Ferroelectric Phase Transition Temperature Tc Research Articles

Broadband and infrared spectroscopy of Ag0.98Li0.02NbO3 ceramics

The dielectric properties of Ag0.98Li0.02NbO3 (ALN2) ceramics were investigated in a broad frequency range (20 Hz – 60 THz). The dielectric spectra of ALN2 ceramics are mainly impacted by electrical conductivity at higher temperatures (above 400 K) and low frequencies (below 100 Hz), ferroelectric domains below ferroelectric phase transition temperature Tc = 330 K and at low frequencies (below 1 MHz), and contribution of the soft ferroelectric mode, the frequency of which is below 50 cm–1. All phononic modes are slightly temperature dependent, thus confirming the influence of Ag, O and Li ions dynamics on the phase transitions. However, the most important contribution to the dynamics of phase transition is made by Nb ions. Ceramics exhibits a huge value of dielectric permittivity and relatively low losses in a microwave frequency range (ε΄ ≈ 250 and ε˝ ≈ 20 at 10 GHz and room temperature), indicating that it is suitable for various microwave dielectric applications.

Effect of bismuth substitution on piezoelectric coefficients and temperature and pressure-dependent dielectric and impedance properties of lead zirconate titanate ceramics

Basic Physics as well as devices fabrication required in-depth analysis of the effect of temperature, pressure, and other physical parameters on dielectric properties to understand the microstructure-property relation. In this regard, we have thoroughly investigated the temperature and pressure dependent ferroelectric, dielectric, and impedance properties of Bi-substituted Pb1-xBi2x/3(Zr0.52Ti0.48)O3 (PBiZT) (x = 0.10 to 0.40) ceramics. The maximum solubility of Bi was found near x = 0.15, and above this composition, it shows a composite phase of PBiZT and ZrO2. The remanent polarization (Pr) increases with decrease in the Bi concentration with the maximum value obtained for x = 0.15, i.e., 40 μC/cm2. Temperature-dependent dielectric constant and tangent loss were measured as function of composition to check the ferroelectric phase transition temperature (Tc) and the operating temperature limit for the material to make a device. The Tc value increases with decrease in bismuth (Bi) concentration which obtained in the range of 350 ℃ - 375 ℃. The tangent loss was observed to be near about 0.01 to 0.03 below 200 ℃ which suggests its suitability for device applications. The temperature-dependent imaginary part of impedance (Z'') was measured at various temperatures with varying frequency in the range of 100 Hz – 1 MHz. The peaks positions of the Z'' vs. frequency shift towards lower temperature side with increasing the value of x from 0.1 to 0.4 near and above Tc. The pressure-dependent dielectric constant (εr) and capacitive reactance (Xc) were also measured for varying x, and it was observed that maximum change in εr and Xc was observed for x = 0.15 i.e. 13% and 10.5%, respectively. The pressure sensitivity was obtained maximum for the composition near the maximum solubility of Bi in PbZr1-xTixO3 (PZT) matrix. The reason behind getting the highest remanent polarization and pressure sensitivity for x = 0.15 is that of the highest piezoelectric coefficients ∼ 130 pC/N.

Ferroelectric and dielectric properties of improper ferroelectric Ca8−xSrx[Al12O24 ](MoO4)2 multilayer capacitors

Improper ferroelectric Ca8−xSrx[Al12O24](MoO4)2 (x = 0–2) multilayer capacitors (MLCs) were fabricated to study their dielectric and ferroelectric properties for their potential practical applications and reversible switching of electric polarization by an electric field. The substitution of Sr for Ca caused a structural change from polar orthorhombic to non-polar cubic phases at 297 K and decreased the ferroelectric phase transition temperature (Tc). Spontaneous polarization up to ∼0.6 μC/cm2 was performed at 297 K in Ca7.5Sr0.5[Al12O24](MoO4)2 and Ca7Sr[Al12O24](MoO4)2 MLCs by poling with an electric field of 10 MV/m. Furthermore, ferroelectric hysteresis loops from the electric polarization switching were successfully demonstrated in Ca8[Al12O24](MoO4)2 MLCs above 469 K by the application of a high electric field over 50 MV/m, and remnant electric polarization of ∼0.59 μC/cm2 was obtained at 573 K. The results introduce a new avenue for possible applications and new functionalities of improper ferroelectrics with an MLC structure.

Dielectric Properties in Transition Metal and Rare‐Earth‐Doped Multiferroic BaTiO3 Nanoparticles

The temperature, doping concentration, size, and magnetic field dependences of the real part of the dielectric function ϵ′ for Fe and Er ion‐doped BaTiO3bulk and nanoparticles—are calculated using microscopic models and the Green's function technique. The Fe ions substitute the Ti ions and cause a tensile strain. The maximum value of ϵ′ and the ferroelectric phase transition temperature TC decrease with increasing the Fe doping concentration. Moreover, ϵ′ decreases with decreasing particle size and increases with increasing an external magnetic field. The latter is an evidence for a strong magnetodielectric effect. By substituting the Ba with Er ions, there appears a strong compressive strain. It causes an increase in the maximum of ϵ′ and TC with increasing the Er ion concentration.

Antiferroelectricity and weak ferromagnetism in rare earth doped multiferroic BiFeO3

The remanent polarization Pr and remanent magnetization Mr for different Sm-concentrations x in Sm-doped BiFeO3 is investigated using a microscopic model and the Green's function technique. For pure BiFeO3 with a rhombohedral symmetry we obtain a hysteresis loop P(E) which is typical for a ferroelectric material. Further increase of x (x = 0.16) leads to a double hysteresis loop, which is typical for an antiferroelectric substance. Sm substitution induces a weak ferromagnetic moment at room-temperature. Pr and Mr versus x show a maximum at x = 0.05. The ferroelectric phase transition temperature TC decreases strongly whereas the magnetic phase transition temperature TN increases slightly with increasing the Sm concentration x.

Properties and structure of magnetic alkali-borosilicate glasses prepared by induction melting

In this contribution we present the first results devoted to studies of structure, morphology and magnetic properties of magnetic alkali-borosilicate glasses prepared by induction melting. This procedure permits to fabricate a large amount of two-phase glasses with magnetic properties due to self-assembled magnetite (Fe3O4) nanoparticles and with branched dendrite-like structure of channels formed by a chemical unstable phase. After withdrawal of this phase we have obtained the porous matrices with two different average pore diameters: ∼6 nm and ∼60 nm. The empty pore spaces in these matrices were filled by ferroelectrics KH2PO4 from water solution. It is shown that application of external magnetic fields above B=1 T results in a linear growth of the ferroelectric phase transition temperature TC on 6 K at B=10 T due to magnetostrictive properties of porous matrices.

Anomalous reduced sound velocity of multiferroic TbMn2O5

The anomalous reduced sound velocity of multiferroic TbMn2O5 (TMO) has been studied using Green's function technique. To achieve this aim, the anharmonic phonon-phonon interaction and the spin-phonon interaction were used. It was shown that the reduced velocity of sound of TMO exhibits a kink at the ferroelectric phase transition temperature TC. This can be explained as an effect of vanishing ferroelectric ordering above TC. It was found that the reduced sound velocity increases with increasing V(3) (the third-order atomic force constants of the anharmonic phonons) in the interval T < TC, whereas the reduced sound velocity remains unchanged in the interval TC < T < TN. It was also found that the reduced sound velocity increases with the increase of V(4) (the fourth-order atomic force constants of the anharmonic phonons) in the interval T < TN. In addition, the ferroelectric phase transition temperature TC decreases when V(4) increases in the interval T < TN. Those theoretical results are in agreement with the experimental data.

Tuning a sign of magnetoelectric coupling in paramagnetic NH2(CH3)2Al1\u2212xCrx(SO4)2\u2009\xd7\u20096H2O crystals by metal ion substitution

Hybrid organometallic systems offer a wide range of functionalities, including magnetoelectric (ME) interactions. However, the ability to design on-demand ME coupling remains challenging despite a variety of host-guest configurations and ME phases coexistence possibilities. Here, we report the effect of metal-ion substitution on the magnetic and electric properties in the paramagnetic ferroelectric NH2(CH3)2Al1−xCrx(SO4)2 × 6H2O. Doing so we are able to induce and even tune a sign of the ME interactions, in the paramagnetic ferroelectric (FE) state. Both studied samples with x = 0.065 and x = 0.2 become paramagnetic, contrary to the initial diamagnetic compound. Due to the isomorphous substitution with Cr the ferroelectric phase transition temperature (Tc) increases nonlinearly, with the shift being larger for the 6.5% of Cr. A magnetic field applied along the polar c axis increases ferroelectricity for the x = 0.065 sample and shifts Tc to higher values, while inverse effects are observed for x = 0.2. The ME coupling coefficient αME = 1.7 ns/m found for a crystal with Cr content of x = 0.2 is among the highest reported up to now. The observed sign change of αME with a small change in Cr content paves the way for ME coupling engineering.

Ferroelectric Gd-doped ZnO nanostructures: Enhanced dielectric, ferroelectric and piezoelectric properties

Gadolinium doped ZnO nanorods were grown by wet chemical method. Line broadening of each diffraction peak was studied to evaluate the crystallite size, lattice strain, stress and energy density. Crystallite size was estimated by Scherrer and Williamson-Hall methods. The size was found to be in good agreement with the results of transmission electron microscopy. As a result of Gadolinium doping, high remnant polarization (Pr = 0.29 μC/cm2) and coercive field (Ec = 16.41 kV/cm) were observed. The decrease in leakage current due to Gadolinium doping makes ZnO a remarkable candidate for ferroelectric capacitor. High ferroelectric phase transition temperature (Tc = 215 °C) and large piezoelectric coefficient (d33 = 45.49 pm/V) were observed which makes Gadolinium doped ZnO sample useful for high temperature non-volatile ferroelectric memory applications.

Crossover between ferroelectric order and dipolar glass disorder in betaine phosphate0.06betaine phosphite0.94mixed crystals

The dielectric properties of mixed betaine phosphate0.06betaine phosphite0.94(BP0.06BPI0.94) ferroelectric crystals were investigated in a wide frequency range from 20 Hz to 3 GHz. Although the dielectric anomaly is clearly observed close to the ferroelectric phase transition temperatureTc= 121 K, the dielectric dispersion is very broad around and below this temperature. Temperature dependences of the dielectric strength, the mean relaxation frequency and the spontaneous polarization cannot be described together according to the Landau theory for ferroelectric phase transition with a single order parameter or according to the quasione-dimensional Ising model. The polarization shows a pronounced distribution in the ferroelectric phase and only a partial ordering of hydrogen atoms occurs down to the lowest temperatures. The unordered hydrogen atoms form a glassy phase and its formation is observed in dielectric spectra at very low temperatures. The proton freezing temperature was estimated.

Amplitude dependences of dielectric permittivity and dielectric losses in a mixed K0.88(NH4)0.12H2PO4 crystal

Amplitude dependences of dielectric permittivity ɛ and dielectric losses tanδ in K0.88(NH4)0.12H2PO4 mixed crystal near the ferroelectric phase transition temperature TC are investigated. At measurement fields E∼ exceeding some critical value, dependence tanδ(E∼) can be described within a model that includes the depinning of domain walls from point lattice defects. The activation field is found to grow appreciably when approaching TC from below, due to the presence of nonpolar regions.

Ferroelectric Phase Transition of Lead Free (1-x)(Na0.5K0.5)NbO3-xLiNbO3Ceramics

Lead-free (1-x)(Na0.5K0.5)NbO3-xLiNbO3, i.e., NKN-LNx (x =0.0, 0.1, 0.2, 0.3, 0.4 mol) was prepared using the conventional solid state reaction method. The effects of LN mixing on the ferroelectric properties of NKN-LNx ceramics were studied using a dielectric constant and P-E (Polarization-electric field) measurements. Ferroelectricity was observed in the composition for x approximately varying between 0.0 and 0.4. Minimum remanent polarization 2Pr=5 C/cm 2 was achieved in the composition for x = 0.2. The ferroelectric phase transition temperature TC increased with increasing LN content. The ferroelectric phase transition of NKN-LNx (x ≥ 0.1) is a second-order phase transition, and that of NKN-LNx (x ≤ 0.2) is a first-order phase transition. These results indicate that the ferroelectric phase transition temperature of NKN-LNx change from that of second-order to weak first-order phase transition according to the LN content.

Synthesis and Dielectric Properties of Mn-Doped BaTi2O5 Ceramics

High-density ceramics of BaTi2O5 have been fabricated by a conventional sintering method using both sol–gel-derived BaTi2O5 powders and MnO2 additives of 0.2–0.8 wt %. The effects of sintering conditions on the densification, microstructural evolution and dielectric properties are investigated. As the effect of Mn addition, the BaTi2O5 phase becomes stable at least up to 1250 °C, and a significant densification is achieved at temperatures as low as 1200–1250 °C. The dielectric constant ε' vs temperature T curve of the MnO2-added ceramics exhibits a broad maximum ε'max at the ferroelectric phase transition temperature TC, which is 140 °C lower than that of the nondoped ceramics. Among the ceramics with different Mn contents, the 0.2 wt % MnO2-added ceramics have the largest ε'max of 470 at 328 °C and the smallest tan δ of <0.05 at a high temperature of around 520 °C at 1 MHz. We observed a ferroelectric D–E hysteresis loop for the first time in the polycrystalline form of BaTi2O5.

Synthesis and Dielectric Properties of Mn-Doped BaTi2O5Ceramics

High-density ceramics of BaTi2O5 have been fabricated by a conventional sintering method using both sol–gel-derived BaTi2O5 powders and MnO2 additives of 0.2–0.8 wt %. The effects of sintering conditions on the densification, microstructural evolution and dielectric properties are investigated. As the effect of Mn addition, the BaTi2O5 phase becomes stable at least up to 1250 °C, and a significant densification is achieved at temperatures as low as 1200–1250 °C. The dielectric constant ε' vs temperature T curve of the MnO2-added ceramics exhibits a broad maximum ε'max at the ferroelectric phase transition temperature TC, which is 140 °C lower than that of the nondoped ceramics. Among the ceramics with different Mn contents, the 0.2 wt % MnO2-added ceramics have the largest ε'max of 470 at 328 °C and the smallest tan δ of <0.05 at a high temperature of around 520 °C at 1 MHz. We observed a ferroelectric D–E hysteresis loop for the first time in the polycrystalline form of BaTi2O5.

Dielectric and Ferroelectric Studies on Sol-Gel Derived Calcium Modified Lead Zirconate Titanate Ceramics

Bulk samples of Pb(1-x)CaxZr0.52Ti0.48O3 (x = 0,0.02,0.04,0.06,0.08,0.1) have been synthesized by sol-gel process using lead acetate trihydrate, calcium acetate hydrate, zirconium propoxide and titanium propoxide as the starting materials. The dependence of ferroelectric phase transition temperature Tc as a function of calcium substitution has been investigated. A linear decrease of Tc is observed with increasing calcium concentration. The variation of room temperature dielectric constant and that of maximum value of dielectric constant corresponding to the phase transition temperature are explained by the microstructural studies. XRD patterns of the samples show that all the compositions sintered at 1200°C are having single phase with perovskite structure. P-E hysteresis loop measurements showed that calcium modified PZT ceramics have higher Pr values than that of pure PZT composition prepared by sol-gel method. The composition with 4 mol% calcium has the highest value of Pr, 25.2 μC/cm2 and lowest Ec, 9.6 kV/cm.

Relaxor-like behavior of BaTiO3 crystals from acoustic emission study

BaTiO 3 crystals were studied by means of the acoustic emission during thermal cycling through 300–700 K range. In addition to pronounced acoustic emission at the ferroelectric phase transition temperature Tc≈400 K, also nucleation of nanoclusters was detected at somewhat smeared Burns temperature Td≈530–570 K and their local freezing at T∗≈506 K. Bias electric field shifts the T∗ and Tc linearly up, T∗ more steeply than Tc. Except for its much faster cluster dynamics than that of classical relaxor materials, BaTiO3 shows many relaxor-like features in its paraelectric phase.

Ferroelectricity in UltrathinBaTiO3Films: Probing the Size Effect by Ultraviolet Raman Spectroscopy

We demonstrate the dramatic effect of film thickness on the ferroelectric phase transition temperature Tc in strained BaTiO3 films grown on SrTiO3 substrates. Using variable-temperature ultraviolet Raman spectroscopy enables measuring Tc in films as thin as 1.6 nm, and a film thickness variation from 1.6 to 10 nm leads to Tc tuning from 70 to about 925 K. Raman data are consistent with synchrotron x-ray scattering results, which indicate the presence of 180 degrees domains below Tc, and thermodynamic phase-field model calculations of Tc as a function of thickness.

Ferroelectricity in chemical nanostructures: proximal probe characterization and the surface chemical environment

Renewed interest in the evolution of the ferroelectric phase transition temperature TC and the character of ordering of ferroelectric polarizations with finite size and shape is driven in part by several recent developments. An expanding array of pathways for producing nano-structured ferroelectric oxides with control of size, shape, and composition has emerged. Experimental characterization methods originally developed for thin films have been extended to ensemble-free investigations of functional properties of individual nanostructures. Progress in understanding the origin and nature of ferroelectric stability in ultra-thin films and nanostructures is reviewed. Specifically, we discuss evidence for a new surface adsorbate-driven mechanism for stabilizing ferroelectricity in nanostructures owing to a combination of recent proximal probe analysis and model calculation results, along with a new experimental paradigm for investigating and exploiting these effects and effects of finite curvature.

Ferroelectric phase transitions in three-component short-period superlattices studied by ultraviolet Raman spectroscopy

Vibrational spectra of three-component BaTiO3/SrTiO3/CaTiO3 short-period superlattices grown by pulsed laser deposition with atomic-layer control have been investigated by ultraviolet Raman spectroscopy. Monitoring the intensity of the first-order phonon peaks in Raman spectra as a function of temperature allowed the determination of the ferroelectric phase transition temperature Tc. Raman spectra indicate that all superlattices remain in the tetragonal ferroelectric phase with out-of-plane polarization in the entire temperature range below Tc. The dependence of Tc on the relative thicknesses of ferroelectric (BaTiO3) to nonferroelectric materials (SrTiO3 and CaTiO3) has been studied. The highest Tc was found in superlattices having the largest relative amount of BaTiO3, provided that the superlattice maintains its coherency with the substrate. Strain relaxation leads to a significant decrease in the ferroelectric phase transition temperature.

Investigation of the Vibrational Spectra of a SbSI (Sb2S3)0.15 Crystals in Harmonic and Anharmonic Approximations

The vibrational spectra of SbSI (Sb2S3)0.15 crystals in harmonic and anharmonic approximations are calculated by Density Functional Theory (DFT) method. Investigation of IR, R and A low frequency normal modes at points k = 0 and k = 0.5 of the Brillouin zone in SbSI (Sb2S3)0.15 crystals reveal that soft behavior of these modes mainly depends on the temperature dependent force constant Cij between Sb and I atoms and symmetry coordinates. Investigation of the mean value potential energy p(z) per one atom dependence on the amplitude of the soft mode B1u symmetry coordinates in c (z) axis direction on the phase transition region have explained why ferroelectric phase transition temperature TC depends on compression in x-y plane of the SbSI chains in SbSI (Sb2S3)x crystals.