Multiple soft-mode vibrations of lead zirconate

A Roadmap for Ferroelectric-Antiferroelectric Phase Transition

Antiferroelectric materials have shown great potential in electronic devices benefiting from the reversible phase transition between ferroelectric and antiferroelectric phases. Understanding the dipole arrangements and clear phase transition pathways is crucial for design of antiferroelectric materials-based energy storage and conversion devices. However, the specific phase transition details remain largely unclear and even controversial to date. Here, we have grown a series of PbZrO3 on SrTiO3 substrates and elucidated the fine atom structures and phase transition pathways using atomic-resolution transmission electron microscopy. Specifically, a roadmap for ferroelectric to antiferroelectric phase transitions, here with increasing film thickness, is determined as ferroelectric rhombohedral (R3c)-ferroelectric monoclinic (Pc)-ferrielectric orthorhombic (Ima2)-antiferroelectric orthorhombic (Pbam), where Pc and Ima2 phases act as structural bridges. Moreover, the phase transition pathway is strongly related to the synergistic effect of oxygen octahedral tilting and cation displacement. These findings provide an insightful understanding for the theories and related properties of antiferroelectrics.

Room-temperature stabilizing strongly competing ferrielectric and antiferroelectric phases in PbZrO3 by strain-mediated phase separation

PbZrO3 has been broadly considered as a prototypical antiferroelectric material for high-power energy storage. A recent theoretical study suggests that the ground state of PbZrO3 is threefold-modulated ferrielectric, which challenges the generally accepted antiferroelectric configuration. However, such a novel ferrielectric phase was predicted only to be accessible at low temperatures. Here, we successfully achieve the room-temperature construction of the strongly competing ferrielectric and antiferroelectric state by strain-mediated phase separation in PbZrO3/SrTiO3 thin film. We demonstrate that the phase separation occurs spontaneously in quasi-periodic stripe-like patterns under a compressive misfit strain and can be tailored by varying the film thickness. The ferrielectric phase strikingly exhibitsa threefold modulation period with a nearly up-up-down configuration, which could be stabilized and manipulated by the formation and evolution of interfacial defects under applied strain. The present results construct a fertile ground for further exploring the physical properties and applications based on the novel ferrielectric phase.

Synergistically Optimizing Pressure-Driven Energy Conversion and Energy-Harvesting Application via Modulating an Antiferroelectric-to-Ferroelectric Overlap Zone in Antiferroelectric Ceramics

Dielectric ceramics with ultrahigh polarization and energy density are the core components used in next-generation pulse power generators based on explosive energy conversion. However, the low polarization of ferroelectric materials and high depolarized pressure hinder their development toward miniaturization, light weight, and integration, while antiferroelectric materials possessing larger nonlinear saturated polarization and rich phase structure are neglected in pulse power energy conversion. Here, an effective strategy of constructing antiferroelectric-to-ferroelectric overlap zone is achieved in binary system (1 - x)(Pb,La)(Zr,Ti)O3-xBa(Al1/2Nb1/2)O3 antiferroelectric ceramics to realize an excellent polarization of 41 μC/cm2 and a large depolarization efficiency of >99% under 150 MPa as well as a record high energy harvesting density of 2.5 J/cm3 under 400 MPa. The excellent comprehensive energy conversion and energy harvesting performance is mainly attributed to the strategy of antiferroelectric-to-ferroelectric overlap zone and improved microdomain density, at which orthorhombic-to-rhombohedral structure evolution is confirmed by transmission electron microscopy, piezo-response force microscopy, and Raman spectrum, resulting in substantially enhanced remanent polarization compared to ferroelectric ceramics. Besides, excellent temperature stability (∼180 °C) and optimized depolarization pressure also support that this binary system is a candidate for energy conversion and energy harvesting application. This work demonstrates that antiferroelectric-to-ferroelectric overlap based on antiferroelectric materials is an excellent strategy to develop dielectric materials with excellent depolarized polarization and energy harvesting density for energy conversion and harvesting.

Renaissance of elemental phosphorus materials: properties, synthesis, and applications in sustainable energy and environment

The polymorphism of phosphorus-based materials has garnered much research interest, and the variable chemical bonding structures give rise to a variety of micro and nanostructures. Among the different types of materials containing phosphorus, elemental phosphorus materials (EPMs) constitute the foundation for the synthesis of related compounds. EPMs are experiencing a renaissance in the post-graphene era, thanks to recent advancements in the scaling-down of black phosphorus, amorphous red phosphorus, violet phosphorus, and fibrous phosphorus and consequently, diverse classes of low-dimensional sheets, ribbons, and dots of EPMs with intriguing properties have been produced. The nanostructured EPMs featuring tunable bandgaps, moderate carrier mobility, and excellent optical absorption have shown great potential in energy conversion, energy storage, and environmental remediation. It is thus important to have a good understanding of the differences and interrelationships among diverse EPMs, their intrinsic physical and chemical properties, the synthesis of specific structures, and the selection of suitable nanostructures of EPMs for particular applications. In this comprehensive review, we aim to provide an in-depth analysis and discussion of the fundamental physicochemical properties, synthesis, and applications of EPMs in the areas of energy conversion, energy storage, and environmental remediation. Our evaluations are based on recent literature on well-established phosphorus allotropes and theoretical predictions of new EPMs. The objective of this review is to enhance our comprehension of the characteristics of EPMs, keep abreast of recent advances, and provide guidance for future research of EPMs in the fields of chemistry and materials science.

Brillouin light scattering in niobium doped lead zirconate single crystal

Brillouin light scattering experiments were performed for lead zirconate single crystals doped with niobium. Special attention was paid to the elastic mode softening near phase transition temperatures. The results are compared with data obtained by Raman light scattering experiments. We observed that the interaction between acoustic and optic modes is responsible for symmetry breaking far above TC, leading to polar regions' appearance. No changes in the acoustic mode frequency and its damping are observed at TC, where ε(T) exhibits a maximum value. The absence of these changes and the central peak observed in Raman experiments suggest that the phase transition at TC is mainly of the order-disorder type. The origin of other phase transitions is discussed as well.

Long-Term Isothermal Phase Transformation in Lead Zirconate

Lead zirconate PbZrO₃ has been the subject of research interest for several dozen years. Recently, even its antiferroelectric properties have started to be questioned, and many researchers still deal with the so-called intermediate phase below Curie temperature (T_C), whose existence is not fully understood. It turns out that PbZrO₃ doped with Nb exhibits below T_C phases with complex domain structures. One of them undergoes self-organization taking place at a constant temperature, and transforms, after several minutes, into a lower phase. This isothermal transition was investigated through dielectric, pyroelectric current and Raman scattering measurements. Discontinuities accompanied it in the permittivity and pyroelectric current. The obtained Raman spectra proved that those discontinuities are strictly linked with the isothermal transition between two intermediate phases. The ordering process in lead sublattice stimulated by thermal fluctuations is discussed as a driving force for this peculiar phenomenon.

Atomic reconfiguration among tri-state transition at ferroelectric/antiferroelectric phase boundaries in Pb(Zr,Ti)O3

Phase boundary provides a fertile ground for exploring emergent phenomena and understanding order parameters couplings in condensed-matter physics. In Pb(Zr_1-xTi_x)O₃, there are two types of composition-dependent phase boundary with both technological and scientific importance, i.e. morphotropic phase boundary (MPB) separating polar regimes into different symmetry and ferroelectric/antiferroelectric (FE/AFE) phase boundary dividing polar and antipolar dipole configurations. In contrast with extensive studies on MPB, FE/AFE phase boundary is far less explored. Here, we apply atomic-scale imaging and Rietveld refinement to directly demonstrate the intermediate phase at FE/AFE phase boundary exhibits a rare multipolar Pb-cations ordering, i.e. coexistence of antipolar or polar displacement, which manifests itself in both periodically gradient lattice spacing and anomalous initial hysteresis loop. In-situ electron/neutron diffraction reveals that the same parent intermediate phase can transform into either FE or AFE state depending on suppression of antipolar or polar displacement, coupling with the evolution of long-/short-range oxygen octahedra tilts. First-principle calculations further show that the transition between AFE and FE phase can occur in a low-energy pathway via the intermediate phase. These findings enrich the structural understanding of FE/AFE phase boundary in perovskite oxides.

Ferroelectric/antiferroelectric phase boundary is both technologically and scientifically important. Here, the authors reveal the structure of intermediate phase involved in classical Pb(Zr_1-xTi_x)O₃.

Structural Phase Transition and In-Situ Energy Storage Pathway in Nonpolar Materials: A Review

Benefitting from exceptional energy storage performance, dielectric-based capacitors are playing increasingly important roles in advanced electronics and high-power electrical systems. Nevertheless, a series of unresolved structural puzzles represent obstacles to further improving the energy storage performance. Compared with ferroelectrics and linear dielectrics, antiferroelectric materials have unique advantages in unlocking these puzzles due to the inherent coupling of structural transitions with the energy storage process. In this review, we summarize the most recent studies about in-situ structural phase transitions in PbZrO₃-based and NaNbO₃-based systems. In the context of the ultrahigh energy storage density of SrTiO₃-based capacitors, we highlight the necessity of extending the concept of antiferroelectric-to-ferroelectric (AFE-to-FE) transition to broader antiferrodistortive-to-ferrodistortive (AFD-to-FD) transition for materials that are simultaneously ferroelastic. Combining discussion of the factors driving ferroelectricity, electric-field-driven metal-to-insulator transition in a (La_1−xSr_x)MnO₃ electrode is emphasized to determine the role of ionic migration in improving the storage performance. We believe that this review, aiming at depicting a clearer structure–property relationship, will be of benefit for researchers who wish to carry out cutting-edge structure and energy storage exploration.

Cooperative Cluster Jahn-Teller Effect as a Possible Route to Antiferroelectricity

We report the observation of an antipolar phase in cubic GaNb_{4}S_{8} driven by an unconventional microscopic mechanism, the cooperative Jahn-Teller effect of Nb_{4}S_{4} molecular clusters. The assignment of the antipolar nature is based on sudden changes in the crystal structure and a strong drop of the dielectric constant at T_{JT}=31  K, also indicating the first-order nature of the transition. In addition, we found that local symmetry lowering precedes long-range orbital ordering, implying the presence of a dynamic Jahn-Teller effect in the cubic phase above T_{JT}. Based on the variety of structural polymorphs reported in lacunar spinels, also including ferroelectric phases, we argue that GaNb_{4}S_{8} may be transformable to a ferroelectric state, which would further classify the observed antipolar phase as antiferroelectric.

Local properties and phase transitions in Sn doped antiferroelectric PbHfO3single crystal

Pb(Hf1-xSnx)O3single crystals withx= 0.23 were characterized using single-crystal x-ray diffraction in the wide temperature range. The information on the structure of two intermediate phases, situated between low temperature antiferroelectric and high temperature paraelectric phases, has been obtained. The lower-temperature intermediate AFE2 phase is characterized by incommensurate displacive modulations in the Pb sublattice. The higher temperature intermediate IM phase is characterized by rotations of oxygen octahedra, primarily in the form of anti-phase tilts, which are also present in the lower-temperature AFE2 phase. For the same crystal,119Sn Mossbauer effect in the temperature range from 300 K to 600 K has been used to study phase transitions mechanism. Two kinds of quadruple splitting have been found. It implies that two different environments of the central Sn ion exist. The observed two kinds of quadruple splitting do not disappear in the whole investigated temperature range which confirm that even far aboveTCthe structure of paraelectric phase is locally non-centrosymmetric.

Unveiling the ferrielectric nature of PbZrO3-based antiferroelectric materials

Benefitting from the reversible phase transition between antiferroelectric and ferroelectric states, antiferroelectric materials have recently received widespread attentions for energy storage applications. Antiferroelectric configuration with specific antiparallel dipoles has been used to establish antiferroelectric theories and understand its characteristic behaviors. Here, we report that the so-called antiferroelectric (Pb,La)(Zr,Sn,Ti)O₃ system is actually ferrielectric in nature. We demonstrate different ferrielectric configurations, which consists of ferroelectric ordering segments with either magnitude or angle modulation of dipoles. The ferrielectric configurations are mainly contributed from the coupling between A-cations and O-anions, and their displacement behavior is dependent largely on the chemical doping. Of particular significance is that the width and net polarization of ferroelectric ordering segments can be tailored by composition, which is linearly related to the key electrical characteristics, including switching field, remanent polarization and dielectric constant. These findings provide opportunities for comprehending structure-property correlation, developing antiferroelectric/ferrielectric theories and designing novel ferroic materials.

The large family PbZrO₃-based solid solutions are usually considered as antiferroelectric materials with specific antiparallel polarization configuration. Here, the authors demonstrate the PbZrO₃-based material has ferrielectric dipoles ordering and configure a clear structure-property relationship.

Rich antiferroelectric phase diagram of antiferroelectric-ferroelectric superlattices: internal electric field- and interface induced phase transitions

We propose a thermodynamic model to the study the antiferroelectric (AFE) phase transitions in antiferroelectric-ferroelectric (AFE-FE) superlattices in which the coupling at the interface between two layers is mediated by local polarizations. Phase diagram of the AFE layer in term of the degree of interfacial effectλand temperatureTinvolving ferrielectric (FI) and ferroelectric (FE) phases is investigated. These two phases are stabilized by the interfacial effect and internal electric field. AFE thicknessLAFEversusTphase diagram is also constructed. Intermediate regions of two-phase coexistence (IM) emerge in theλ-TandLAFE-Tphase diagrams, if certain interface propertiesλand layer thicknessLAFEcriteria are met. These IM regions are metastable states, which exist as a transition state between two phases. A tricritical point locates at the boundaries across the FI, IM and FE phases is found in theLAFE-Tphase diagram. Competition among the internal electric field due to the electrostatic coupling, the FE ordering arises from the interfacial effect and the antiferroelectric ordering within the AFE layer giving rises to the rich AFE phase diagram.

Enhanced Energy Storage Density of Lead Lutetium Niobate Crystals by Electric Field-Induced Secondary Phase Transition via Na/La Codoping

As emerging materials for capacitor applications, antiferroelectric (AFE) materials possess high energy storage density. AFE single crystals are conducive to studying the physical mechanism of AFE response. However, the preparation of AFE single crystals is a huge and long-standing challenge. Herein, we report the effect of Na/La codoping on the energy storage properties and phase transition of Pb(Lu_1/2Nb_1/2)O₃ (PLN) AFE single crystals. An enhanced recoverable energy storage density of 4.81 J/cm³ with a high energy efficiency of 82.36% is obtained, which is much larger than that of the PbZrO₃- and PLN-based AFE crystals. Two superlattice reflections, which stem from the A-site Pb²⁺ ions and the ordered B-site ions, are identified by X-ray diffraction and selected-area electron diffraction. The domain structures demonstrated a high temperature stability of the AFE phase. A secondary ferroelectric phase transition is induced after codoping, resulting in a sharp improvement of polarization (12.5 μC/cm²), which contributes to the enormous enhancement of energy storage density. This multiphase transition is explained using the modified Ginzburg-Landau-Devonshire phenomenology.

Defects and Lattice Instability in Doped Lead-Based Perovskite Antiferroelectrics: Revisited

This paper is a summary of earlier results that have been completed with recent investigations on the nature and sequence of phase transitions evolving in the antiferroelectric PbZrO3 single crystals doped with niobium and Pb(Zr0.70Ti0.30)O3 ceramics doped with different concentration of Bi2O3. It was found that these crystals undergo new phase transitions never observed before. To investigate all phase transitions, different experimental methods were used to characterize the crystal properties. Temperature and time dependencies have been tentatively measured in a wide range, including a region above Tc, where precursor dynamics is observed in the form of non-centrosymmetric regions existing locally in crystal lattices. Also, coexistence of antiferroelectric phase and one of the intermediate phases could be observed in a wide temperature range. The phase transition mechanism in PbZrO3 is discussed, taking into account the local breaking of the crystal symmetry above Tc and the defects of crystal lattices, i.e., those generated during crystal growth, and intentionally introduced by preheating in a vacuum or doping with hetero-valent dopant.

Archetypal Soft-Mode-Driven Antipolar Transition in Francisite Cu_{3}Bi(SeO_{3})_{2}O_{2}Cl

Model materials are precious test cases for elementary theories and provide building blocks for the understanding of more complex cases. Here, we describe the lattice dynamics of the structural phase transition in francisite Cu_{3}Bi(SeO_{3})_{2}O_{2}Cl at 115 K and show that it provides a rare archetype of a transition driven by a soft antipolar phonon mode. In the high-symmetry phase at high temperatures, the soft mode is found at (0,0,0.5) at the Brillouin zone boundary and is measured by inelastic x-ray scattering and thermal diffuse scattering. In the low-symmetry phase, this soft-mode is folded back onto the center of the Brillouin zone as a result of the doubling of the unit cell, and appears as a fully symmetric mode that can be tracked by Raman spectroscopy. On both sides of the transition, the mode energy squared follows a linear behavior over a large temperature range. First-principles calculations reveal that, surprisingly, the flat phonon band calculated for the high-symmetry phase seems incompatible with the displacive character found experimentally. We discuss this unusual behavior in the context of an ideal Kittel model of an antiferroelectric transition.

Self-organization into ferroelectric and antiferroelectric crystals via the interplay between particle shape and dipolar interaction

Ferroelectricity and antiferroelectricity are widely seen in various types of condensed matter and are of technological significance not only due to their electrical switchability but also due to intriguing cross-coupling effects such as electro-mechanical and electro-caloric effects. The control of the two types of dipolar order has practically been made by changing the ionic radius of a constituent atom or externally applying strain for inorganic crystals and by changing the shape of a molecule for organic crystals. However, the basic physical principle behind such controllability involving crystal-lattice organization is still unknown. On the basis of a physical picture that a competition of dipolar order with another type of order is essential to understand this phenomenon, here we develop a simple model system composed of spheroid-like particles with a permanent dipole, which may capture an essence of this important structural transition in organic systems. In this model, we reveal that energetic frustration between the two types of anisotropic interactions, dipolar and steric interactions, is a key to control not only the phase transition but also the coupling between polarization and strain. Our finding provides a fundamental physical principle for self-organization to a crystal with desired dipolar order and realization of large electro-mechanical effects.

Critical scattering and incommensurate phase transition in antiferroelectric PbZrO3 under pressure

Antiferroelectric lead zirconate is the key ingredient in modern ferroelectric and piezoelectric functional solid solutions. By itself it offers opportunities in new-type non-volatile memory and energy storage applications. A highly useful and scientifically puzzling feature of this material is the competition between the ferro- and antiferroelectric phases due to their energetic proximity, which leads to a challenge in understanding of the critical phenomena driving the formation of the antiferroelectric structure. We show that application of hydrostatic pressure drastically changes the character of critical lattice dynamics and enables the soft-mode-driven incommensurate phase transition sequence in lead zirconate. In addition to the long known cubic and antiferroelectric phases we identify the new non-modulated phase serving as a bridge between the cubic and the incommensurate phases. The pressure effect on ferroelectric and incommensurate critical dynamics shows that lead zirconate is not a single-instability-driven system.

A unified description of the double perovskite family Sr2MWO6 within a rigid ion model

The sequence of phase transitions and structural instabilities of the Sr₂MWO₆ double perovskites are investigated using a rigid ion model. The parametrization of the short range empirical potential allows the control of the cation sizes by means of independent parameters, and in particular, the effective size of the M cation can be tuned to reproduce the behaviour of the whole family. The coupling of symmetry modes and its role in the stability of the phases are discussed, and molecular dynamics simulations are carried out to determine structural phase transitions as a function of temperature. A satisfactory agreement between experiments and ab initio calculations is obtained for the relevant range of ionic radii and temperatures, indicating that the range of stability of the structures is mainly governed by steric effects.

Lattice dynamics in the paraelectric phase of PbHfO₃ studied by inelastic x-ray scattering

We report the results of an inelastic x-ray scattering study of the lattice dynamics in the paraelectric phase of the antiferroelectric lead hafnate PbHfO3. The study reveals an avoided crossing between the transverse acoustic and transverse optic phonon modes propagating along the [1 1 0] direction with [1  -1 0] polarization. The static susceptibility with respect to the generally incommensurate modulations is shown to increase on cooling for the entire Γ-M direction. We consider different approaches to the data analysis that correspond to different models for the temperature evolution of the dynamic susceptibility function. A number of similarities and differences between the lattice dynamics of PbHfO3 and PbZrO3 are described.

Phase transitions and interrelated instabilities in PbHfO3 single crystals

PbHfO(3) is investigated theoretically and experimentally with respect to possible precursor effects starting in the paraelectric phase far above the cubic to tetragonal phase transition temperature. The theoretical modeling within the polarizability model predicts a giant softness of the system with spatially large polar and antiferrodistortive domain formation which compete with each other. These predictions are substantiated by the experiments, where the softness and the precursor effects are confirmed by birefringence, dielectric permittivity measurements and elastic properties by Brillouin scattering. The intermediate phase is found to have the polar nature confirmed by P-E hysteresis loop measurements, which is another manifestation of the competition between interrelated instabilities, namely a polar one and an antiferroelectric one.