectric and ferromagnetic properties of 0.75BiFeO₃–0.25BaTiO₃ have been controlled by Na doping, and the maximum values of magnetization and polarization were observed at 5.0 mol%.     

Sol-gel synthesis,structural, morphological and magnetic properties of BaTiO3–BiMnO3 solid solutions

In this study, solid solutions of (1-x)BaTiO₃-xBiMnO₃ have been synthesized by an aqueous sol-gel method. It was determined that single-phase compounds can be obtained up to x = 0.6 and with further increase in percentage of BiMnO₃ component additional crystal phases were detected. Perovskite crystal structure was determined for all synthesized compounds regardless of chemical composition. Raman spectra of synthesized solid solutions showed gradual change of the shape with an increase of BiMnO₃ fraction. It was demonstrated that partial substitution of BaTiO₃ by BiMnO₃ led to the drastic growth of grains of the end products. Magnetization measurements showed that all BiMnO₃-containing samples are characterized by paramagnetic behavior. Clear correlation between magnetization values and composition of the materials was observed, magnetization values increased with increasing of BiMnO₃ content in solid solutions.     

Energy storage performance of BiFeO3–SrTiO3–BaTiO3 relaxor ferroelectric ceramics

Dielectric capacitors reveal great potential in the application of high power and/or pulsed power electronic devices owing to their ultrafast charge–discharge rate and ultrahigh power density. Among various dielectric capacitors, the environment-friendly lead-free dielectric ceramics have drawn extensive investigations in recent years. Nevertheless, the relatively small recoverable energy storage density (W_rec) is still an obstacle for their application. Herein, the (0.55−x)BiFeO₃–0.45SrTiO₃–xBaTiO₃ ternary ceramics with 0.1 wt% MnO₂ were prepared by the solid-state reaction, and achieved enhanced relaxor behavior as well as breakdown strength E_b. As a result, the x = 0.12 ceramic exhibited superior comprehensive energy storage performance of large E_b (50.4 kV/mm), ultrahigh W_rec (7.3 J/cm³), high efficiency η (86.3%), relatively fast charge–discharge speed (t_0.9 = 6.1 μs) and outstanding reliability under different frequency, fatigue, and temperature, indicating that the BiFeO₃-based relaxor ferroelectric ceramics are prospective alternatives for electrostatic energy storage.     

Electrical properties and phase of BaTiO3–SrTiO3 solid solution

It has been known that ABO₃ type perovskite ferroelectrics, such as BaTiO₃ (BTO) and SrTiO₃ (STO), form a complete solid solution. In this study, Ba_1?xSr_xTiO₃ (BST, x=0.0–1.0) solid solution were sintered by a solid-state reaction method using BTO and STO raw powders with appropriate chemical composition. The crystal structure was investigated by a Rietveld refinement method; Fullprof, using X-ray diffraction data. Within the reasonable goodness of fit, tetragonal symmetry was found in BST with x≤0.2, while BST with x≥0.4 were found to be cubic symmetry. However, Ba_0.7Sr_0.3TiO₃ was difficult to decide whether it is cubic or tetragonal because of large uncertainties after final fitting. The composition ratios calculated from the fitted occupancies match well with those measured by EDS within experimental uncertainties. Remnant polarizations of BST with x<0.3 decrease with increasing Sr concentration. Furthermore, measured phase transition temperatures and maximum dielectric constant decrease as increasing Sr concentration. Measured electrical properties of BST were match well with the structural refinement investigations.     

Phase structure and energy storage performance for BiFeO3–BaTiO3 based lead-free ferroelectric ceramics

Recently, BiFeO₃–BaTiO₃ (BF-BT) lead-free ferroelectric ceramics have been widely concerned and deemed as one of the most promising candidates for lead-free energy-storage material because of their high spontaneous polarization and excellent energy storage properties. Herein, a series of Bi_1-zLa_zFeO₃-xBaTiO₃+yMnO₂ (BL_zF-xBT-yMn at 0.45 ≤ x ≤ 0.60 mol, 0.0 ≤ y ≤ 0.4% mol and z = 0 or 0.02 mol) ceramics were prepared to reveal their energy-storage performance. With increasing x, the breakdown strength (BDS) increases, while the maximum polarization (P_max), remanent polarization (P_r), and the difference value between P_max and P_r (ΔP) decrease. Because of the high BDS and ΔP, a large energy storage density W_re = 1.08 J/cm³ is achieved in BF-0.48BT ceramics as the electric field is 130 kV/cm. In addition, with increasing y and z, the increasing BDS and ΔP have been observed. Due to the improvement in BDS and ΔP, an excellent W_re = 1.22 J/cm³ was achieved in Bi_1-zLa_zFeO₃-xBaTiO₃+yMnO₂ ceramics at x = 0.48, y = 0.3% and z = 0.02. This work provides the clue for application of the high-power-energy BF-BT ceramics.     

Room-temperature multiferroic characteristics and unique vortex domain structures of h-Yb1−xInxFeO3 solid solutions

Hexagonal rare-earth ferrites (h-RFeO₃) have attracted much scientific attention due to their room-temperature multiferroicity. However, it is still a hard job to obtain h-RFeO₃ bulk materials because of the meta-stability of such hexagonal phase, and the evaluation of room-temperature ferroelectric and magnetoelectric characteristics in such materials is also a challengeable issue. In the present work, Yb_1−xIn_xFeO₃ ceramics with the stable hexagonal structure were obtained by introducing chemical pressure, where the unique ferroelectric domain structures of sixfold vortex combined with tenfold vortex with a typical size of ~400 nm were determined. Symmetry of the present system evolved from centrosymmetric orthorhombic Pbnm (x = 0–0.4) to non-centrosymmetric hexagonal P6₃cm (x = 0.5 and 0.6) with a ferroelectric polarization up to 3.2 μC/cm², and finally to centrosymmetric hexagonal P6₃/mmc (x = 0.7 and 0.8). The Curie point decreased monotonically from 723 K to a temperature below room temperature with increasing x, and the antiferromagnetic phase transition above room temperature was determined for all compositions. Meanwhile, a large linear magnetoelectric coefficient (α_ME) up to 0.96 mV/cm Oe was obtained at room temperature, and this indicated the great application potential for magnetoelectric devices.     

Phase evolution and origin of the high piezoelectric properties in lead-free BiFeO3–BaTiO3 ceramics

La-substitution effects for Bi³⁺-site in 0.7Bi_1.03(1-x)La_xFeO₃-0.3BaTiO₃ (abbreviated as BF30BT-100xLF with x = 0.00, 0.01, 0.035, 0.06, 0.07 and 0.10) ceramics were investigated systematically. All ceramics were synthesized by a conventional solid-state reaction method and quenched in water from its sintering temperature. The crystal structure Rietveld refinement shows that undoped BF30BT ceramic exhibited dominant rhombohedral (R) symmetry and gradually changed to the tetragonal (T) phase with La-doping. However, for x ≥ 0.07 compositions the lattice distortions (c_T/a_T and 90°−α_R) significantly decrease as a result crystal structures become close to the cubic-like (CL) phase. Hence, two different morphotropic phase boundaries (MPBs) were reported for BF30BT-100xLa ceramic system; one MPB-I between the R and T phases and the other MPB-II between T and CL phases. The largest direct piezoelectric coefficient (d₃₃ = 274 pC/N) with a high Curie temperature (T_C = 532 °C) for BF30BT-1LF composition was obtained due to the typical MPB-I between R and T phases. However, a maximum electric field-induced strain (S_max = 0.27%) with a high converse piezoelectric coefficient (d₃₃* = 500 pm/V) for BF30BT-7LF ceramic was mainly attributed to the MPB-II of T + CL phases and soft ferroelectric switching properties.     

Coexistence of three ferroelectric phases and enhanced piezoelectric properties in BaTiO3–CaHfO3 lead-free ceramics

Perovskite ferroelectrics possess the fascinating piezoelectric properties near a morphotropic phase boundary, attributing to a low energy barrier that the results in structural instability and easy polarization rotation. In this work, a new lead-free system of (1-x)BaTiO₃-xCaHfO₃ was designed, and characterized by a coexistence of ferroelectric rhombohedral-orthorhombic-tetragonal (R-O-T) phases. With the increase amount of CaHfO₃ (x), a stable coexistence region of three ferroelectric phases (R-O-T) exists at 0.06 ≤ x ≤ 0.08. Both large piezoelectric coefficient (d₃₃～400 pC/N), inverse piezoelectric coefficient (d₃₃^*～547 pm/V) and planar electromechanical coupling factor (k_p～58.2%) can be achieved for the composition with x = 0.08 near the coexistence of three ferroelectric phases. Our results show that the materials with the composition located at a region where the three ferroelectric R-O-T phases coexist would have the lowest energy barrier and thus greatly promote the polarization rotation, resulting in a strong piezoelectric response.     

High piezoelectric properties in 0.7BiFeO3–0.3BaTiO3 ceramics with MnO and MnO2 addition

BiFeO₃–BaTiO₃–based solid solutions are promising candidates for high–temperature piezoelectric devices because of their high Curie temperature (T_C) and considerable electrical properties. Here, we reported an optimum composition of 0.7Bi(Fe_0.999Mn_0.001)O₃–0.3BaTiO₃ ceramic with a large piezoelectric constant (d₃₃) of 230 pC/N and a high T_C of 505 °C, which was attributed to the intentional introducing of the ceramic with MnO and MnO₂ mixture. Furthermore, an in situ d₃₃ measurement was carried out, demonstrating excellent thermal stability for the 0.7Bi(Fe_0.999Mn_0.001)O₃–0.3BaTiO₃ specimen. The d₃₃ remained above 200 pC/N in the temperature range of 25 °C–400 °C and its fluctuation was less than ± 15 %. It was determined that the high d₃₃ in the 0.7BiFe_0.999Mn_0.001)O₃–0.3BaTiO₃ ceramic originated from a synergistic effect of rhombohedral distortion, intrinsic response, and ferroelectric order. The findings establish a solid correlation between electrical properties and phase/domain structure, and provide a novel approach to improve the piezoelectric properties for BiFeO₃–BaTiO₃–based ceramics.     

High-temperature 0.5BiFeO3–0.5PbFe0.5Nb0.5O3 multiferroic: Microstructure,ferroelectric properties,and Mössbauer effect

Research findings of the microstructure, dielectric, ferroelectric characteristics, and Mössbauer effect of solid solution ceramics with 0.5BiFeO₃–0.5PbFe_0.5Nb_0.5O₃ composition in a wide temperature range are presented. The examined ceramic chip surface allows one to draw conclusions about the internal homogeneity of grains and the absence of pores inside them. It was shown that Fe³⁺ iron cations in the material are valence and they are found only in seven locally different states, which is associated with disorder in the solid solution structure. The Néel temperature is T_N ~ 445 K. The anomalous behavior at T < 30 K becomes clear when analyzing the dielectric spectra of 0.5BiFeO₃–0.5PbFe_0.5Nb_0.5O₃ ceramics in the range of 10 … 1000 K. It is explained by the appearance of a spin-glass state in the object. The presence of contributions to the dielectric response in ceramics at T > 300 K is revealed. It is claimed that the ferroelectric–relaxor → paraelectric phase transition caused the low-temperature contribution, and the second one is a manifestation of the Maxwell–Wagner polarization and the corresponding non-Debye type dielectric relaxation. The causes of the revealed regularities and the prospects for using the material in the thin films form are discussed.     

Piezoelectric properties and temperature stabilities of Mn- and Cu-modified BiFeO3–BaTiO3 high temperature ceramics

The structures, microstructures, electrical properties and the thermal stability have been investigated for the MnO₂-doped (1 ? x)BF–xBT system and the MnO₂ and CuO-doped (1 ? x)BF–xBT system, where x ranges from 0.25 to 0.35. The XRD analysis shows that the two systems have a single perovskite phase, and the MnO₂ and CuO-doped (1 ? x)BF–xBT system has a morphotropic phase boundary (MPB) with the coexistence of rhombohedral and pseudo-cubic phases in the system about x = 0.325. The addition of small amount of CuO was quite effective to lower the sintering temperature. The diffusive phase transition characteristics were observed in the MnO₂-doped (1 ? x)BF–xBT system and a normal ferroelectric phase transition characteristics were observed in the MnO₂ and CuO doped (1 ? x)BF–xBT system. Compared with the MnO₂ doped (1 ? x)BF–xBT system, the ?_m, Curie temperature (T_c), depoling temperature (T_d), and piezoelectrical properties were improved evidently with the MnO₂ and CuO doping.     

Structural modification and evaluation of dielectric and ferromagnetic properties of Ce-modified BiFeO3–BaTiO3 ceramics

An investigation on Rare earth constituent Ce incorporated BiFeO₃–BaTiO₃ ceramics has been focused in the present study. The ceramic samples of (Bi₀.₇Ba_0.3)_1-xCe_x(Fe₀.₇Ti_0.3)O₃ (x = 0–0.12) were formulated adopting the cost-effective solid-state sintering method. The influence of aliovalent Ce ions on the structural, microstructural, dielectric, ferromagnetic, and optical properties of BiFeO₃–BaTiO₃ was evaluated in this paper. The coexistence of the Tetragonal and the Rhombohedral phases was established by the Rietveld refinement process. The refined crystallographic parameters showed maximum cell volume (V_cell) and the highest percentage of the Rhombohedral phase for x = 0.06; and consequently, the ceramic exhibited the topmost dielectric constant of 946 at x = 0.06. The scanning electron microscopy of the samples revealed the manifestation of polygonal grain morphology. Besides, remarkably improved ferromagnetic properties were evinced for Ce doped ceramics. The magnitude of saturation (M_s) and remnant (M_r) magnetizations were boosted from 0 emu/g and 0.0019 emu/g to 0.9186 emu/g and 0.3745 emu/g respectively with increasing x from 0 to 0.12. Additionally, the optical band gaps of all the samples were evaluated and found to be in the range of 2.941–3.077 eV.     

Structural properties of multiferroic 0.5BiFeO3–0.5Pb(Fe0.5Nb0.5)O3 solid solution

Magnetoelectric multiferroics are very promising materials because of their practical applications and fundamental interests. The most widely studied magnetoelectric oxides are ABO₃ perovskites. In the paper structural properties of BiFeO₃ and Pb(Fe_0.5Nb_0.5)O₃ solid solution are described. The material crystallizes in rhombohedral R3c crystal structure which parameters are presented. Mössbauer spectroscopy was used to study local changes in an iron environment due to Fe/Nb substitution and hyperfine interaction parameters of different local surroundings of iron atoms are presented. The random distribution of B-site sublattice cations was confirmed. Ab initio calculations of the studied solid solution were conducted and theoretical crystal structure parameters were compared with the experimental data. The theoretical magnetic and electric properties are discussed. The local iron magnetic moments were estimated and their dependence on the local surrounding changes is shown. The calculated electrons densities and Bader's topological analysis were used to describe chemical bonding properties.     

Structure–property relationships in BaTiO3–BiFeO3–BiYbO3 ceramics

Ba_1?xBi_xTi_1?xYb_x/2Fe_x/2O₃ ceramics were fabricated by the solid state reaction method. X-ray diffraction analyses show 0 ≤ x ≤ 0.04 ceramics to have an average crystal structure described by the non-centrosymmetric tetragonal P4 mm space group, whereas x ≥ 0.08 ceramics are consistent with a centrosymmetric cubic perovskite (space group Pm-3 m). Coexistence of both tetragonal and cubic symmetries is observed for x = 0.06. Raman spectroscopy analysis corroborate a change in average structure with increasing x, but also show the local crystal symmetry for x ≥ 0.08 ceramics to deviate from the idealized cubic perovskite structure. Dielectric data show a ferroelectric-to-relaxor crossover, which occurs in conjunction with the change in both the average and local crystal symmetry as indicated by X-ray and Raman data. For x ≥ 0.08, ceramics exhibit relaxor behavior, which is also accompanied by a shift of the permittivity maxima towards higher temperatures with increasing x.     

Room temperature multiferroic properties of BiFeO3–MnFe2O4 nanocomposites

The novel functionalities of multiferroic magneto-electric nanocomposites have spawned substantial scope for fast-paced memory devices and sensor applications. Following this, herein we report the development of nanocomposites with soft ferromagnetic MnFe₂O₄ and ferroelectric BiFeO₃ to fabricate a system with engineered multiferroic properties. A modified sol-gel route called Pechini method is demonstrated for the preparation of the (1-x) BiFeO₃-x MnFe₂O₄ (x = 10%, 30%, 50%, 70%) nanocomposites. The crystallographic phase, structure, and morphology are characterized by XRD, FESEM, and HRTEM. The accurate crystallite size and lattice strain are determined by Williamson-Hall plot method and a comparative study with Scherer's equation is carried out. TEM image evidences the interface between BiFeO₃ and MnFe₂O₄ nanoparticles in the composite. The room temperature magnetic response reveals the strong dependence of magnetic saturation, remanent magnetization, and coercivity of the nanocomposites on MnFe₂O₄ addition. The dielectric response and impedance analysis of the prepared nanocomposites are observed. The electrical performance of the composite is affected by grain, grain boundaries, and oxygen vacancies. The unsaturated P-E loops exhibit the leaky ferroelectric behavior for the nanocomposite. The intrinsic magnetoelectric coupling between ferroelectric BiFeO₃ and ferromagnetic MnFe₂O₄ has been determined by varying H_dc/H_ac and its maximum coupling coefficient (α) is found to be 25.39 mV/cmOe for 70% BiFeO₃ -30% MnFe₂O₄ nanocomposite. These distinctive and achievable characteristics of the nanocomposite would enable the designing of magnetic field sensors, spintronic devices, and multiferroic memory devices.