Perovskite Unit Cell Research Articles

Investigating structural, dielectric and energy storage properties of NaNbO3 based ferroelectric ceramics

Sodium niobate (NaNbO3) based dielectric materials are getting recognition for the electric energy storage applications due to their promising ferroelectric/anti-ferroelectric properties. In the present work, (0.95-x)NaNbO3-0.05SrTiO3-xBi(Fe1/3Zn1/3Ti1/3)O3 (abbreviated as NNSTBFZT, x=0.04, 0.07 and 0.10) lead-free ferroelectric ceramics were synthesized using conventional solid-state reaction technique. A magnificent recoverable energy storage density (Wrec) and an ultrahigh energy storage efficiency (η) have been observed for x4 (∼1.05 J/cm3) and x10 (∼85 %) samples, respectively which reliant on the non-equivalent replacement of ions at the A and B sites of perovskite unit cell. Further, the phase investigation has been studied using X ray diffraction and Raman spectroscopy, which elucidate the effect of incorporation of the substituent ions at host matrix properly. A moderate dielectric constant with ultralow losses has been obtained and all the samples show diffuse phase transition, which is in favor of enhanced energy storage parameters. Moreover, the morphological analysis indicates decrease in grain size ∼ 1 µm, which also reflects the increased breakdown strength responsible for enhancing energy storage capacity. Therefore, proposed sodium niobate based ceramics become favorable for high energy dielectric capacitors applications.

Growth of KNbO3 Single Crystals by the Flux Method Using KBO2 as a Flux

KNbO3 single crystals are grown by the self-flux method using K2CO3 as a flux, but often suffer from discolouration. In this work, KNbO3 single crystals were grown by the flux method using KBO2 as a flux. KNbO3 powder was prepared by the solid-state reaction of K2CO3 and Nb2O5. KBO2 was fabricated by the reaction of K2B4O7·4H2O and K2CO3. Single crystals of KNbO3 were grown in a Pt crucible and the structure and dielectric properties of the single crystals were investigated. X-ray diffraction showed the KNbO3 single crystals to have an orthorhombic Cmm2 perovskite unit cell at room temperature. The existence of ferroelastic domains was revealed by transmission electron microscopy. Electron probe microanalysis showed the single crystals to be stoichiometric and contain small amounts of B. Differential thermal analysis, Raman scattering and impedance spectroscopy were used to study the phase transitions. KBO2 may be a suitable flux for the growth of KNbO3 single crystals.

Pressure-induced band gap engineering and enhanced optoelectronic properties of non-toxic Ca-based perovskite CsCaCl3: Insights from density functional theory

This study investigates the impact of hydrostatic pressure on the structural, electronic, optical, and mechanical properties of the cubic halide perovskite CsCaCl3. The pressure effect reduces the interatomic distance, which has a significant impact on this perovskite's lattice constant and unit cell volume. The electronic band gap narrows under pressure from the ultraviolet to the visible range, making it simpler to transport electrons from the valence band to the conduction band and increasing the efficiency of optoelectronic devices. Additionally, at around 50 GPa pressure, the band gap nature changes from indirect to direct, making the material better suited for use in optoelectronic applications. A deep optical study has shown that CsCaCl3 might be used in surgical tools, integrated circuits, QLED, OLED, solar cells, waveguides, and solar heat reduction materials. The mechanical stability of CsCaCl3 under the full range of applied pressure is confirmed by the elastic constants, which perfectly follow the Born stability criteria, the CsCaCl3 phase is proven to be stable across the whole applied pressure range by the tolerance factor "t", and support the thermodynamic stability achieved by the negative values of formation enthalpy. External pressure has a major impact on the mechanical properties, making this compound more ductile.

Modulation of electric and magnetic properties in 0.70BiFeO3-0.30BaTi0.9O3 ceramics with controlled oxygen vacancies and Fe2+/Fe3+ ratio

The physical properties of transition-metal oxides are closely related to defect concentrations. Herein, the modulation of oxygen vacancies and Fe2+/Fe3+ ratio on the electric and magnetic properties in Ti non-stoichiometry 0.70BiFeO3-0.30BaTi0.9O3 ceramics were investigated systematically. As the concentration of oxygen vacancies decreases, grain size increases and the perovskite unit cell with rhombohedral phase becomes more distorted. The maximum dielectric constant gradually increases and the relaxation factor decreases with decreasing oxygen vacancy concentration. The enhancement of ferroelectricity is consistent with the degree of the distortion of the perovskite lattice, and the reduction of internal bias field comes from the reduction of the defect dipoles in the ceramics. At low oxygen partial pressure, Fe2+ defects are preferred and form the double exchange interaction between Fe2+ and Fe3+ ions, which will enhance the magnetization.

Ordered deficient perovskite La2/3TiO3 films grown via molecular beam epitaxy

As the parent compound of a promising solid electrolyte material Li3xLa2/3−xTiO3, the perovskite La2/3TiO3 has potential for advancing research on Li-intercalated ionic conductors. Epitaxial La2/3TiO3 films have been grown by molecular beam epitaxy using a growth process consisting of deposition and annealing cycles, with in situ monitoring by electron diffraction. X-ray absorption spectroscopy confirms the tetravalent state of Ti in La2/3TiO3, and the as-grown films are insulating. X-ray diffraction reveals the presence of half-order peaks, indicating a doubling of the pseudocubic perovskite unit cell due to the ordering of La vacancies in alternating A-site layers. These results demonstrate that single-phase, vacancy-ordered epitaxial films of La2/3TiO3 can be stabilized with excellent crystalline and electronic properties over wafer-sized areas, making possible Li-ion intercalation studies in films with well-defined domain boundary properties. Such boundaries are known to profoundly influence Li-ion conduction within the material. Understanding the effects of domain boundaries on Li-ion conduction could lead to improvements in solid-state battery technology and pave the way for the development of more efficient and safer energy storage devices.

Superior Capacitive Energy-Storage Performance in Pb-Free Relaxors with a Simple Chemical Composition

Chemical design of lead-free relaxors with simultaneously high energy density (Wrec) and high efficiency (η) for capacitive energy-storage has been a big challenge for advanced electronic systems. The current situation indicates that realizing such superior energy-storage properties requires highly complex chemical components. Herein, we demonstrate that, via local structure design, an ultrahigh Wrec of 10.1 J/cm3, concurrent with a high η of 90%, as well as excellent thermal and frequency stabilities can be achieved in a relaxor with a very simple chemical composition. By introducing 6s2 lone pair stereochemical active Bi into the classical BaTiO3 ferroelectric to generate a mismatch between A- and B-site polar displacements, a relaxor state with strong local polar fluctuations can be formed. Through advanced atomic-resolution displacement mapping and 3D reconstructing the nanoscale structure from neutron/X-ray total scattering, it is revealed that the localized Bi enhances the polar length largely at several perovskite unit cells and disrupts the long-range coherent Ti polar displacements, resulting in a slush-like structure with extremely small size polar clusters and strong local polar fluctuations. This favorable relaxor state exhibits substantially enhanced polarization, and minimized hysteresis at a high breakdown strength. This work offers a feasible avenue to chemically design new relaxors with a simple composition for high-performance capacitive energy-storage.

Direct Observation of Transient Structural Dynamics of Atomically Thin Halide Perovskite Nanowires

Halide perovskite is a unique dynamical system, whose structural and chemical processes happening across different timescales have significant impact on its physical properties and device-level performance. However, due to its intrinsic instability, real-time investigation of the structure dynamics of halide perovskite is challenging, which hinders the systematic understanding of the chemical processes in the synthesis, phase transition, and degradation of halide perovskite. Here, we show that atomically thin carbon materials can stabilize ultrathin halide perovskite nanostructures against otherwise detrimental conditions. Moreover, the protective carbon shells enable atomic-level visualization of the vibrational, rotational, and translational movement of halide perovskite unit cells. Albeit atomically thin, protected halide perovskite nanostructures can maintain their structural integrity up to an electron dose rate of 10,000 e-/Å2·s while exhibiting unusual dynamical behaviors pertaining to the lattice anharmonicity and nanoscale confinement. Our work demonstrates an effective method to protect beam-sensitive materials during in situ observation, unlocking new solutions to study new modes of structure dynamics of nanomaterials.

Local Ionic Displacements and Polarization at the Paraelectric‐Ferroelectric Heterointerface: Effect of Octahedral Distortion

AbstractAt the ferroelectric ABO3 perovskite heterointerface, the broken translational symmetry significantly alters the octahedral network and cation sublattices. Experimentally revealing the interaction between the polarization and atomic structure parameters is beneficial not only for understanding the novel physical phenomena but also for realizing exotic physical functionalities at the materials surface science. Here, it is demonstrated that the induced polarization can be realized by the cooperative interaction between the octahedral distortion and the cation sublattices at the paraelectric‐ferroelectric heterointerface in the (K,Na)NbO3 epitaxial thin film. It is found that the cation–cation displacement scales with the tetragonality of the perovskite unit cell but not with the polarization, which is minimal in the substrate but increases substantially in the thin film. Nevertheless, the oxygen octahedral distortion in the substrate, accompanied by octahedral rotation, is stronger than that in the film, resulting in a translated and stable polarization across the interface. These results demonstrate the octahedral network plays a significant role in the ferroelectric polarization and can also greatly advance the understanding of the heterointerface science for ferroelectrics.

Ferroelectric polarization of tetragonal BiFeO3—an approach from DFT calculations for BiFeO3–BaTiO3 superlattices—

Density functional theory calculations are conducted for tetragonal BiFeO3–BaTiO3 superlattices to investigate the influence of electronic structures on ferroelectric spontaneous polarization (P s). When the number of the perovskite unit cells in one layer (N) is decreased below 10, the P s starts to decrease from the volume-averaged one (50.9 μC cm−2) and eventually becomes half at N = 1. In the BiFeO3 cell (N = ∞) with a large P s (73.3 μC cm−2), a covalent bond arising from a Bi_6p-O_2p orbital interaction is extended through a Bi–O network and stereo-chemical lone-pair electrons of Bi are accommodated in the opposite direction of the polar c axis. In the superlattice with N = 1, the Bi–O network cannot be developed by the presence of Ba, and then the Bi–O bond becomes ionic. We show that the large P s of the BiFeO3 cell originates from the Bi_6p-O_2p mixing superimposed on the stereo-chemical nature of the lone-pair electrons of Bi.

Industrially Produced Fe- and Mn-Based Perovskites: Effect of Synthesis on Reactivity in Three-Way Catalysis: Part 2.

Mn-based perovskites obtained by two different industrial procedures [flame spray pyrolysis (FSP) and co-precipitation (COP)] have been extensively compared in terms of chemical, structural, and morphological properties with the aim of evaluating how the upscale of complex catalysts can affect the functionality. The transition between laboratory and production scale is, in fact, usually not straightforward. The catalytic activity was tested focusing on reactions of relevance in the abatement of pollutants. In particular, CO-assisted NO reduction (which could be also considered as a model reaction) and reactions with a synthetic automotive exhaust mixture, including 10% steam and oxygen, were carried out. The development of three-way catalysts is still a relevant question: noble metal-free, efficient catalysts are even more necessary in hybrid vehicles. For this purpose, the catalytic activity of the samples has been correlated with the characterization results and thus with the peculiar aspects of the production method. Relevant differences have been observed between COP and FSP catalysts, in terms of the specific surface area, surface composition, and presence of surface-active sites. Also, the different efficiencies of inserting dopants in the perovskite unit cell and thus in reducibility and ion mobility are relevant. Despite having the same composition and crystalline structure, the catalytic activity and the effect of pre-treatments are observed to depend on the production procedure.

Polarization and Dielectric Properties of BiFeO3-BaTiO3 Superlattice-Structured Ferroelectric Films.

Superlattice-structured epitaxial thin films composed of Mn(5%)-doped BiFeO3 and BaTiO3 with a total thickness of 600 perovskite (ABO3) unit cells were grown on single-crystal SrTiO3 substrates by pulsed laser deposition, and their polarization and dielectric properties were investigated. When the layers of Mn-BiFeO3 and BaTiO3 have over 25 ABO3 unit cells (N), the superlattice can be regarded as a simple series connection of their individual capacitors. The superlattices with an N of 5 or less behave as a unified ferroelectric, where the BaTiO3 and Mn-BiFeO3 layers are structurally and electronically coupled. Density functional theory calculations can explain the behavior of spontaneous polarization for the superlattices in this thin regime. We propose that a superlattice formation comprising two types of perovskite layers with different crystal symmetries opens a path to novel ferroelectrics that cannot be obtained in a solid solution system.

Gold-Cage Perovskites: A Three-Dimensional AuIII-X Framework Encasing Isolated MX63- Octahedra (MIII = In, Sb, Bi; X = Cl-, Br-, I-).

The Cs8AuIII4MIIIX23 (M = In3+, Sb3+, Bi3+; X = Cl-, Br-, I-) perovskites are composed of corner-sharing Au-X octahedra that trace the edges of a cube containing an isolated M-X octahedron at its body center. This structure, unique within the halide perovskite family, may be derived from the doubled cubic perovskite unit cell by removing the metals at the cube faces. To our knowledge, these are the only halide perovskites where the octahedral sites do not bear an average 2+ charge. Charge compensation in these materials requires a stoichiometric halide vacancy, which is disordered around the Au atom at the unit-cell corner and orders when the crystallization is slowed. Using X-ray crystallography, X-ray absorption spectroscopy, and pair distribution function analysis, we elucidate the structure of this unusual perovskite. Metal-site alloying produces further intricacies in this structure, which our model explains. Compared to other halide perovskites, this class of materials shows unusually low absorption onset energies ranging between ca. 1.0 and 2.4 eV. Partial reduction of Au3+ to Au+ affords an intervalence charge-transfer band, which redshifts the absorption onset of Cs8Au4InCl23 from 2.4 to 1.5 eV. With connected Au-X octahedra and isolated M-X octahedra, this structure type combines zero- and three-dimensional metal-halide sublattices in a single material and stands out among halide perovskites for its ordering of homovalent metals, ordering of halide vacancies, and incorporation of purely trivalent metals at the octahedral sites.

Ruddlesden‐Popper Manganites: Tailoring c‐Axis Orientation in Epitaxial Ruddlesden–Popper Pr0.5Ca1.5MnO4 Films (Adv. Mater. Interfaces 7/2021)

For Ruddlesden-Popper Pr0.5Ca1.5MnO4 thin films grown on STO (100) the c-axis lies in-plane and two possible configurations are observed. On the atomic level the film consists of perovskite (yellow) and rock-salt (blue) sub unit cell blocks forming a network throughout the film. Both lattice and packing induced stress fields result in formation of perpendicularly aligned cracks (mirroring the micro-structure). More details can be found in article number 2002049 by Sarah Hoffmann-Urlaub and co-workers.

Design of dispersion-free dielectrics as engineering perovskite unit cells of KCa2Li(n-3)NbnO3n+1 ceramics

Dion–Jacobson-phase perovskites have received continuous attention owing to their impressive properties such as the ability to convert them into a two-dimensional (2D) form. We report a new Dion–Jacobson phase-layered perovskite KCa2LiNb4O13 ceramics prepared by a solid-state synthesis method. The KCa2LiNb4O13 ceramic exhibited a large dielectric permittivity of 61 at 1 MHz, which is three times higher than that of KCa2Nb3O10 (εr = 20), owing to an increase in the number of octahedral units (n), which causes considerably high polarization in perovskite lattices. Moreover, as engineering octahedral units by lithium ion addition instead of sodium ions, KCa2LiNb4O13 ceramic showed improved and stable dielectric properties over a wide frequency range. Moreover, the exfoliated form of 2D Ca2LiNb4O13 nanosheets was successfully synthesized via a two-step cation exchange method.

Photovoltaic Recovery of All Printable Mesoporous‐Carbon‐based Perovskite Solar Cells

Lead halide perovskites attract much attention in recent years as a realistic solution for efficient and low‐cost solar cells. One of the interesting solar cell structures is the fully mesoporous‐carbon‐based perovskite solar cells. The mesoporous layers can be fabricated entirely by screen printing with the potential for upscaling. Herein, the two‐step deposition of perovskite in mesoporous‐carbon‐based perovskite solar cells is studied. The influence of the dipping time on the photovoltaic parameters is investigated using charge extraction and intensity‐modulated photovoltage spectroscopy (IMVS) measurements. A power conversion efficiency of 15% is observed for cells fabricated using two‐step deposition which is one of the highest reported for this solar cell structure. Stability characterizations at maximum power point (MPP) tracking show degradation with time, however a complete recovery of the devices in the dark is revealed. Analyzing the mechanism for this shows that the perovskite's unit cell shrinks during the recovery process due to internal stress relief. This interesting phenomenon opens the possibility to optimize the stability of these solar cells for commercial applications.

Improving electrical properties and toughening of PZT-based piezoelectric ceramics for high-power applications via doping rare-earth oxides

Ceramics show brittle nature because of ionic and covalent bondings. However, the need for reliable high-power applications with vibrational displacements under high electric fields and stress loadings demands robust piezoelectric ceramics. In this work, three series of Lead Zirconium Titanate-based piezoelectric ceramics for high-power applications are synthesized via solid solution reaction technique. The microstructures and phases are manipulated by doping different rare-earth oxides Nd2O3, Dy2O3 and Yb2O3. Although all as-sintered piezoelectric ceramics show a perovskite structure with a coexistence of rhombohedral and tetragonal phases, phase fraction, grain size distribution and attendant electrical and mechanical properties vary depending on the doping species and contents. The comprehensive properties of optimized piezoelectric ceramics for high-power applications are: d33 = 292–308 pC/N, kp = 54.2–57.0%, Qm = 320–400, ε33T/ε0 = 1140–1210 and KIC = 1.55–1.60 MPa m1/2. These properties are improved by 19.0%–27.0% compared to the original composition without doping. The toughening mechanism by doping rare-earth oxides is discussed. Nd3+, Dy3+ and Yb3+ substitution and compensation at A-sites of perovskite unit cells generate extrinsic vacancies besides the intrinsic vacancies caused due to PbO evaporation during sintering, which make the domain wall highly mobile and result in softened ferroelectrics. Our results show an efficient route to toughen the PZT-based piezoelectric ceramics and enhance electrical properties synergistically via doping appropriate amounts of rare-earth oxides.

Orientation effects and links between hydrostatic parameters in piezo-active 2–0–2 composites

The paper reports results on the large hydrostatic parameters of novel three-component 2–2-type composites based on [011]-poled relaxor-ferroelectric single crystals. The role of the rotation of the main crystallographic axes X and Y in forming the considerable hydrostatic piezoelectric response is studied. The composite consists of the single-crystal and 0–3 corundum ceramic/polyethylene layers that are regularly arranged along the non-polar axis. 0.72Pb(Mg1/3Nb2/3)O3–0.28PbTiO3 and 0.93Pb(Zn1/3Nb2/3)O3–0.07PbTiO3 single crystals poled along [011] of the perovskite unit cell are considered as main piezoelectric components of the composites with 2–0–2 connectivity. Links between the hydrostatic parameters (piezoelectric coefficients and squared figure of merit and electromechanical coupling factor ) are analysed by taking into account specifics of the microgeometry and elastic anisotropy of the 0–3 layer. Due to the large values of the aforementioned hydrostatic parameters, the studied 2–0–2 composites are of interest for hydroacoustic and piezoelectric energy-harvesting applications.

Influence of Na and Na/Fe doping on the dielectric constant, ferromagnetic and sunlight photocatalytic properties of BaTiO3 perovskite

Perovskite BaTiO3, Ba0.98Na0.02TiO3, Ba0.98Na0.02Ti0.97Fe0.03O3 and Ba0.98Na0.02Ti0.95Fe0.05O3 ceramics have been prepared by solid state reaction method. The XRD analysis of these compositions revealed that a single-phase was formed having tetragonal crystal structure. A contraction of the perovskite unit cell volume was observed upon incorporation of 2 ​at% Na and 3 or 5 ​at% Fe. This contraction confirms that Na+ ions substitute Ba2+ cation while Fe3+ ions replace Ti4+ of BaTiO3. Inclusion of Na+ ions into BaTiO3 increases the dielectric constant while Fe3+ codoping decreases it. On contrast, incorporation of Fe3+ ions enhances the ferromagnetic properties of BaTiO3. Na/Fe codoping leads to formation of oxygen vacancies in BaTiO3 accompanied by new energy bands in visible light region. We show the suitability of such band-engineered Na/Fe codoped BaTiO3 for sunlight photocatalytic application. The results of photocatalytic experiments showed that both Ba0.98Na0.02Ti0.97Fe0.03O3 and Ba0.98Na0.02Ti0.95Fe0.05O3 exhibit higher photocatalytic activity than undoped and Na doped BaTiO3.

Broadband critical dynamics in disordered lead-based perovskites**Article In Memoriam of Professor Roger A Cowley.

Materials based on the cubic perovskite unit cell continue to provide the basis for technologically important materials with two notable recent examples being lead-based relaxor piezoelectrics and lead-based organic–inorganic halide photovoltaics. These materials carry considerable disorder, arising from site substitution in relaxors and molecular vibrations in the organic–inorganics, yet much of our understanding of these systems derives from the initial classic work of Prof. Roger A Cowley, who applied both theory and neutron scattering methods while at Chalk River Laboratories to the study of lattice vibrations in SrTiO3. Neutron scattering continues to play a vital role in characterizing lattice vibrations in perovskites owing to the simple cross section and the wide range of energy resolutions achievable with current neutron instrumentation. We discuss the dynamics that drive the phase transitions in the relaxors and organic–inorganic lead-halides in terms of neutron scattering and compare them to those in phase transitions associated with a ‘central peak’ and also a soft mode. We review some of the past experimental work on these materials and present new data from high-resolution time-of-flight backscattering spectroscopy taken on organic–inorganic perovskites. We will show that the structural transitions in disordered lead-based perovskites are driven by a broad frequency band of excitations.

Phase Transitions in the Metastable Perovskite Multiferroics BiCrO3 and BiCr0.9Sc0.1O3: A Comparative Study

The temperature behavior of the crystal structure as well as dielectric and magnetic properties of the perovskite bismuth chromate ceramics with the 10 mol % Cr3+-to-Sc3+ substitution were studied and compared with those of the unmodified compound. Using a high-pressure synthesis, BiCrO3 and BiCr0.9Sc0.1O3 were obtained as metastable perovskite phases which are monoclinic C2/c with the √6ap × √2ap × √6ap superstructure (where ap is the primitive perovskite unit-cell parameter) under ambient conditions. At room temperature, the unit cell volume of BiCr0.9Sc0.1O3 is ∼1.3% larger than that of BiCrO3. Both perovskites undergo a reversible structural transition into a nonpolar GdFeO3-type phase (orthorhombic Pnma, √2ap × 2ap × √2ap) in the temperature ranges of 410-420 K (BiCrO3) and 470-520 K (BiCr0.9Sc0.1O3) with a relative jump of the primitive perovskite unit cell volume of about -1.6 and -2.0%, respectively. Temperature dependences of the complex dielectric permittivity demonstrate anomalies in the phase transition ranges. The Pnma-to-C2/c crossover in BiCrO3 is accompanied by a decrease in the direct current (dc) conductivity, while in BiCr0.9Sc0.1O3 the conductivity increases. The onset of an antiferromagnetic order in BiCr0.9Sc0.1O3 is observed at the Néel temperature (TN) of about 85 K as compared with TN = 110 K in BiCrO3. In contrast to BiCrO3, which exhibits a spin reorientation at Tsr ∼ 72 K, no such a transition occurs in BiCr0.9Sc0.1O3.