Ferroelectric-paraelectric Transition Research Articles

New insights into BaTi1–x Sn x O3 (0 ≤ x ≤ 0.20) phase diagram from neutron diffraction data

Neutron powder diffraction (NPD) was employed to further investigate the BaTi1−x Sn x O3 (BTS) system previously studied by X-ray diffraction. The room-temperature phase compositions and crystal structures of BTS samples with x = 0, 0.025, 0.05, 0.07, 0.10, 0.12, 0.15 and 0.20 were refined by the Rietveld method using NPD data. It is well known that barium titanate powder (x = 0) crystallizes in the tetragonal P4mm space group. The crystal structures of the samples with 0.025 ≤ x ≤ 0.07 were refined as mixtures of P4mm and Amm2 phases; those with x = 0.1 and 0.12 show the coexistence of rhombohedral R3m and cubic phases, while the samples with x = 0.15 and 0.20 crystallize in a single cubic Pm{\overline 3}m phase. Temperature-dependent NPD was used to characterize the BaTi0.95Sn0.05O3 sample at 273, 333 and 373 K, and it was found to form single-phase Amm2, P4mm and Pm{\overline 3}m structures at these respective temperatures. The NPD results are in agreement with data obtained by differential scanning calorimetry and dielectric permittivity measurements, which show a paraelectric–ferroelectric transition (associated with structural transition) from Pm{\overline 3}m to P4mm at about 353 K followed by a P4mm to Amm2 phase transition at about 303 K.

Structural transformations in Pb1 − xBi4 + xTi4 − xMnxO15(x = 0.2 and 0.4): a Raman scattering study

The temperature evolution of the Raman scattering of Pb1 − xBi4 + xTi4 − xMnxO15 (x = 0.2 and 0.4) on cooling from 850 to 140–130 K was studied in order to elucidate the effect of B-site Mn3+ doping accompanied by the increase in the A-site Bi3+ content on the structural transformations in four-layer Aurivillius-type compound (An − 1Bi2BnO3n + 3, n = 4). The ferroelectric–paraelectric transition (Tc ~800 K for x = 0.2 and ~765 K for x = 0.4) is well mirrored by the Raman scattering near 60 cm−1 arising from the so-called rigid layer mode. The temperature dependence of the phonon mode near 42 cm−1 arising from A-site Pb/Bi displacements indicates a second structural transformation near 570 K and 400 K for x = 0.2 and 0.4, respectively, similar to that observed for Mn-free PbBi4Ti4O15. This structural alteration resembles a thermodynamically second-order phase transition for all three compounds and the critical temperature (Ta) decreases with the increase in the A-site Bi3+ amount, related to the heterovalent substitution (Mn3+ for Ti4+) on the B site; Ta = 600, 570, 400 K for x = 0, 0.2, 0.4, respectively. The BO6 tilting and bending mode near 220 cm−1 also shows an anomaly at Ta, and, thus, this second structural transformation was attributed to subtle rearrangements of A-site cations accompanied by octahedral BO6 tilting in the perovskite-like blocks. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.

Strain‐Induced Structural Phase Transition and its Effect on Piezoelectric Properties of (BZT‐BCT)‐(CeO2) Ceramics

A strain‐induced structural phase transition and its subsequent effect on the piezoelectric properties of BZT‐BCT–x CeO2 (x = 0–0.1 wt%) ceramics have been reported in this manuscript. BZT‐BCT–x wt% CeO2 lead‐free piezoceramics were successfully been prepared using sol–gel method. X‐ray diffraction spectra of BZT‐BCT–x wt% CeO2 (x = 0–0.10 wt%) ceramics showed a purely single‐phase perovskite structure and a change in crystal structure between tetragonal and rhombohedral phases with increasing CeO2 content. The strain‐induced structural phase transition has resulted in excellent piezoelectric properties, d33 = 673 pC/N, kp ~ 56%, and Tc = 110°C for BZT‐BCT–0.08 wt% CeO2 ceramics. Greatly improved temperature stability of the piezoelectric properties has been observed over a temperature range between 20°C and 90°C. The ferroelectric–paraelectric transition temperature, Tc obtained in this study is 17°C higher than those reported earlier.

Pressure effects on model ferroelectricBiFeO3−PbTiO3: Multiple phase transitions

Pressure-dependent powder x-ray diffraction, Raman spectroscopy, and dielectric constant measurements are performed on 0.$65{\mathrm{BiFeO}}_{3}\text{\ensuremath{-}}0.35{\mathrm{PbTiO}}_{3}$. Four structural transitions are reported in 0--34 GPa range. The transitions are observed at 0.7(1), 0.8(1), 3.4(1), and 6.1(1) GPa, where the system transformed from ambient tetragonal $P4mm$ to a mixed phase of tetragonal and rhombohedral $(P4mm+R3c)$, followed by transition to rhombohedral $R3c$, cubic Pm$3m$ and again to rhombohedral $R3c$ structure, respectively. The transformation to the cubic structure coincides with the ferroelectric-paraelectric transition. Higher-pressure cubic-to-rhombohedral transition shows a slight softening of Raman modes along with an enhancement in spontaneous strain. The high-pressure rhombohedral phase shows signatures of another ferroic transition.

Damping Constant (Linewidth) and the Relaxation Time of the Brillouin LA Mode for the Ferroelectric-Paraelectric Transition in PbZr1-xTixO3.

The damping constant (linewidth) of the longitudinal acoustic (LA) mode is calculated as a function of temperature using the observed Brillouin frequencies of this mode from the literature for the ferroelectric-paraelectric transition ( TC = 657 K) in PbZr1-xTixO3 ( x = 0.45 ). For this calculation of the damping constant, the pseudospin-phonon coupled model and the energy fluctuation model are used by fitting to the observed data for the Brillouin frequencies of the LA mode in the ferroelectric ( ) and paraelectric ( T > TC) phases of this compound ( x = 0.45 ). Values of the activation energy are deduced for both ferroelectric and paraelectric phases. The relaxation time is also obtained by means of fitting to the observed data from the literature for the inverse relaxation time at various temperatures in the paraelectric phase of PbZr1-xTixO3. The temperature dependences of the damping constant and of the relaxation time with the values of the activation energy that we have calculated indicate that the pseudospin-phonon coupled model and the energy fluctuation model are capable of describing the ferroelectric-paraelectric transition ( TC = 657 K) in PbZr1-xTixO3 ( x = 0.45 ) adequately.

Structure, ferroelectric ordering, and semiempirical quantum calculations of lanthanide based metal-organic framework: [Nd(C4H5O6)(C4H4O6)][3H2O

We investigate the structure and ferroelectric behavior of a lanthanide based metal-organic framework (MOF), [Nd(C4H5O6)(C4H4O6)][3H2O]. X-ray crystal structure analyses reveal that it crystallizes in the P41212 space group with Nd centres, coordinated by nine oxygen atoms, forming a distorted capped square antiprismatic geometry. The molecules, bridged by tartrate ligands, form a 2D chiral structure. The 2D sheets are further linked into a 3D porous framework via strong hydrogen-bonding scheme (O-H…O ≈ 2.113 Å). Dielectric studies reveal two anomalies at 295 K and 185 K. The former is a paraelectric-ferroelectric transition, and the later is attributed to the freezing down of the motion of the hydroxyl groups. The phase transition is of second order, and the spontaneous polarization in low temperature phase is attributed to the ordering of protons of hydroxyl groups. The dielectric nonlinearity parameters have been calculated using Landau– Devonshire phenomenological theory. In addition, the most recent semiempirical models, Sparkle/PM7, Sparkle/RM1, and Sparkle/AM1, are tested on the present system to assay the accuracy of semiempirical quantum approaches to predict the geometries of solid MOFs. Our results show that Sparkle/PM7 model is the most accurate to predict the unit cell structure and coordination polyhedron geometry. The semiempirical methods are also used to calculate different ground state molecular properties.

Effect of Na on the structural and electrical properties of lead-free sodium potassium niobate ceramics

K1-xNaxNbO3 (where x = 0.48, 0.53, 0.56 and 0.60) (KNN) perovskite structured ferroelectric ceramics were prepared by the solid-state reaction method. X-ray diffraction patterns indicate that single orthorhombic phase was formed for KNN. Dielectric anomalies around 200°C and 370°C were identified as ferroelectric-ferroelectric and ferroelectric-paraelectric transition temperatures for KNN compound. The electrical behavior of KNN ceramics was studied by the help of impedance spectroscopy technique in the different temperature range. The impedance response in KNN ceramics could be resolved into a single contribution, associated with the bulk/grain. The conductivity studies confirm that the activation energies are strongly frequency dependent. A decreasing trend of Z′ with rise in temperature suggests the presence of negative temperature coefficient of resistance (NTCR) in the material in the low frequency region but tends to merge in the high frequency region at almost all temperatures.

On Mn2+ EPR Probing of the Ferroelectric Transition and Absence of Magnetoelectric Coupling in Dimethylammonium Manganese Formate (CH3)2NH2Mn(HCOO)3, a Metal–Organic Complex with the Pb-Free Perovskite Framework

We employ electron paramagnetic resonance (EPR) of Mn2+ as a spin probe to study the paraelectric–ferroelectric transition in dimethylammonium manganese formate, [(CH3)2NH2]Mn(CHCO2)3, (DMMnF), which is considered a model metal–organic framework (MOF) with a Pb-free perovskite architecture. We study the variation of the Mn2+ EPR line shape and intensity at the X-band (∼9.5 GHz) over 80 to 300 K. The peaks are essentially Lorentzian, implying electron spin exchange at frequencies greater than 9.5 GHz. On cooling, an anomalous increase in the peak width is noted at 185 K but no anomalous change in the normalized, double-integrated EPR signal intensity around the TC, indicating that DMMnF is transparent to microwave electric fields with a clear lack of magnetoelectric coupling, in contrast to an earlier report. Our analysis enables us to estimate change in lattice strain related to the ferroelectric transition, information that is difficult to obtain by other techniques.

BiFeO3 codoping with Ba, La and Ti: Magnetic and structural studies

Conventional solid state reaction method, from oxides and carbonates, was employed to prepare bismuth (Bi)-based multiferroic systems. The undoped BiFeO3 (BFO) and the codoped system with Ba, La and Ti (Bi[Formula: see text]BaxFe[Formula: see text]TiyO3, Bi[Formula: see text]BaxLazFe[Formula: see text]TiyO3) with x,y,[Formula: see text] were prepared stoichiometrically and sintered at two different temperatures. The structural and magnetic properties were investigated at room temperature. XRD measurements confirm the obtaining of the rhombohedral perovskite structure of the BFO family system. For the undoped system, some reflections of undesired phases are present for two different sintering temperatures, while for the doped system only one phase is present for both temperatures. The magnetic characterization at room temperature revealed remarkable differences between the ceramic samples. The results show that for undoped BFO system, spontaneous magnetization is not observed at room temperature. Nevertheless, in doped one, a well-defined ferromagnetic behavior is observed at room temperature, possible, due to the suppression of the spatially modulated spin structure of BFO promoted by the reduction of the rhombohedral distortion and the weakening of the Bi–O bonds. The XPS results confirm the presence of oxygen vacancies and the coexistence of Fe[Formula: see text] and Fe[Formula: see text] in all the studied samples. Calorimetric measurements reveal that the dopant incorporation has not a direct effect in Néel temperature but possibly yes in ferroelectric-paraelectric transition.

High pressure single crystal x-ray and neutron powder diffraction study of the ferroelectric–paraelectric phase transition in PbTiO3

The results obtained by high pressure neutron powder diffraction and single-crystal x-ray diffraction for the P4mm–Pmm phase transition in the prototype ferroelectric perovskite lead titanate are shown. Neutron diffraction is found to be strongly sensitive to the dipolar moment in the PbTiO3 unit cell due to the gradual reduction of the displacement of the Ti and O atoms from centrosymmetric positions in the cubic perovskite structure which exhibits anti-phase scattering of Pb, Ti and O atoms. From applying both techniques, the anomalously high Debye–Waller factor for the lead atoms confirms the disordered character of the cubic phase. High pressure single crystal x-ray diffraction also perfectly describes the ferroelectric–paraelectric transition and will be the technique of choice to solve higher pressure structures for PbTiO3.

Structural, Optical, and Electrical Characterization of Yttrium-Substituted BiFeO3 Ceramics Prepared by Mechanical Activation.

Ceramics of Bi(1-x)Y(x)FeO3 solid solutions (x = 0.02, 0.07, and 0.10) have been prepared by mechanical activation followed by sintering. The effect of yttrium content on the structural, electrical, and optical properties of the materials has been studied. Thus, single-phase solid solutions with rhombohedral R3c structure have been achieved for x = 0.02 and 0.07, while for x = 0.10 the main R3c phase has been detected together with a small amount of the orthorhombic Pbnm phase. Multiferroic properties of the samples, studied by differential scanning calorimetry (DSC), showed that both T(N) and T(C) (temperatures of the antiferromagnetic-paramagnetic and ferroelectric-paraelectric transitions, respectively) decrease with increasing yttrium content. The nature of the ferroelectric-paraelectric transition has been studied by temperature-dependent X-ray diffraction (XRD), which revealed rhombohedral R3c to orthorhombic Pbnm phase transitions for x = 0.07 and 0.10. On the other hand, for x = 0.02 the high-temperature phase was indexed as Pnma. Optical properties of the samples, as studied by diffuse reflectance spectroscopy, showed low optical band gap that decreases with increasing yttrium content. Prepared ceramics were highly insulating at room temperature and electrically homogeneous, as assayed by impedance spectroscopy, and the conductivity increased with x.

Metallic-like to nonmetallic transitions in a variety of heavily oxygen deficient ferroelectrics

The coupling between ferroelectric distortions and electron transport is an important factor in understanding ferroelectric/noncentrosymmetric materials with metallic conductivities and ferroelectric-based thermoelectrics. Here, multiple d0 ferroelectrics with a variety of crystal structures are doped via oxygen deficiency, resulting in metallic-like conduction in the paraelectric state. It is found that most of the studied systems show a metallic-like to nonmetallic transition near the paraelectric-ferroelectric transition. The metallic-like to nonmetallic transition temperature can be shifted using mechanisms that shift the paraelectric-ferroelectric transition temperature. It was found that the metallic-like to nonmetallic transition temperature could be shifted from 373 K to 273 K by varying (Ba1−xSrx)TiO3−δ from x = 0 to x = 0.3 and x = 1. The most probable mechanism for ferroelectric-electron transport coupling was determined to be Anderson localization associated with polarization with short-range order.

Low dielectric loss and good thermal stability of Eu and Ti co-doped K0.5Na0.5NbO3 ceramics

In order to develop dielectric ceramics with temperature-stable permittivity characteristics, Eu and Ti co-doped K0.5Na0.5NbO3 [(1 − x)K0.5Na0.5NbO3-xEuTiO3] ceramics were prepared by a conventional solid state route. The effect of EuTiO3 on the phase structure and dielectric performance of KNN ceramics were investigated. The temperature dependence of the relative permittivity illustrated that the small amount of EuTiO3 depressed the dielectric peak of ferroelectric-paraelectric transition. Therefore, the temperature stability and relative permittivity of ceramics were effectively improved. As x = 0.03, the ceramic showed superior dielectric temperature stability (±10 %), very low dielectric loss (0.008 at 250 °C) and a relatively high permittivity (>1500 at 250 °C) over the temperature range from ~100 to 350 °C. It suggests that this ceramic is a promising candidate for high-temperature capacitor application.

Formation mechanism and characteristics of lanthanum-doped BaTiO3 powders and ceramics prepared by the sol–gel process

Pure and lanthanum-doped barium titanate nanopowders described by two different formulae, as Ba1−xLaxTiO3, for lower La concentrations (0≤x≤0.005) and Ba1−xLaxTi1−x/4O3 for higher La concentration (x=0.025) were prepared by an alkoxide sol–gel method. Single phase compositions were obtained after annealing at 900°C for 2h, in air. The increase of the lanthanum content causes structural and morphological changes in the oxide powders, including the evolution of the unit cell from tetragonal toward a cubic symmetry, the particle size decrease and a higher aggregation tendency.SEM investigations of the ceramics sintered at 1300°C for 4h indicate significant changes of the microstructural features (strong decrease of the average grain size and increase of the intergranular porosity) with the raise of La amount. Lanthanum addition to barium titanate prepared by sol–gel induces a more significant shift of the Curie temperature toward lower values, than that one reported in literature for ceramics of similar compositions, but processed by the conventional solid state method. The compositions with smaller La amount (x≤0.005) show semiconducting properties at room temperature and high relative dielectric permittivity values, while the undoped ceramics and those doped with higher La content (x=0.025) are good dielectrics. The ceramic with x=0.025 exhibits acceptable low losses, a very diffuse ferroelectric–paraelectric transition and Curie temperature closed to the room temperature, being thus susceptible for high tunability applications.

Structural, magnetic and electrical properties of BiFeO3 co-substituted with Pr–Mn

Samples of stoichiometry Bi1−xPrxFe1−xMnxO3 (x=0, 0.10, 0.15, 0.20) have been synthesized by solid-state reaction method. Fe is found to be in the mixed and single oxidation states for pure and substituted samples, respectively. Mn is in +3 state, as inferred from XPS. XRD analysis indicates the formation of stoichiometric phase with control over the Bi and Fe rich secondary phases. The x=0.10 sample shows thermal stability and characteristic ferroelectric–paraelectric transitions are observed in all the samples, from the thermal studies. Effect of substitution on the grain size distribution is not observed in the SEM picture. Magnetization values from the hysteresis curves obtained at the maximum applied field, increases with substitution indicating the effect of Pr–Mn in BiFeO3. The substituted samples show different response in resistivity and dielectric behaviour than the pure sample, as understood from impedance spectroscopy data. Defects, and charge carriers resulting from Pr–Mn substitution are responsible for this change in conductivity.

Investigation on impedance response, magnetic and ferroelectric properties of 0.20(Co1−xZnxFe2−yMnyO4)–0.80(Pb0.70Ca0.30TiO3) magnetoelectric composites

In the present work, we report on structural, dielectric, multiferroic, magnetoelectric properties and impedance spectroscopic studies of 0.20(Co1−xZnxFe2−yMnyO4)–0.80(Pb0.70Ca0.30TiO3) (x = 0.4, y = 0; x = 0, y = 0.2 and x = 0.4, y = 0.2) composites. The composites were prepared by metallo-organic decomposition (MOD) route. The coexistence of a cubic spinel phase and a tetragonal phase in the composites is confirmed by the X-ray diffraction measurement. Scanning electron micrographs show well distributed ferrite and ferroelectric phases in the composites. The dielectric properties of the composites have been studied in a wide range of frequency at different temperatures. The temperature dependent dielectric properties of the composites reveal that the ferroelectric–paraelectric transition temperature lies in the range of 499–546 °C. The impedance spectroscopic measurements of the composites indicate non-Debye type dielectric relaxation. The grain and grain boundary contributions in the composites have been separated using impedance spectroscopy analysis. The magnetization hysteresis (M−H) loop of the composites shows ferrimagnetic behavior with saturation magnetization values Ms ∼12–13.4 emu/g and coercive field Hc ∼71–273.5 Oe. The Curie temperature of the composites as determined from the inverse susceptibility vs. temperature curves is in the range of 297–480 °C. All the composites exhibit typical ferroelectric hysteresis loops indicating the presence of spontaneous polarization. A maximum value of magnetoelectric coupling co-efficient α ∼17.4 mV/cm Oe has been observed for the composite with x = 0.4 and y = 0.

Investigation of the Ferroelectric Paraelectric Phase Transition in Bulk and Confined Sodium Nitrite

Levanyuk-Sannikov model is used to describe the ferroelectric-paraelectric transition in bulk and confined sodium nitrite (NaNO2). It is found on the basis of this model that the phase transition of NaNO2 for the bulk material is weakly first order whereas for the confined material it is a second order transition. Calculations are performed and the fitted parameters are obtained using this model for the ferroelectric-paraelectric transition in NaNO2. Our results indicate that the Levanyuk-Sannikov model describes adequately the observed behaviour of the ferroelectric-paraelectric transition in NaNO2.

Tuning the functional properties of PMN-PT single crystals via doping and thermoelectrical treatments

Single crystals based on solid solutions of lead-magnesium-niobate (PMN) and lead titanate (PT) have emerged as highly promising multifunctional systems combining piezoelectric, pyroelectric, and electro-optic properties that surpass by far those of the best known lead-zirkonium-titanate ceramics. In this paper we present new findings on how the phase transition temperature and the dielectric and ferroelectric properties can be tuned depending on crystal composition, orientation, and thermoelectrical treatment. Mn-doped and pure 0.72PbMg1/3Nb2/3O3-0.28PbTiO3 (0.72PMN-0.28PT) single crystals with ⟨111⟩ and ⟨001⟩ orientations were investigated. A special attention was devoted to field cooling (FC), i.e., cooling under electric field from different temperatures. The results illustrate different findings that were not reported before: the Curie temperature, i.e., ferroelectric-paraelectric transition temperature, is enhanced after field cooling of the Mn-doped, ⟨001⟩-oriented crystal while such a shift is not observed in the ⟨111⟩-oriented and the non-doped crystals. In addition, substantial polarization suppression occurs in the Mn-doped crystals upon FC from high temperature regardless of orientation. Based on piezoforce microscopy of the domain structure that shows suppression of domain growth following field cooling from 200 °C, we propose a mechanism for polarization suppression based on domain pinning by charged defects. The practical importance of our results lies in showing the opportunity offered by a proper choice of crystal composition and poling conditions for tuning the functional properties of PMN-PT single crystals for a specific application. This should contribute to the understanding of their properties towards advanced sensor and transducers devices.

Electron transport properties of composite ferroelectrics

We study electron transport in composite ferroelectrics —materials consisting of metallic grains embedded in a ferroelectric matrix. Due to its complex tunable morphology the thermodynamic properties of these materials can be essentially different from bulk or thin-film ferroelectrics. We calculate the conductivity of composite ferroelectrics by taking into account the interplay between charge localization, multiple grain boundaries, strong Coulomb repulsion, and ferroelectric order parameter. We show that the ferroelectricity plays a crucial role on the temperature behavior of the conductivity in the vicinity of the ferroelectric-paraelectric transition.

Pressure effect on structural and vibrational properties of Sm-substituted BiFeO3

The structural and vibrational properties of 5% Sm-substituted BiFeO3 under pressure are investigated using synchrotron X-ray diffraction and Raman scattering measurements. The results yield the pressure-induced structural phase transitions from the polar R3c phase to the orthorhombic Pnma phase commencing at 3.9 and being complete at 7.6 GPa, where there is a region of the coexistence of the R3c and Pnma phases. This structural transition is companied by the ferroelectric-paraelectric transition for the Sm-substituted BiFeO3. We find that the Sm substitution leads to lower transition pressure compared to that of the pure BiFeO3 system due to the substitution-induced chemical pressure. Our results do not suggest the pressure-induced reentrance of ferroelectricity in the model multiferroic BiFeO3 in the pressure range studied.