Morphotropic Boundary Research Articles

Ultralow frequency dielectric dispersion in PZT + PFS ferroelectric ceramic

The depolarization decaying currents were measured for the PZT compound with morphotropic composition modified by the Fe 1/3 Sb 2/3 cations, i.e., Pb [( Fe 1/3 Sb 2/3)x Ti y Zr z] O 3 with x + y + z = 1, x = 0.1, y = 0.44 (morphotropic boundary) and y = 0.47. The measurements were performed at room temperature and a few temperatures above (up to 473 K), as well as at a few lower temperatures (down to 77 K). The samples were poling at fields 0.02 kV/cm and 0.2 kV/cm at higher and lower temperatures, respectively. In the high temperature range, the time dependence depolarization current follows the fractional power law with two different exponents (n < 1 and m > 1), while in the low temperature range with a single exponent (n). The appropriate procedure has been used to transform the results obtained in the time domain into the frequency domain in order to calculate real and imaginary parts of the dielectric permittivity in the frequency range 5 × 10-5-0.1 Hz. The possible dielectric relaxation mechanisms have been discussed.

Influence of gallium substitutions on the structure, dielectric and piezoelectric properties of BiScO3–PbTiO3 ceramics

Ceramics with compositions close to the morphotropic boundary in the (1 − x)Bi(Sc1−yGay)O3–xPbTiO3 system were prepared by the conventional solid-state reaction method. The influence of substituting Ga3+ cations for Sc3+ cations on the structural parameters and the dielectric and piezoelectric properties was studied. It was proved that variations in the relative content of the tetragonal and rhombohedral phases depended on the tolerance-factor t value determined by the B-cation substitutions. The highest values of the piezoelectric coefficient d33 were observed in the compositions with the largest proportion of the tetragonal phase.

Bistability of ferroelectric domain walls: Morphotropic boundary and strain effects

The internal structure of neutral 180 degrees domain walls in perovskite-type ferroelectrics is studied in terms of Landau theory taking into account electromechanical coupling. The study is focused on the wall bistability, a factor of potential interest for information storage. A strong impact of elastic effects on the wall structure is demonstrated. It is shown that the conclusion derived earlier by Houchmandzadeh et al. [J. Phys.: Condens. Matter 3, 5163 (1991)], neglecting the electrostictive coupling, that all the domain walls near the boundary between two ordered phases become bistable may not hold due to the elastic effects. Criteria for domain-wall bistability are formulated in terms of the materials thermodynamic properties and the wall orientation. The obtained general results are applied to the analysis of bistability of 180 degrees domain walls in Pb(Zr-c, Ti1-c)O-3 near the tetragonal-rhombohedral morphotropic boundary. It is shown that, on the tetragonal side, the electrostrictive interaction suppresses the wall bistability that was predicted in terms of the theory neglecting the elastic effects. On the rhombohedral side, the domain walls are found bistable or not depending on the anisotropy of the correlation energy, the information on which is not presently available. It is also shown that, in the rhombohedral phase, the anisotropy of the correlation energy results in appearance of additional polarization component in the plane of the wall.

Ferroelastic aspects of relaxor ferroelectric behaviour in Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 perovskite

Elastic and anelastic properties of poled and depoled single crystals of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 with compositions close to the morphotropic boundary have been investigated over the temperature range 5–700 K by resonant ultrasound spectroscopy (RUS) at frequencies of 0.1–1.2 MHz. Steep elastic softening occurs in a temperature interval of at least 250 K as the Vogel-Fulcher freezing interval and cubic → tetragonal transition point, Tc, are approached from above. This is understood in terms of coupling between acoustic modes and central peak mode(s) associated with dynamic polar nano regions (PNR's) below the Burns temperature. Acoustic losses occur in a temperature interval of ∼50 K above Tc, associated with slowing down of the PNR dynamics. The cubic ↔ tetragonal and tetragonal ↔ rhombohedral transitions are accompanied by steep minima in elastic properties, closely analogous to the pattern of softening and stiffening observed in sequences of improper ferroelastic transitions in other perovskites. Variations in the magnitudes of acoustic losses at T < Tc correlate with the density of ferroelastic twin walls, from lowest for [001]c-poled and [111]c-poled crystals in the stability fields of the tetragonal and rhombohedral phases, respectively, to highest for unpoled crystals. A simple model of Debye-like peaks in acoustic loss near 100 K has yielded activation energies and attempt frequencies in the same range as those observed from dielectric data in the Vogel-Fulcher freezing interval. These highlight the fact that, in addition to conventional ferroelectric/ferroelastic twin walls, relaxor ferroelectrics contain local structural heterogeneities coupled to strain, which are probably related to the presence of static PNR's preserved even in poled crystals. RUS also provides a convenient and effective means of determining the mechanical quality factor of relaxor ferroelectrics, as functions of both poling history and temperature.

Influence of Complex Additives on Structure, Microstructure, Phase Transitions and Dielectric Properties of BiScO3-PbTiO3 Ceramics

Ceramic solid solutions from the region of morphotropic boundary on the base of 0.36BiScO3-0.64PbTiO3, modified by complex additives of Bi2O3 and Ni2O3, Bi2O3 and MnO2, and LiF additives in amounts up to 5 w. % have been prepared by the conventional solid state reaction method. Decreasing of the phase formation temperature accompanies by increasing in density of the doped ceramics have been revealed. Influence of additives on microstructure, dielectric, ferroelectric properties, and effects of dielectric relaxation have been revealed.

Giant anhysteretic response of ferroelectric solid solutions with morphotropic boundaries: the role of polar anisotropy

Computer modeling and simulation for the Pb(Zr1-xTix)O3 (PZT) system reveal the role of polar anisotropy on the giant anhysteretic response and structural properties of morphotropic phase boundary (MPB) ferroelectrics. It is shown that a drastic reduction of the composition-dependent polar anisotropy near the MPB flattens energy surfaces and thus facilitates reversible polarization rotation. It is further shown that the polar anisotropy favors formation of polar domains, promotes phase decomposition and results in a two-phase multidomain state, which response to applied electric field is anhysteretic when the polar domain reorientation is only caused by polarization rotation other than polar domain wall movement. This is the case for the decomposing ferroelectrics under a poling electric field with the formation of a two-phase multidomain microstructure, wherein most domain walls are pinned at the two-phase boundaries. Indication of the microstructure dependence of the anhysteretic strain response opens new avenues to improve the piezoelectric properties of these materials through the microstructure engineering.

Spreading resistance microscopy of polycrystalline and single-crystal ferroelectric films

Thin films based on lead zirconate titanate with stoichiometric composition near the morphotropic boundary have been studied using atomic-force microscopy methods. The dependence of the local conductivity on the local polarization direction has been observed for all samples, independently of substrate type, deposition method, and film thickness. It has been shown that the current response to the applied voltage exhibits a long current relaxation, about several tens of seconds, which is two to three orders of magnitude greater than the current relaxation time in an external circuit, associated with the ferroelectric domain switching. The conductivity features have been explained by recharging of traps localized at ferroelectric grain boundaries near electrodes and involved in polarization charge screening.

Rhombohedral and Monoclinic Phases of PZT near the Antiferroelectric and the Morphotropic Boundaries

Anelastic and dielectric spectroscopy measurements are presented, which, together with previous measurements [1], clarify some controversial aspects of the phase diagram of PbZr1xTixO3close to the border with the antiferroelectric (AFE) phase, and at the morphotropic phase boundary (MPB). No evidence is found of a border separating monoclinic (M) from rhombohedral (R) phases, supporting recent structural studies according to which the two phases coexist, with the fraction of M prevailing near the MPB. A large frequency independent softening at the MPB indicates a genuine M phase over only finely twinned R phase. A new phase transition is found in both the anelastic and dielectric spectra atx= 0.1, at a temperatureTITbetween the Curie temperatureTCand the boundaryTTto the phases with tilted octahedra. Such a diffuse transition is interpreted as onset of disordered tilts, which finally become ordered belowTT. In this manner, the phase diagram of PZT is rationalised with respect to the common tendency of perovskites to undergo tilting when the mismatch between the cation sizes exceeds a threshold.

Phase Diagram of the Ferroelectric Perovskite (Na<sub>0.5</sub>Bi<sub>0.5</sub>)<sub>1−<i>x</i></sub>Ba<sub><i>x</i></sub>TiO<sub>3 </sub>by Anelastic and Dielectric Relaxation Measurements

Anelastic and dielectric relaxation measurements have been carried out on poled and unpoled samples of the ferroelectric perovskite (Na0.5Bi0.5)1−xBaxTiO3(NBT-xBT), with composition in the range between pure NBT and the morphotropic phase boundary, 0 ≤x≤ 0.08. The complex elastic compliance spectra contain clear indications of both the rhombohedral/tetragonal and tetragonal/cubic transitions, allowing the determination of the phase diagram, which is difficult to obtain by diffraction techniques due to the very low distortions in both the tetragonal and rhombohedral phases and to the structural disorder in the Na/Bi sublattice. An extensive study is made for concentrations in the region of the morphotropic boundary (x∼ 0.06) in order to find possible signatures of monoclinic phase, as for the case of PbZr1-xTixO3(PZT). The main features in the anelastic curves are compared with those in the dielectric spectra and are tentatively related to different modes of octahedral rotations and polar cation shifts.

Impact of Strain Effects on the Stability of Ising Walls in Ferroelectrics

The stability of Ising 180-degree domain walls with respect to appearance of chirality is analytically studied in tetragonal perovskite-type ferroelectrics taking into account electro-mechanical coupling. It is shown that the widely used approximation neglecting the elastic effects may lead to qualitatively wrong results. Sufficient condition for the stability of Ising walls is formulated only in terms of elastic and electrostrictive properties, regardless the correlation energy. We demonstrate that elasticity stabilizes Ising walls in tetragonal Pb(Zr1-cTic)O3 and makes chiral walls unfavorable for any composition of the material (factor c), while nonelastic model predicts chirality of 180-degree walls near the morphotropic boundary.

Octahedral tilting, monoclinic phase and the phase diagram of PZT

Anelastic and dielectric spectroscopy measurements on PbZr1−xTixO3 (PZT) close to the morphotropic (MPB) and antiferroelectric boundaries provide new insight into some controversial aspects of its phase diagram. No evidence is found of a border separating monoclinic (M) from rhombohedral (R) phases, in agreement with recent structural studies supporting a coexistence of the two phases over a broad composition range x < 0.5, with the fraction of M increasing toward the MPB. It is also discussed why the observed maximum of elastic compliance appears to be due to a rotational instability of the polarization linearly coupled to shear strain. Therefore it cannot be explained by extrinsic softening from finely twinned R phase alone, but indicates the presence also of M phase, not necessarily homogeneous. A new diffuse transition is found within the ferroelectric phase near x ∼ 0.1, at a temperature TIT higher than the well established boundary TT to the phase with tilted octahedra. It is proposed that around TIT the octahedra start rotating in a disordered manner and finally become ordered below TT. In this interpretation, the onset temperature for octahedral tilting monotonically increases up to the antiferroelectric transition of PbZrO3, and the depression of TT(x) below x = 0.18 would be a consequence of the partial relief of the mismatch between the average cation radii with the initial stage of tilting below TIT.

PLZT-based photovoltaic Piezoelectric Transformer with light feedback

Piezoelectric Transformer (PT) converts an electrical AC input voltage into ultrasonic vibrations and reconverts back to an output as AC voltage. Hard lead zirconate titanate (PZT) ceramics is typically used for fabrications of such devices. In case of lanthaniun ion La3+ addition in PZT solid solution we can achieve piezoelectric ceramics with good transparency exhibiting both optical Pockels and Kerr effects. Values of these coefficients in the PLZT system are much bigger than in LiNbO3 or SBN single crystals. Among the various PLZT compositions 8/65/35, near the morphotropic boundary, exhibit large electrooptic effect and thus have found applications in light shutters and displays. In the present study we have investigated radial mode piezoelectric transformer based on optically transparent PLZT8/65/35 ceramics. The effect of the UV light generated photovoltage and photostriction on the efficiency and voltage step-up ratio of piezoelectric transformer have been demonstrated. Novel functions of this device is proposed by superimposing two sophistically coupled effects of piezoelectricity and photostriction.

Phase Transitions, Magnetic and Piezoelectric Properties of Rare‐Earth‐Substituted BiFeO3 Ceramics

The concentration range of the stability of polar (R3c) and antipolar phases in the Bi1−xRExFeO3 (RE–La ‐ Dy) solid solutions has been determined by X‐ray study of the polycrystalline samples. Both polar and antipolar phases become less stable with a decrease of the rare earth ionic radii. It is stimulated by a reduction of the rare‐earth ions polarizability with a decrease in ionic radii. The antipolar phase is characterized by a weak ferromagnetic state, whereas the polar one exhibits dominantly antiferromagnetic behavior near the polar‐antipolar morphotropic boundary. The local piezoelectric response decreases with increase in antipolar phase content in the mixed polar‐antipolar structural state. It is suggested that the piezoelectric activity is associated with polar (R3c) phase.

Enhanced piezoelectric and magnetic properties of Bi1−xCaxFe1−x/2Nbx/2O3 solid solutions

An investigation of the crystal structure, the magnetic and the piezoelectric properties of polycrystalline Bi1-xCaxFe1-x/2Nbx/2O3 (I) and Bi1-xPbxFe1-x/2Nbx/2O3 (II) systems was performed by x-ray diffraction, Mössbauer spectroscopy, vibrating sample magnetometry, and piezoresponse force microscopy. It is shown that an increasing niobium content induces a polar-to-nonpolar morphotropic boundary near x = 0.19 (I) and x = 0.3 (II). Within the polar region of the system (I), (0.15 ≤x ≤0.18), the solid solutions are homogeneous weak ferromagnets whereas compounds of the system (II) do not exhibit an appreciable spontaneous magnetization. It is assumed that chemical substitutions leading to a decrease of the initial volume of the unit cell favor the stabilization of the weak ferromagnetic state within the rhombohedral ferroelectric phase. The piezoresponse is significantly enhanced near the morphotropic boundary. The piezoelectric properties of the parent antiferromagnet BiFeO3, harboring a cycloidal spatially modulated spin structure, are compared with those of the polar weak ferromagnet Bi0.82Ca0.18Fe0.91Nb0.09O3.

Phase Transitions, Piezo- and Ferroelectric Properties of BiScO3-PbTiO3 Solid Solutions

Ceramic solid solutions (1-x)(Bi0.9Nd0.1)(Sc0.9B0.1)O3 - xPbTiO3 (x = 0.60–0.66; B - Lu, Yb, Er, Y) based on the compositions close to the morphotropic boundary, have been prepared. To improve the phase content and the resistance of ceramics, Bi2O3 and MnO2 additives were used. Phase composition, structure, microstructure, phase transitions, ferroelectric and piezoelectric properties have been studied. The 1st order ferroelectric phase transitions are marked by peaks near 700–750 K in dielectric permittivity and dielectric loss temperature dependences. Effect of dielectric relaxation was observed, its relation to composition of ceramics determined. The Curie temperature TC increase was observed with increasing the PbTiO3 constituent, while the TC decrease is typical of the A- or B-cations doped compositions. Introduction of MnO2 and/or Bi2O3 additives lead to the decrease of total conductivity and dielectric losses to more than one order at elevated temperatures. Enhanced d33 and kt values were measured in modified ceramics.

Raman Spectroscopy Study of Na1/2Bi1/2TiO3–BaTiO3 Lead-Free Single Crystal Relaxor Piezoelectrics

Novel lead-free piezoelectric single-crystal with a morphotropic boundary composition (0.94Na1/2Bi1/2TiO3–0.06BaTiO3) was characterized by dielectric spectroscopy, far-IR reflectivity and Raman spectroscopy. Analysis of polarized Raman spectra suggests that the three principal Raman bands are associated with polar optic modes.

A morphotropic phase boundary system based on polarization rotation and polarization extension

Many ferroelectric solid solutions exhibit enhanced electromechanical properties at the morphotropic boundary separating two phases with different orientations of polarization. The mechanism of properties enhancement is associated with easy paths for polarization rotation in anisotropically flattened free energy profile. Another mechanism of properties enhancement related to free energy flattening is polarization extension. It is best known at temperature-driven ferroelectric-paraelectric phase transitions and may lead to exceedingly large properties. Its disadvantage is temperature instability of the enhancement. In this paper a temperature-composition phase diagram is proposed that exhibits compositionally driven-phase transitions with easy paths for both polarization rotation and polarization extension.

Dependences of the dielectric and pyroelectric properties of (1 − x)PbFe1/2Nb1/2O3−x PbTiO3 ferroelectric ceramics solid solutions on the PbTiO3 content in a compositional range of 0 ≤ x ≤ 0.08

(1 − x)PbFe1/2Nb1/2O3−xPbTiO3 (PFN-xPT) ferroelectric ceramics with low dielectric loss and pronounced anomalies of dielectric and pyroelectric properties near the ferroelectric-ferroelectric phase transition was obtained by doping with lithium. The temperature dependence of the position of the morphotropic boundary between the rhombohedral (monoclinic) and tetragonal phases in the x-T phase diagram of the PFN-xPT system was determined for the first time.

Thermodynamics of polar anisotropy in morphotropic ferroelectric solid solutions

A classical 2-4-6 Landau polynomial is used to describe the generic pseudo-binary composition–temperature (c–T) diagram of ferroelectric solid solutions that display a morphotropic boundary (MB). This polynomial is separated into an isotropic part that depends on the absolute value of the polarization and an anisotropic part that depends also on the direction of polarization. Composition and temperature are taken as external thermodynamic variables and the direction of the polarization vector is treated as an internal relaxing variable of the system. Conditions leading to the observation of stable rhombohedral (FR), tetragonal (FT) and orthorhombic (FO) phases in the c–T plane are determined. In the lowest-order approximation, the isothermal composition change producing the FR to FT transition at the MB is dictated by the condition that the polarization anisotropy energy vanishes. A small lifting of the orientational degeneracy of the polarization leads to differing phase diagram topologies wherein a stable FO phase may interleave the FR and FT phase fields along the MB line. The theory simultaneously accounts for disparate physical phenomena observed in morphotropic ferroelectric solid solutions, which include a sharp reduction in the ferroelectric domain size and the appearance of a dipole glass state connected with intermediate ferroelectric phases, and large extrinsic contributions to the electromechanical properties near the MB. The curving of the MB line, the nearly continuous transitions between adjacent ferroelectric phases and the change in the order of the paraelectric to ferroelectric transitions with composition also arise naturally in the theory.

Ferroelectric solid solutions with morphotropic boundary: Rotational instability of polarization, metastable coexistence of phases and nanodomain adaptive states

Ferroelectric solid solutions with different symmetry phases at opposite sides of the perovskite-based pseudo-binary phase diagram are considered. An analysis of the concentration dependence of polar anisotropy shows conditions under which the anisotropy changes sign, assuming zero value. The orientational stability of a polarization was investigated near this point, which is also a position of so-called morphotropic boundary (MB). The symmetry conditions resulting in the high-symmetry ferroelectric phases with finite temperature–composition stability field as well as the thermodynamic conditions providing their metastable two-phase coexistence are formulated. A loss of orientation stability around MB results in a divergence of fluctuations of polarization direction and a peak of dielectric susceptibility. The calculated energy of a 90° domain wall demonstrates that this energy is drastically reduced or even vanishes at the MB. The latter leads to a miniaturization of the domain structure to the nano-scale level and the formation of a mixed adaptive state. The miniaturization explains the observed special properties of ferroelectrics near MB that are extrinsic for the nanodomain adaptive state. It also explains the observed diffraction patterns generated by nanodomains of the mixed state. A coherent scattering generated by nanodomains mimics diffraction patterns that could be confused with a pattern generated by a homogeneous monoclinic phase.