Heterogeneous domain configurations in ferroelectric crystals during thermal depolarization

The phase transition and domain configurations in emerging ternary Mn‐doped Pb(In_1/2Nb_1/2)O₃–Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ ferroelectric crystals have been investigated by high‐resolution X‐ray diffraction. Phase transition sequences of cubic→rhombohedral and monoclinic A→tetragonal→cubic were determined under zero‐field‐cooling and zero‐field‐heating after poling, respectively. Most importantly, a monoclinic‐like heterogeneous domain configuration was observed in the conventional high‐symmetry tetragonal phase field due to the coexistence of multiscale tetragonal variants. Our results provide further evidence that superlattices with low‐symmetry can be assisted by tetragonal nano‐twins.     

Hierarchical Domain Structure of Adaptive MB Phase in Pb(Mg1/3Nb2/3)O3–32%PbTiO3 Single Crystal

The domain structure in Pb(Mg_1/3Nb_2/3)O₃–32%PbTiO₃ single crystal has been investigated with analytical electron microscopy. It is revealed that the M_B phase is an adaptive phase composed of 71° rhombohedral nanodomains and that the nanodomain walls are parallel to {110} planes by a combination of convergent beam electron diffraction and through-focus high-resolution image series. The findings suggest and are supportive of the viewpoint that the M_A phase could also be an adaptive phase composed of 109° rhombohedral nanodomains with domain walls parallel to {001} planes.     

Effect of mechanical stress on phase stability and polarization states in ferroelectric barium titanate and lead titanate

The effect of normal and shear stress on phase transitions in BaTiO₃ and PbTiO₃ has been investigated using a modified Landau–Ginzburg–Devonshire phenomenological model based on assumption of constant stress boundary conditions. Stress–temperature phase diagrams have been developed, and the influence of stress on polarization stability has been analyzed. The results show monoclinic phases with various polarization states absent in stress-free BaTiO₃ may exist under uniaxial, biaxial, anisotropic three-dimensional, and shear stress conditions. For PbTiO₃, our calculations show that, under normal stress new phases cannot be generated and the only stable ferroelectric phase has tetragonal symmetry, but under shear stress orthorhombic, rhombohedral, and monoclinic phases can be stabilized.     

In-Situ Measurement of the Stress-Induced Phase Transformations in Magnesia-Partially-Stabilized Zirconia Using Raman Spectroscopy

Raman spectroscopy has been used to examine the connection between room-temperature creep and stress-induced phase transformations in TS-Grade Mg-PSZ. The in-situ phase measurements obtained show that the tetragonal phase transforms to both the orthorhombic and monoclinic structures under stress. The effect of precipitate orientation is indicated by the variation of creep strain with monoclinic volume fraction.     

Symmetry-bridging phase as the mechanism for the large strains in relaxor-PbTiO3 single crystals

Relaxor-PbTiO₃ piezoelectric single crystals have been of a great interest, since the discovery of ultrahigh piezoresponse demonstrated in <001> -oriented crystals of the composition at the rhombohedral side of morphotropic phase boundary. It has been proposed that the exceptionally large piezoelectric properties should originate from an electric-field-induced polarization rotation that involves a reversible phase transformation between rhombohedral and tetragonal via monoclinic symmetry. However, this commonly accepted polarization rotation mechanism has its limit in explaining still the excellent piezoelectricity even at a small excitation field far below the coercive field. Here, we show by a comparative study using single crystals from two distinct processing techniques, the polarization rotation has, if ever, little influence on the strain properties of <001 > -oriented rhombohedral relaxor-PbTiO₃ single crystals. Instead, they may come from a reversible shear-mode piezoelectric contribution from electric-field-susceptible ‘symmetry-bridging’ unit-cell-level phases, the polarization direction of which spans monoclinic symmetry.     

Electrical performance and ferroelectric phase transition characteristic in different oriented 0.73PMN-0.27PT single crystals

The electrical and optical properties of (001)- and (110)-oriented 0.73 Pb(Mg_1/3Nb_2/3)O₃-0.27PbTiO₃ single crystals are systematically investigated at various temperatures, both of which present a series of ferroelectric phase transition processes. Dielectric performance measurements reveal that the ferroelectric phase transition occurs over a temperature range, rather than at one temperature point. By testing the ferroelectric hysteresis P–E curves as well as bipolar and unipolar electric field-induced strain S–E curves, the values of remnant polarization, coercive field, maximum strain, and converse piezoelectric constant d₃₃^* change considerably near the phase transition temperatures. Simultaneously, the 0.73PMN-0.27PT single crystals with (001)- and (110)-orientations under a low electric field show ultrahigh d₃₃^* values of 3540 and 2817 pm/V, respectively, which can be attributed to the electric field-induced monoclinic and orthorhombic phases, respectively. The series of ferroelectric phase transitions upon heating, that is, from rhombohedral ferroelectric to monoclinic/orthorhombic, followed by from monoclinic/orthorhombic to tetragonal, and finally from tetragonal to cubic paraelectric, are further investigated via polarized light microscopy and Raman spectroscopy.     

Spectroscopic Assessments of Domain Texture in Barium Titanate: II. Cathodoluminescence Analysis

An analytical procedure has been developed according to basic principles of electro-stimulated luminescence spectroscopy aimed at quickly visualizing with high spatial resolution the domain texture developed in tetragonal BaTiO₃ (BT) materials. In the first part of the paper, the relative intensity of the cathodoluminescence (CL) emission has been systematically collected from different crystallographic planes of BT single-crystal and modeled as a function of anisotropic crystal properties. In this context, an analytical expression has been put forward for quantitatively describing the response function of the CL probe, which links the intensity emission to refractive indexes and absorption coefficients pertaining to different planes of the perovskitic crystal. In the second part of the paper, the CL method is applied to visualize with nanometer-scale resolution the domain texture developed in a polycrystalline BT sample. This study demonstrates that CL spectroscopy is a valuable and efficient tool for the assessment of domain orientation in ferroelectric materials. The CL method possesses both wide-range screening capacity and the scanning flexibility of conventional scanning electron microscopy, coupled with a spatial resolution that is comparable with that obtainable by scanning probe microscopy techniques.     

〈111〉 Rotation Twins in an Orthorhombic LaGaO3 Perovskite

Phase-transformation-induced twins in pressureless-sintered lanthanum gallate (LaGaO₃) ceramics have been analyzed using transmission electron microscopy. Twins are induced by solid-state phase transformation upon cooling from rhombohedral ( r , R 3 c ) to orthorhombic ( o , Pnma ) symmetry at 145°C. Domains with a 150°–60°–150° configuration were frequently detected when viewed along [210]. This observation representing the co-existence of the {121} and {123} twins is suggested by analyzing corresponding selected area diffraction patterns across the domain boundaries. The former, with the twin plane lying on {121}, is the reflection type whose twin variants are related by mirror plane symmetry. The latter, although its nature was confirmed by tilting experiments along an unsplit row of reflections, exhibits characteristic crystallographic orientation relationships that are distinctive from those of the {121} twins. The twin laws represented in the matrix form are also derived accordingly from corresponding orientation relationships. Crystallographic analysis indicates that these domains commonly possess an orientation relationship that can be described by the twofold rotation axis about 〈111〉 lost upon the rhombohedral→orthorhombic phase transition. They are therefore the 180° parallel-rotation twin, with the twin axis 〈111〉 lying in {123}. Twins generated by the r → o phase transition between crystals of non-group–subgroup relations are discussed in terms of an intermediate metastable cubic phase of the lowest common supergroup symmetry.     

Dilatometric studies on structural phase transitions in (NH4)2 SnBr6 crystal

Thermal expansion along several directions was measured in the temperature range including the two transition temperatures. The orientation of the domain walls in two lower temperature phases was examined with a polarizing microscope. The results, together with the experiments so far reported, suggest the possibility that the symmetry of the phase between -116 °C and -129 °C is trigonal, in contrast to being tetragonal as previously conjectured.     

Stress-Induced Transformation During Subcritical Crack Growth in Partially Stabilized Zirconia

Fracture surfaces of a commercial partially stabilized ZrO₂ that had undergone subcritical crack growth in H₂0 were characterized by scanning and transmission electron microscopy. At high stress intensities (≥4.6 MPa·m^−1/2), fracture was primarily trans granular, and coherent tetragonal ZrO₂ precipitates had undergone a martensitic transformation to monoclinic symmetry. At lower stress intensities, where power-law crack growth occurred, fracture suflaces were primarily inter-granular, and the tetragonal ZrO₂ had transformed to a new ZrO₂ polymorph with orthorhombic symmetry.     

Thermodynamics of Ferroelectric Solid Solutions with Morphotropic Phase Boundaries

A thermodynamic analysis is presented for the diffusionless phase diagrams of ferroelectric solid solutions that display a morphotropic phase boundary (MPB) separating adjacent tetragonal and rhombohedral phases. Equations are developed for the shape of the MPB, the locations of triple and tricritical points, and for the line along which the anisotropy of polarization vanishes. The appearance of lower symmetry orthorhombic and monoclinic phases is considered and the topologies of energy surfaces in the region of the phase diagram where these phases may stabilize are illustrated. The theory is applied to the solid solution of lead zirconate with lead titanate (PZT) and relationships between polar anisotropy and the transformation strain, dielectric susceptibility and piezoelectric properties, are discussed. The analysis is used to reproduce phase boundary lines for solid solutions of lead titanate with lead magnesium niobate (PMN‐PT) and lead zinc niobate (PZN‐PT) and composition–temperature diagrams along isopleths in the ternary system PMN‐PZT are estimated. The anisotropies of polarization in solid solutions based on lead titanate and barium titanate are contrasted. The results provide a thermodynamic framework useful for guiding experimental investigations of ferroelectric solid solutions and for generating energy functions used in constitutive modeling and phase field simulations of microstructure and properties.     

Phase-Transformation-Induced Twinning in Orthorhombic LaGaO3: {121} and [010] Twins

Pressureless-sintered lanthanum gallate (LaGaO₃) ceramics have been analyzed using X-ray diffractometry (XRD) and scanning electron and transmission electron microscopy (SEM and TEM). Twins are induced by solid-state phase transformation upon cooling from rhombohedral     to orthorhombic ( o , Pnma ) symmetry at ∼145°C. Two types of transformation twins have been identified, and they co-exist in some of the grains. The {121} twins are generated by loss of the mirror plane symmetry upon phase transformation. The twin planes are on {121} about which the two crystal parts are related by a mirror operation. The fact that twins are of reflection type is also confirmed by a tilting experiment in the microscope. The other, often termed the     -type rotation, is a rotation twin induced by loss of fourfold rotation axes upon phase transition. The spot splitting of 2φ≈0.22° because of the orthorhombic obliquity (φ) is evidenced from the corresponding electron diffraction patterns (SADPs) where triplet reflection spots are clearly identified. The {121} twin domain boundaries exhibit δ-fringes. The fault vector across twin boundaries R =ɛ     is determined by applying the invisibility criteria of 2π g · R =0, or 2 n π. Such a translation is not related in a simple way to the structure of LaGaO₃, and the length of the fault vector is not a fractional of the lattice displacement vector.     

New Lead‐Free (1 − x)(K0.5Na0.5)NbO3–x(Bi0.5Na0.5)ZrO3 Ceramics with High Piezoelectricity

For enhancing the piezoelectric properties of ceramics (Bi_0.5Na_0.5)ZrO₃ (BNZ) was used to partially substitute (K_0.5Na_0.5)NbO₃ (KNN). The addition of BNZ changes the symmetry of KNN ceramics from orthorhombic to tetragonal, and finally to rhombohedral phase. A new phase boundary with both rhombohedral–orthorhombic and orthorhombic–tetragonal phase transitions near room temperature is identified for KNN–0.050BNZ ceramics, where optimum electrical properties were obtained: d₃₃ = 360 pC/N, k_p = 32.1%, ε_r = 1429, tanδ = 3.5%, and T_C = 329°C. The results indicated a new method for designing high‐performance lead‐free piezoelectric materials.     

Structure,Phase Transition,and Electronic Properties of K1−xNaxNbO3 Solid Solutions from First‐Principles Theory

With extensive first‐principles calculations, we investigated the geometric structure, phase transition, and electronic properties of orthorhombic, monoclinic, and tetragonal K_1?xNa_xNbO₃ (KNN) as functions of the Na content. We found that KNN undergoes an orthorhombic‐to‐monoclinic‐to‐orthorhombic phase transition when the Na content is gradually increased. We also found that the polarization vector of the monoclinic phase can be rotated more easily than those of the orthorhombic and tetragonal phases, giving rise to an enhanced piezoelectric response of the monoclinic KNN. Furthermore, our calculations provide an interpretation for the experimentally observed unusual broad peak of the KNN piezoelectric parameters.     

Structural Changes by Compressive Stresses of 2.0-mol%-Yttria-Stabilized Tetragonal Zirconia Polycrystals

Reorientation of the tetragonal (002) peak in tetragonal ZrO₂ polycrystals (TZP) (so-called domain switching) was studied by XRD and residual stress measurement using TZP specimens containing 2.0 mol% Y₂O₃ that had undergone mechanical and thermal treatments and compressive stress. The observed domain switching was due to a preferred transformation of the tetragonal phase caused by compressive stress above 70 MPa leading to a remnant c-axis orientation normal to the compressive direction. Domain switching did not depend on thermal stress but arose directly from the tetragonal phase with little relation to monoclinic phase.     

Electric field induced phase transition and accompanying giant poling strain in lead-free NaNbO3-BaZrO3 ceramics

A series of phase transitions in (1-x)NaNbO₃-xBaZrO₃ ((1-x)NN-xBZ) ceramics was observed from antiferroelectric orthorhombic phase to ferroelectric orthorhombic phase and finally into ferroelectric rhombohedral phase with increasing x. An electric field induced irreversible phase transition was found in different compositions, irrespective of their virgin phase structures. Particularly, an antiferroelectric orthorhombic phase is irreversibly transformed into a ferroelectric monoclinic phase within 0.02?≤?x?≤?0.05, leading to a giant poling strain of ～0.58%. This is much larger than that observed in ferroelectric orthorhombic (0.06?≤?x?≤?0.07) and rhombohedral phases (0.08?≤?x?≤?0.11) suffering from an irreversible ferroelectric-ferroelectric (monoclinic) phase transition. The synchrotron x-ray diffraction and the measurement of longitudinal and transverse strains suggest that this irreversible phase transition should involve not only a distinct volume expansion, but also an obvious lattice elongation. The present study demonstrates a unique nature of the composition and field dependent phase stability and an underlying mechanism of giant poling strains in NN-BZ ceramics.     

Issue Information

Cover Photograph:  Residual planar strain for all possible 90° domain walls in a tetragonal ferroelectric. Minima show the planes on which permissible strain-free domain walls are expected to form. [ https://doi.org/10.1111/jace.17555 ].

     

Occurrence of Pseudotetragonal Mullite

Mullite-type phases with about 74 wt% Al₂O₃ and 26 wt% SiO₂ were prepared from tetraethyl orthosilicate and aluminum butylate between 950° and 1000°C. The lack of 120/210, 240/420, 041/401, and 250/520 reflection pair splitting on the X-ray diffractograms indicates tetragonal symmetry, whereas normal mullite is orthorhombic. We believe that the tetragonal character of the phase is due to twinning and/or domain formation of orthorhombic structural units in an elementary cell scale. Therefore, the mullite-type phase should be designated as pseudotetragonal rather than tetragonal. According to our present knowledge, pseudotetragonal mullite is formed from highly reactive metal organic compounds only. The phase is metastable and transforms gradually to orthorhombic mullite at temperatures above 1000°C.     

Structural and Mechanical Property Changes in Toughened Magnesia-Partially-Stabilized Zirconia at Low Temperatures

The mechanical properties of high-toughness magnesia-partially-stabilized zirconia were found to be dramatically altered by a single cooling cycle between room temperature and − 196°C. Raman spectroscopy and X-ray diffraction were used to correlate the changes in mechanical properties with structural changes that occur at temperatures below ∼− 100°C. Most of the tetragonal precipitates that are responsible for toughening transformed to an orthorhombic phase with unit-cell volume intermediate between those of the tetragonal and monoclinic phases. The orthorhombic phase was stable with heating to 300°C, but it transformed back to the tetragonal structure when heated to 400°C. Surprisingly, the orthorhombic phase was not readily transformable by stress, with the consequence that, after the cooling cycle, most of the high-toughness properties of the original tetragonal-containing material were lost.     

Synthesis of the Orthorhombic Phase of 2Y˙ZrO2

A mixture of tetragonal and monoclinic 2Y˙ZrO₂ (2 mol% Y₂O₃–ZrO₂) powder was treated from 400° to 800°C and from 4 to 7 GPa for 30 min. The products were identified by powder XRD, Raman spectroscopy, and TEM. Results indicated that an orthorhombic phase was synthesized at T=400° to 600°C and P>4 GPa. The lattice parameters were obtained as a=0.505, b=0.525, and c=0.509 nm; the density was 6.17 Mg/m³. The orthorhombic phase always coexisted with the tetragonal phase in the products. The amounts of the tetragonal phase before and after treatment remained largely unchanged, whereas the amount of new orthorhombic phase was nearly the same as the decreased amount of the monoclinic phase. It was assumed, therefore, that only the monoclinic phase transformed into the orthorhombic phase.