Presence Of Polar Nanoregions Research Articles

Enhanced energy-storage performance in BNT-LST-based ceramics via polarization optimization and breakdown strength regulation

Dielectric storage ceramics are widely used in electronic products due to their speed charge-discharge rate. However, the low recoverable energy density, which is attributing to the high remanent polarization and low breakdown electric field, results in an inferior efficiency, which hinders their applications. Here, superior energy storage properties were achieved in the pseudo-cubic 0.5(Bi0.5Na0.5)TiO3–0.5(La0.1Sr0.8)TiO3-δ (BNT-0.5LST) ceramics at 323 kV/cm, and the recoverable energy density, energy efficiency, and breakdown strength in BNT-0.5LST ceramics were severally enhanced 10 times, 5.9 times and 1.3 times than those of BNT-0.1LST ceramics. Slim P-E loops and high polarization (28.13 μC/cm2) originated from the presence of polar nano-regions (PNRs), and the PNRs with weakly couple with each other decreased the remanent polarization. Large breakdown strength attributed to the broad band gap (3.302 eV) and refined grain (1.59 μm). Transmission electron microscopy proved the presence of PNRs in the pseudo-cubic matrix phase. The results have provided a novel route for fabricating the storage ceramics with the outstanding energy storage properties.

Фазовые превращения в ниобате бария-стронция SBN-50 в интервале температур от 80 до 700 K по данным спектроскопии КРС

The dynamics lattice of the Sr0.5Ba0.5Nb2O6 (SBN-50) ceramic was studied using Raman spectroscopy in the temperature range of 80-700 K with a step of 10 K. The analysis of the Raman spectra in the range ν = 50-1000 cm-1 revealed features in the temperature dependence behavior of the optical modes frequencies associated with the displacement of Ba, Sr and Nb atoms from their average positions in the crystal lattice, which is associated with a macroscopic phase transition from the paraelectric to the ferroelectric phase in the vicinity of 390 K. It is shown that anomalies at T~ 565 K on the temperature dependences of the frequency and line half-width corresponding to the oscillations of the NbO6 octahedron can be caused by the presence of polar nanoregions in the paraelectric phase in SBN-50. When the sample was cooled to a temperature of 80 K, the behavior was recorded in the Raman spectra, indicating the occurrence of a structural phase transition in the ferroelectric phase in the vicinity of 190 K. The reasons for the revealed patterns are discussed.

Phase transformations in barium-strontium niobate SBN-50 in the temperature range from 80 K to 700 K according to Raman spectroscopy data

The lattice dynamics of the Sr0.5Ba0.5Nb2O6 (SBN-50) ceramic was studied using Raman spectroscopy in the temperature range of 80-700 K with a step of 10 K. The analysis of the Raman spectra in the range ν=50-1000 cm-1 revealed features in the temperature dependence behavior of the optical modes frequencies associated with the displacement of Ba, Sr and Nb atoms from their average positions in the crystal lattice, which is associated with a macroscopic phase transition from the paraelectric to the ferroelectric phase in the vicinity of 390 K. It is shown that anomalies at T~565 K on the temperature dependences of the frequency and line half-width corresponding to the oscillations of the NbO6 octahedron can be caused by the presence of polar nanoregions in the paraelectric phase in SBN-50. When the sample was cooled to a temperature of 80 K, the behavior was recorded in the Raman spectra, indicating the occurrence of a structural phase transition in the ferroelectric phase in the vicinity of 190 K. The reasons for the revealed patterns are discussed. Keywords: ferroelectric, Raman scattering, SBN, tetragonal tungsten bronze.

Measurement of the Quadratic Electro-Optic Coefficient of KTN Crystal with an Electro-Optic Modulation System in the Presence of Polar Nano-Regions

An electro-optic modulation system was adopted for measuring the quadratic electro-optic coefficient of KTN crystal. Theoretical analysis and experimental results verified the feasibility of this method. The quadratic electro-optic coefficient of a KTN crystal chip, which has a Curie temperature of 0 °C, was measured using this system in the temperature range of 2 °C to 18 °C (Tc = 0 °C). The influences of temperature, AC voltage and frequency on the quadratic electro-optic coefficient were discussed. It was found that the relaxation effect of PNRs (polar nano-regions) played an important role in the determination of the quadratic electro-optic coefficient of KTN crystal.

Phase formation and relaxor properties of lead-free perovskite ceramics on the base of sodium-bismuth titanate (Na0.5Bi0.5)(Ti,Mg)O3

Effects of cation substitutions on crystal structure parameters, microstructure, dielectric and ferroelectric properties of ceramic solid solutions (Na0.5Bi0.5)(Ti1-xMgx)O3 with x = 0 ÷ 0.1 were studied. Phase transitions near 400 K demonstrate a pronounced relaxor behavior typical for (Na0.5Bi0.5)TiO3 (NBT) based compositions conditioned by presence of polar nanoregions in nonpolar matrix. Changes in the perovskite structure parameters, microstructure, and functional properties of Mg2+ modified NBT ceramics confirmed the influence of the acceptor dopant amount on their properties. Increase in total conductivity at high temperatures and effects of dielectric relaxation were observed with increasing amount of magnesium cations indicating enhancement of ionic conductivity.

The piezo-optical effect in Na0.5Bi0.5TiO3 crystals

The work reports some aspects of the effect of applied axial pressure on birefringence in Na0.5Bi0.5TiO3 crystals, which have peculiarities of the macroscopic properties in a large extent due to the coexistence of nanoregions of different phases in a wide temperature range. The birefringence ∆n is investigated by the polarization-optical method in temperature range 25–500 °C. The temperature behavior of spontaneous birefringence in single domain crystals is associated with the structure phase transitions and the “isotropization point” and depend on the way used to switch the sample into the single domain state. Also, the influence of axial pressure on ∆n of samples preliminary switched to the single domain state is studied. It is discovered that application of the axial pressure (6.25–16.6 bar) along [100] direction does not show significant changes of birefringence in temperature range 25–500 °C. While the application of axial pressure (2.5–25 bar) along [110] direction leads to exhibition of significant piezo-optical effect. It is supposed that both the presence of polar nanoregions and changes of their orientation make contribution to the piezo-optical effect.

Origin of the large electrostrain in BiFeO3-BaTiO3 based lead-free ceramics

The large electrostrain (1 − x)BiFeO3-0.3BaTiO3-xNd(Li0.5Nb0.5)O3 ceramics is explained by the presence of polar nano-regions which distort in the direction of the applied field within multiple local symmetries.

Optimum discharge energy density at room temperature in relaxor K1/2Bi1/2TiO3 for green energy harvesting

Lead-free polycrystalline K1/2Bi1/2TiO3 was prepared by the solid state reaction method. Experimentally observed frequencies of Raman modes signified its tetragonal phase, and matched reasonably well with theoretically calculated values. The relaxor nature of this material was observed in the temperature-dependent real part of the permittivity and dielectric loss curve. The value of the degree of diffuseness (1.99) was estimated from the modified Curie–Weiss law confirmed its relaxor behavior. The validation of this behavior was justified by the Vogel–Fülcher relation. The shoulder in the imaginary part of the modulus (M″) and permittivity (ε″) spectra revealed the presence of polar nano regions (PNRs). The evidence of PNRs was detectable above freezing temperatures, and became weaker when the temperature exceeded Tm (temperature at the maximum of the dielectric constant). The electric field-induced polarization and strain curve showed the stabilization of the long-range ferroelectric order of the specimen at room temperature. Moreover, the discharge energy density and strain were 0.46 J cm−3 and 0.12%, respectively, at the maximum application of the electric field of 115 kV cm−1 at room temperature.

Electric field dependences of Curie and Néel phase transition temperatures in magnetoelectric relaxor multiferroic Pb(Fe0.5 Ta0.5 )O3 crystals seen via acoustic emission

Pb(Fe0.5Ta0.5)O3 magnetic and ferroelectric relaxor multiferroic crystals have been studied under both temperature and dc external electric field conditions by means of acoustic emission technique. All the characteristic relaxor points as Burns Td ≈ 610 K and intermediate T* ≈ 500 K temperatures, as well as both Curie ferroelectric cubic-tetragonal Tc1 ≈ 259 K and tetragonal–monoclinic Tc2 ≈ 201 K structural and Neel antiferromagnetic–paramagnetic TN ≈ 187 K phase transitions have been successfully detected. It is shown that, when an external dc electric field is applied, both Tc1 and Tc2 exhibit a nonlinear or V-shape behavior and attain the sharp minima at threshold temperatures Tth1 ≈ 247 K and Tth2 ≈ 200 K with the threshold field Eth ≈ 0.1 kV cm−1, as the field enhances, while their acoustic emission count rate monotonically increases. It is shown, too, that the TN behaves linearly in dependence on dc electric field with a negative slope of about −20 K cm kV−1, while its acoustic emission count rate monotonically decreases. All these data are discussed from a viewpoint of presence of polar nanoregions and magnetoelectric coupling effect in Pb(Fe0.5Ta0.5)O3 crystals.

Temperature-dependent Raman scattering study of tin modified PbZrO3 and PbHfO3 single crystals

Using Raman scattering spectroscopy, the sequence of phase transitions in antiferroelectric PbZrO3 and PbHfO3 single crystals modified with Sn ions was investigated. As a consequence of the cation replacement (partial substitutions of Zr4+ or Hf4+ with isovalent Sn4+) the changes that took place in the different phase transition temperatures were reported. The phase transition temperatures for tin content of x=0.04 and 0.22 in PbZrO3 and x=0.025 in PbHfO3 were studied in the temperature range 300–600K. The modifications of the Raman spectra were observed in details in the vicinity of two structural phase transitions occurring in the crystals: antiferroelectric (AFE1)–antiferroelectric (AFE2) phase transition and antiferroelectric (AFE2)–paraelectric (PE) phase transition. The partial replacement of Zr or Hf ions with Sn ones does not change the nature of the phase transitions but widens the temperature range of intermediate phase and shifts AFE2–PE phase transition toward lower temperatures. It was shown that the presence of polar nanoregions (PNRs) in the high-temperature phase is the common feature of all three samples. The Raman measurements were supplemented by dielectric and differential scanning calorimetry (DSC) measurements. Excellent agreement for transition temperatures obtained by means of these three techniques suggests that Sn ions placed in octahedra are uniformly distributed in the material without introducing any mechanical stress.

Phase coexistence and transformations in field-cooled ternary piezoelectric single crystals near the morphotropic phase boundary

Structural phase transformations in (100)-oriented Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals have been investigated by X-ray diffraction. A cubic (C) → tetragonal (T) → monoclinic-C (MC) transformation sequence was observed in the field-cooled condition. Two phase coexistence regions of C + T and T + MC were found. In addition to an increase in the C → T phase transition temperature and a decrease of the T → MC one, a broadening of the coexistence regions was also found with increasing field. This broadening can be explained by the presence of polar nano regions within the C, T, and MC phase regions.

Ferroelectric Phase Transitions Viewed Via Nonlinear Dielectric Response

Experimental data on the nonlinear dynamic responses of barium titanate (discontinuous ferroelectric phase transition, PT), triglycine sulphate (continuous PT), as well as the prototype relaxor lead magnoniobate, and the weakly doped quantum paraelectric K0.995Li0.005TaO3 are presented. The classic ferroelectrics display all features of the third order dielectric susceptibility, χ 3 , as predicted by mean-field theory (including the change of sign of χ 3 at the continuous PT). On the other hand, the positive sign of χ 3 measured on the relaxor PMN is due to the presence of polar nanoregions and their specific contribution to the dielectric response measured at enhanced electric field.

High-temperature macroscopic piezoelectricity in Nb-dopedPbZr1−xTixO3ceramics driven by the existence of polar regions

The dielectric, elastic, and piezoelectric properties of a niobium-doped $\mathrm{Pb}{\mathrm{Zr}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{3}$ ceramic with a composition located near the morphotropic phase boundary have been studied from $25\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. A strong softening of the elastic coefficients was observed far below the temperature corresponding to the dielectric permittivity maximum, ${T}_{\ensuremath{\epsilon}\mathrm{max}}=366\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. The steep decrease of the stiffness coefficient ${c}_{44}$ from $400\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}350\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ is probably a stepwise anomaly which is expected at an improper ferroelastic transition. In addition, it was found that all macroscopic piezoelectric activity had disappeared well above ${T}_{\ensuremath{\epsilon}\mathrm{max}}$. This is explained by the combined effects of compositional fluctuations in the $\mathrm{Zr}∕\mathrm{Ti}$ sublattice and the presence of polar nanoregions. The elastic properties have an anomaly at the temperature at which the lattice becomes predominantly centrosymmetric. The piezoelectric activity vanishes when the polar nanoregions reach a critical size below which they can no longer interact coherently through the nonpolar matrix to induce a macroscopic piezoelectric response.

Piezoelectric activity and phase transitions in (PMN)0.69(PT)0.31 single crystal

Piezoelectric and dielectric investigations have been performed on a (PMN)0.69(PT)0.31 single crystal. Low frequency (100 Hz) dielectric permittivity measurements revealed distinct anomaly at 129°C (T εmax) corresponding to the structural transformation from the tetragonal to cubic phase. Two other anomalies have been detected at 90 and 96°C. After preliminary polarization in the d.c. electric field, switched on above T εmax and switched off inside the tetragonal phase, the piezoelectric activity has been observed in function of temperature. Values of the piezoelectric resonance frequencies changed markedly at 96°C (on cooling) and 124°C (on heating) showing clear softening of the elastic properties near these temperatures. Values of the piezoelectric and electromechanical coupling coefficients obtained were respectively of the order of 800 pCN−1 (d 31) and 0.35 (k 31). Piezoelectric activity was detected tens of degrees above the temperature T εmax and disappeared at temperature at which the dispersion of the dielectric permittivity due to the presence of polar nanoregions is negligible. It was found that strong softening of the elastic properties accompanies phase transitions to the tetragonal and monoclinic phase.