The effect of a constant electric field on the domain structure of ferroelectric PbSc0.5Nb0.5O3 crystals

The effect of a constant electric field on the domain structure formation in ferroelectric lead scandium niobate (PbSc_0.5Nb₀₅O₃) crystals was studied by a polarization-optical method. It is shown that the crystals contain 71° and 180° domains with the boundaries representing (100) and (110) crystal planes.     

Structural,Electrical and Magnetic Behaviour of FeTe0.5Se0.5 Superconductor

We report the synthesis as well as structural and physical properties of the bulk polycrystalline FeTe and FeTe_0.5Se_0.5 compounds. These samples are synthesised by the solid state-reaction method via vacuum encapsulation. Both studied compounds are crystallized in a tetragonal phase with space group P4/nmm. The parent FeTe compound shows an anomaly in resistivity measurement at around 78 K, which is due to the structural change along with a magnetic phase transition. The superconductivity in the FeTe_0.5Se_0.5 sample at 13 K is confirmed by the resistivity measurements. DC magnetisation along with an isothermal (M–H) loop shows that FeTe_0.5Se_0.5 possesses bulk superconductivity. The upper critical field is estimated through resistivity ρ (T,H) measurements using Gingzburg–Landau (GL) theory and is above 50 T with 50 % resistivity drop criterion. The origin of the resistive transition broadening under magnetic field is investigated by thermally activated flux flow. The magnetic field dependence of the activation energy of the flux motion is discussed.     

Role of strain and lattice distortion on ferroelectric and piezoelectric properties of bismuth magnesium zirconate substituted sodium bismuth titanate ceramics

Effect of weak ferroelectric perovskite, bismuth magnesium zirconate [Bi(Mg_0.5Zr_0.5)O₃] substitution in lead-free sodium bismuth titanate [(Na_0.5Bi_0.5)TiO₃] ceramics is studied. Influence of substitution on intrinsic and extrinsic contribution and impact on ferroelectric and piezoelectric properties are investigated. Improved spontaneous polarization (P_s), increased remnant polarization (P_r), decreased coercive field (E_c) and high piezoelectric coefficient (d₃₃) are obtained for x = 0.01 mole fraction of Bi(Mg_0.5Zr_0.5)O₃ substitution due to decrease in rhombohedral lattice distortion and homogeneous strain. Small rhombohedral lattice distortion (δ_r) and minimum homogeneous strain (δ) are the primary intrinsic parameters which favours the extrinsic parameters such as mobility of non-180° domain reorientation, domain switching and domain wall motion. Enhanced mobility softens the coercive field and increases remnant polarization to maximum. Reduced rhombohedral lattice distortion, low strain and enhanced mobility are the key factors for enhanced piezoelectric constant, highest remnant polarization and decreased coercive field in non-MPB (1 ? x)(Na_0.5Bi_0.5)TiO₃–xBi(Mg_0.5Zr_0.5)O₃ solid solutions.     

Relaxation of magnetoresistance of single-crystalline (La0.5Eu0.5)0.7Pb0.3MnO3 in a pulsed magnetic field

The magnetoresistance (MR) of substituted lanthanum manganite (La_0.5Eu_0.5)_0.7Pb_0.3MnO₃ has been measured in a pulsed magnetic field with amplitude H = 250 kOe at various temperatures. It is established that temperature dependence of the MR relaxation parameter τ(T) is correlated with temperature dependence of the electric resistance R(T). A mechanism of relaxation is proposed that is related to the relaxation of conducting and dielectric phases in the volume of a sample under the conditions of phase separation. It is shown that the behavior of τ is related to the number of phase boundaries in the volume.     

Improvement on ferroelectric and piezoelectric properties of (K0.5Na0.5)NbO3 ceramic with Sr0.53Ba0.47Nb2O6 addition

(1 ? x)(K_0.5Na_0.5)NbO₃–xSr_0.53Ba_0.47Nb₂O₆ [(1 ? x)KNN–xSBN] ceramics were synthesized by solid-state reaction technique. X-ray diffraction analysis of samples indicated that a single orthorhombic perovskite phase was formed as the x value is ≤0.02. Optimized piezoelectric properties with d ₃₃ = 126 pC/N, K _p = 0.39, Q _m  = 201 were obtained for 0.98KNN–0.02SBN ceramic. The dielectric properties studies illustrated that both peaks of orthorhombic to tetragonal (T _O–T ) and ferroelectric tetragonal to paraelectric cubic (T _C ) phase transition shifted to lower temperature. The maximum remanent polarization (P _r  = 22.5 μC/cm²) for 0.98KNN–0.02SBN was obtained by the polarization versus electric field (P–E) researches. AC conductivity of samples increased with increasing the temperature. The calculated activation energy of the dc conductivity was 0.9654 eV, which may be due to thermal activation.     

(Na0.5K0.5)NbO3–LiTaO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics were prepared in the system (1−x)(Na_0.5K_0.5)NbO₃–xLiTaO₃ following the conventional mixed oxide route. The effect of cationic substitution of lithium for sodium and potassium in the A sites and tantalum for niobite in the B sites in (Na_0.5K_0.5)NbO₃ (NKN) perovskite lattice on symmetry and physical properties were investigated. The materials with perovskite structure are in orthorhombic phase when x<5 mol.% and transform to tetragonal phase when x>6 mol.%; when x≥8 mol.%, a K₃Li₂Nb₅O₁₅ phase with tetragonal tungsten bronze structure begins to appear and becomes dominant with increasing content of LiTaO₃. A morphotropic phase boundary (MPB) between orthorhombic and tetragonal phases appears at x=5–6 mol.%. Analogous to Pb(Zr,Ti)O₃, the piezoelectric and electromechanical properties are enhanced for compositions near the morphotropic phase boundary. Piezoelectric constant d₃₃ values reach ∼200 pC/N. Electromechanical coefficients of planar mode reach ∼36%, respectively. Our results show that (Na_0.5K_0.5)NbO₃ with small amount of LiTaO₃ (x=7 mol.%) is a good lead-free piezoelectric ceramic.     

Phase, microstructure and dielectric properties of 0.94Bi0.5Na0.5TiO3-0.06BaTiO3 ceramics prepared by sol-gel technique

0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ ceramics were fabricated by sol-gel technique. The XRD results revealed the formation of a single phase perovskite structured Bi_0.5Na_0.5TiO₃-BaTiO₃ at 600 °C. The SEM images showed dense microstructure and the optimum density of the ceramics sintered at 1100 °C was 5.2 g/cm³. The saturation polarization (P _s ) was found to be increased with increasing temperature while the remnant polarization (P _r ) was found to be increased gradually and then decreased abruptly near 85 °C, which could be attributed to the phase transformation. The coercive electric field (E _c ) was found to be decreased gradually with increasing temperature. The maximum value of dielectric constant (? _r ) at room temperature was 800 and dielectric loss at 1 MHz was 0.07.     

Giant strain in Nb-doped Bi0.5(Na0.82K0.18)0.5TiO3 lead-free electromechanical ceramics

The effect of Nb substitution on the crystal structure, ferroelectric, and electric field induced strain properties of Bi_0.5(Na₈₂K_0.18)_0.5TiO₃ (BNKT) ceramics has been investigated. The coexistence of rhombohedral and tetragonal phases was found in undoped BNKT ceramics, however, Nb doping induced a phase transition to a pseudocubic phase with high electrostriction coefficients. When 3 mol% Nb was substituted on Ti ions, the electric field induced strain was markedly enhanced up to S_max/E_max = 641 pm/V, which is higher than those previously reported on non-textured lead-free electromechanical ceramics.     

Fabrication of lead-free (Na0.82K0.18)0.5Bi0.5TiO3 piezoelectric nanofiber by electrospinning

(Na_0.82K_0.18)_0.5Bi_0.5TiO₃ nanofibers were synthesized by sol-gel process and electrospinning. Scanning electron microscopy was used to verify that the diameters and lengths are in the range of 150-600 nm and several hundreds of micrometer. Perovskite structure and grain size (20-70 nm) were verified by X-ray diffraction and transmission electron microscopy. The high effective piezoelectric coefficient d₃₃ (96 pm/V) was measured by scanning force microscopy. It may be attributed to easily tilting the polar vector of domain for an electric field and the increase in the number of possible spontaneous polarization direction near the rhombohedral-tetragonal morphotropic phase boundary. The research shows that there are potentional applications for (Na_0.82K_0.18)_0.5Bi_0.5TiO₃ nanofiber in nanoscale lead-free piezoelectric devices.     

Structural and dielectric properties of ferroelectric Sr1−xBaxBi2(Nb0.5Ta0.5)2O9 and Sr0.5Ba0.5Bi2(Nb1−yTay)2O9 ceramics

Ferroelectric Sr_1−xBa_xBi₂(Nb_0.5Ta_0.5)₂O₉ and Sr_0.5Ba_0.5Bi₂(Nb_1−yTa_y)₂O₉ were synthesized by solid state reaction route. X-ray diffraction studies confirm the formation of single phase layered perovskite solid solutions over a wide range of compositions (x=y=0.0, 0.25, 0.50, 0.75 and 1). The lattice parameters and the Curie temperature (T_c) have been found to have linear dependence on x and y. Transmission electron microscopy (TEM) suggest the lowering of orthorhombic distortion with increasing Ba²⁺ substitution. Variations in microstructural features as a function of x and y were monitored by scanning electron microscopy (SEM). The dielectric constant at room temperature increases with increase in both x and y. Interestingly Ba²⁺ substitution on Sr²⁺ site induces diffused phase transition and diffuseness increases with increasing Ba²⁺ concentration.     

Ferromagnetism and high magnetic moments induced by Ba substitution for Ca in charge ordered Nd0.5Ca0.5MnO3

The magnetic properties of the Nd_0.5Ca_0.5−xBa_xMnO₃ series have been investigated. Similar to the Pr_0.5Ca_0.5−xBa_xMnO₃ series (Raveau et al., J. Phys.: Condens. Matter 15 (2003) 7055), the substitution of Ba for Ca can also induce a ferromagnetic (FM) ground state and sharp magnetization steps at low temperature (2.5 K) in the Nd ones. The FM fraction first quickly increases with x due to the local ‘counter-distortion’ effect introduced by barium cations, reaches a maximum value of 26% for x = 0.03, and then slowly decreases as more Ba is introduced where the A-site size mismatch has increased dramatically. In comparison with the Pr series, the Nd series exhibit a greater ability to induce ferromagnetism. When the magnetic field is increased, for low substitution levels (x < 0.06) the latter series show higher magnetizations and lower critical fields where the magnetization steps appear. A possible interpretation for this behavior is that the Nd-Mn magnetic interactions can destabilize the antiferromagnetic structure and favour the development of FM domains.     

Origin of the large strain response in tenary SrTi0.8Zr0.2O3 modified Bi0.5Na0.5TiO3–Bi0.5K0.5TiO3 lead-free piezoceramics

The perovskite oxides (1 ? x)Bi_0.5(Na_0.9K_0.1)_0.5TiO₃–xSrTi_0.8Zr_0.2O₃ (SZT1000x, x = 0, 0.2, 0.4, 0.6, 0.8, and 1 %) were prepared via the conventional solid-state reaction method. The room temperature ferroelectric P–E loops coordinate with polarization current density J–E curves illustrated the changes of ferroelectric domains and polar nanoregions under different driving fields exhaustively. The composition and electric field dependent strain behavior of this system were investigated to develop a lead-free piezoelectric material with a large strain response at a lower electric field. A large strain of 0.44 % (S _max/E _max = 744 pm/V) at an applied field of 50 kV/cm was obtained at the composition of 0.6 mol% SZT. Temperature-dependent hysteresis measurements reveal the primary origin of the large strain is due to the presence of a nonpolar phase at a zero field. Upon the application of an electric field, the nonpolar phase that can easily transform into a long-range ferroelectric phase, and then brings the system back to its unpoled state once the applied electric field is removed. Notably, the electric field required to deliver large strains is reduced to 40 kV/cm while the S _max/E _max reached up to 717 pm/V, indicating that the developed material is highly promising for actuator applications.     

Dielectric and piezoelectric studies of perovskite-tungsten bronze structured (1 − x)[0.5PMN-0.5PZT]-xPBBiN nanoceramic composites by high-energy mechanical activation technique

In this paper, phase development, dielectric and piezoelectric properties of nanocomposites consisting of perovskite structured PMN-PZT and tungsten bronze structured PBBiN synthesized via high energy mechanical activation technique were examined as a function of x in (1 ? x)(0.5PMN-0.5PZT)-xPBBiN with a stoichiometric formula as (1 ? x)[0.5Pb(Mg_0.33Nb_0.67)O₃-0.5Pb(Zr_0.53Ti_0.47)O₃]-x[Pb_0.59Ba_0.38Bi_0.02Nb₂O₆]. It was observed that the high-energy mechanical activation technique has greatly improved the reactivity of the precursors by reducing the phase formation temperatures and eliminating unwanted secondary phases and liquid phase sintering as x increased. Powder X-ray diffraction studies of the ternary system revealed the perovskite cubic (PMN-PZT) coexisted with tungsten bronze orthorhombic (PBBiN) phase. The average particle size ranged from 22 to 81 nm. A combination of both perovskite and tungsten bronze grains revealed intragranular and intergranular growth which accelerated densification and homogeneity in the nanocomposite. The dielectric (ε_RT = 2,248) and piezoelectric properties (d ₃₃ = 412 pC/N and k _p = 0.446) obtained were maximum at x = 0.4 which could be suitable for possible electromechanical and energy harvesting applications.     

Influence of Ag-doping in La0.5Sr0.5CoO3 on Its Structural and Magnetic Properties

The structural and magnetic properties of the mixed valent cobaltites La_0.5Sr_0.5?x Ag _x CoO₃ (0≤x≤0.15) with perovskite structure have been investigated. X-ray diffraction (XRD) analysis using Rietveld refinement shows that all our samples crystallize in the orthorhombic structure with $R\overline{3}c$ space group. Although the unit cell volume remains almost constant, the Co–O bond length and the Co–O–Co bond angle are sensitive to the Ag addition. All our studied samples undergo a paramagnetic-ferromagnetic transition with decreasing temperature. The zero field cooled (ZFC) and field cooled (FC) magnetization curves at 50 mT of La_0.5Sr_0.5CoO₃ (x=0) sample exhibits thermomagnetic irreversibility indicating frustration and competition of both antiferromagnetic and ferromagnetic interactions. It is suggested that Co³⁺ ions are in the intermediate spin state but Co⁴⁺ ions stay in a mixture of intermediate and high spin states. Ag addition leads to dramatic changes in magnetic properties. An interesting result has been obtained for La_0.5Sr_0.4Ag_0.1CoO₃, where the ZFC and FC curves coincide. For La_0.5Sr_0.35Ag_0.15CoO₃ (x=0.15) sample, the observed thermomagnetic irreversibility is much higher than that observed in x=0 sample. Hysteresis loops were recorded for x=0, 0.05, 0.1, and 0.15. The coercitive field is found to increase with Ag content reaching 0.26 T for x=0.15 whereas the remanent magnetization decreases.     

Domain structure of [(Na0.7K0.2Li0.1)0.5Bi0.5]TiO3 ceramics studied by piezoresponse force microscopy

The domain structure of lead-free ceramics [(Na_0.7K_0.2Li_0.1)_0.5Bi_0.5]TiO₃ was studied by piezoresponse force microscopy (PFM) method. The complicated curved domain structure was observed in the ceramics, and there are some nano domains in the sub-microsized domains, which indicate the relaxor nature of the material. The mechanism for the strong relaxation of the material was discussed in the letter. The reversal behavior of the domain was also studied by PFM method. Only part of the domains reversed after the poling process, and domains of the ceramics reversed back from the center of the domains at first.     

90‡ domain reversal in Pb(Zr x Ti1−x )O3 ceramics

A simple and direct method has been proposed, which may be used for quantitatively distinguishing the mechanisms of domain reorientation processes in polycrystalline materials. Using this method, the 90 domain reorientation in the Pb(Zr _x Ti_1–x )O₃ ceramic under an electric field was examined through the X-ray diffraction analysis. It was found that polarization switching in the PZT ceramic with a composition near the morphotropic phase boundary, is predominantly controlled by the two successive 90 domain processes rather than only the 180 domain reversal process. Experimental results also indicate that the coercive field of ferroelectric ceramics is related to the cooperative deformation associated with each grain. This cooperative deformation arises from the 90 domain-reversal process.     

Influence of sintering temperature on piezoelectric properties of (K0.5Na0.5)NbO3-LiNbO3 lead-free piezoelectric ceramics

Lead-free piezoelectric ceramics (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ have been synthesized by traditional ceramics process without cold-isostatic pressing. The effect of the content of LiNbO₃ and the sintering temperature on the phase structure, the microstructure and piezoelectric properties of (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics were investigated. The result shows that the phase structure transforms from the orthorhombic phase to tetragonal phase with the increase of the content of LiNbO₃, and the orthorhombic and tetragonal phase co-exist in (K_0.5Na_0.5)NbO₃-LiNbO₃ ceramics when the content of LiNbO₃ is about 0.06 mol. The sintering temperature of (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ decreases with the increase of the content of LiNbO₃. The optimum composition for (1 − x)(K_0.5Na_0.5)NbO₃-xLiNbO₃ ceramics is 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃. The optimum sintering temperature of 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃ ceramics is 1080 °C. Piezoelectric properties of 0.94 (K_0.5Na_0.5)NbO₃-0.06LiNbO₃ ceramics under the optimum sintering temperature are piezoelectric constant d₃₃ of 215 pC/N, planar electromechanical coupling factor k_p of 0.41, thickness electromechanical coupling factor k_t of 0.48, the mechanical quality factor Q_m of 80, the dielectric constant of 530 and the Curie temperature T_c = 450 °C, respectively. The results indicate that 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃ piezoelectric ceramics is a promising candidate for lead-free piezoelectric ceramics.     

Structural and dielectric properties of La substituted polycrystalline Ca(Ti0.5Fe0.5)O3

Polycrystalline Ca_1?x La _x (Ti_0.5Fe_0.5)O₃ were prepared by the standard solid state reaction technique. Pellet shaped samples prepared from each composition were sintered at 1573 K for 5 h. The X-ray diffraction analysis indicated the formation of a single-phase orthorhombic structure. The lattice parameters as well as densities increased but the average grain sizes decreased with the increase of La content. The dielectric measurements were carried out at room temperature as a function of frequency and composition. The experimental results revealed that dielectric constant (?′) decreased but dielectric loss (tan δ) and ac electrical conductivity (σ _ac ) increased as frequency increased. The composition dependence of ?′ and tan δ indicated that they decreased with the increase of La content. The σ _ac was derived from the dielectric measurements and it is concluded that the conduction in the present samples is due to mixed polarons hopping.     

The effect of a constant electric field on the structural parameters of ferroelectric PbSc<Subscript>0.5</Subscript>Nb<Subscript>0.5</Subscript>O<Subscript>3</Subscript> crystals

The effect of a constant electric field on the polarization and structural parameters of ferroelectric lead scandium niobate (PbSc_0.5Nb_0.5O₃) crystals was studied using X-ray diffraction. Application of a constant electric field leads to splitting of the Bragg diffraction peaks. It is shown that this behavior is most probably explained by the formation of a new phase.     

Mechanical and electrical switching of local ferroelectric domains of K<Subscript>0.5</Subscript>Bi<Subscript>4.5</Subscript>Ti<Subscript>4</Subscript>O<Subscript>15</Subscript> film

Ferroelectric/Piezoelectric K_0.5Bi_4.5Ti₄O₁₅ (KBT) film was fabricated by pulsed laser deposition method and confirmed by ferroelectric, dielectric measurements and local butterfly-type piezoresponse hysteresis loops. Importantly, ferroelectric domain switching by both electrical field and mechanical force in KBT film was demonstrated. The dark and bright contrast represents the PFM response of the up and down polarized domains, which can be written by a dc bias of ±12 V or a mechanical force of 40–50 nN. The successful demonstration of mechanical force switching of ferroelectric domain in KBT film other than electric field provides a novel mean for information storage and sensors.