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In quantum materials, phenomena occurring at the subatomic level can manifest as properties on a macroscopic scale. The exploration of quantum effects in materials with nontrivial band topology and strongly correlated electrons holds great promise for technological advancement in fields such as quantum computing. This thesis examines properties of three such quantum materials using computational methods.The primary material system investigated is strontium titanate, an incipient ferroelectric that gives rise to an unconventional superconducting state at exceptionally low doping levels. The polar phase can be stabilized through strain or chemical substitution. Remarkably, superconductivity is enhanced within the polar phase, suggesting that the polar instability plays a pivotal role in the superconducting pairing mechanism. We develop a simplified free energy model combined with statistical mechanics methods to assess the character of the polar transition, which we find to be neither order-disorder nor displacive. We explore the effects of doping on the structural phase transitions and find that, in agreement with experiment, the polar distortion and formation of polar nanodomains are suppressed in the presence of free carriers, while antiferrodistortive order remains essentially unchanged. The single-domain nature and insensitivity to doping suggest that the antiferrodistortive order does not play an important role in Cooper pairing. By calculating electronic properties in the polar phase, we analyze parameters that are relevant to superconductivity such as the density of states at the Fermi level, the Rashba splitting of the energy bands, and the Migdal ratio.We explore the chemical phase space of the naturally occurring minerals herbertsmithite [ZnCu3(OH)6Cl2] and Zn-substituted barlowite [ZnCu3(OH)6BrF], which both feature perfect kagome layers of spin-1/2 copper ions and display experimental signatures consistent with a quantum spin liquid state at low temperatures. To identify other possible candidates within this material family, we perform a systematic first-principles combinatorial exploration of structurally related compounds [ACu3(OH)6B2 and ACu3(OH)6BC] by substituting nonmagnetic divalent cations (A) and halide anions (B, C). We select several promising candidate materials that we believe deserve further attention.Finally, we examine CsV3Sb5, a member of the AV3Sb5 (A = K,Rb,Cs) family of kagome metals, whose low-energy physics is dominated by an unusual charge density wave phase. We elucidate the nature of the charge density wave order parameter using first-principles density functional theory calculations which support the findings of experimental coherent phonon spectroscopy measurements. Through our study of the structural phase space of CsV3Sb5, we find that the charge density wave can be described as tri-hexagonal ordering with interlayer modulation along the c-axis.First-principles techniques are often limited by their inability to incorporate the effects of temperature and disorder. Here, we augment first-principles density functional calculations using statistical mechanics methods such as the Metropolis Monte Carlo algorithm and Langevin dynamics to incorporate temperature effects on large, disordered supercells to simulate the thermal phase space of strontium titanate. The chemical phase of the herbertsmithite material family is systematically explored through high-throughput first-principles pseudo-convex hull calculations and an assessment of defect formation energy. We use frozen phonon calculations to investigate the structural phase space of CsV3Sb5 and find that the charge density wave order expands beyond the previously studied 2x2x1 construction. Our techniques can be applied more broadly to other material systems to expand the capabilities of computational methods to accurately capture thermal effects and structural disorder in quantum materials.

Oxide materials, including perovskite oxides, are well known for hosting a range of properties, such as ferroelectricity and superconductivity, which are significant for both fundamental understanding and technological applications. The interplay among structural, orbital, charge, and spin factors in these oxides provides a platform for studying and manipulating material properties that can be incorporated into devices for daily life, scientific studies, and industry. This motivation drives the development of atomic-level understanding of the phenomena exhibited by oxide materials, helping in the formulation of design principles to achieve desired properties. In this dissertation, I will explore different properties, including ferroelectricity, piezoelectricity, and magnetism, exhibited by various oxide materials with complex crystal structures through first principles density functional theory calculations and group theoretic arguments.Layered perovskites are structures formed by the rearrangement of perovskite structures, with other structural layers interspersed between them. Various families of layered perovskite oxide ferroelectrics demonstrate a coupling between polarization and structural order parameters, particularly involving octahedral rotation distortions. The Aurivillius-phase oxides SrBi2B2O9 (B=Ta, Nb) are recognized for exhibiting such order-parameter couplings and are renowned for their excellent room temperature ferroelectric performance. After providing a brief introduction to the physics of layered oxides in Chapter 1 and the theoretical methods used in our research in Chapter 2, I will discuss the results of our calculations which explore the ferroelectric switching processes of SrBi2B2O9 to identify low-energy switching paths in Chapter 3. This chapter also elucidates the roles of coupled order parameters in determining the complex ferroelectric domain structure in SrBi2 B2 O9.Non-polar structural distortions, like octahedral rotations, present additional degrees of freedom for manipulating ferroelectric properties in Aurivillius phases compared to traditional ferroelectrics such as BaTiO3 and PbTiO3. In Chapter 4, the results of first-principles calculations on the Aurivillius-phase oxide Bi2WO6 are explored. The chapter elucidates the ferroelectric switching mechanism and inves- tigates the coupling of electrical polarization with the isolated spin of a magnetic dopant (Fe3+) in Bi2WO6. It details the change in spin directionality along the ferroelectric switching pathways and emphasizes the roles of crystalline symmetry in such coupling.In addition to demonstrating geometry-driven ferroelectricity, layered perovskites like Ruddlesden-Popper phases also exhibit piezoelectricity. However, there has been minimal study of piezoelectricity in Ruddlesden-Popper phases. Chapter 5 will highlight our efforts to establish a microscopic understanding of piezoelectricity by exploring the roles of the interplay between strain and structural distortions on piezoelectric coefficients in representative Ruddlesden-Popper phases, including Ca3Ti2O7, Sr3Zr2O7, and Sr3Sn2O7.Some layered oxides host intriguing phenomena like frustrated magnetism together with the ferroelectricity. Recent experiments have highlighted the rare earth oxide TbInO3 for its simultaneous manifestation of improper ferroelectricity and frustrated magnetism, making it a potential spin liquid candidate material of interest to the materials science and physics communities. While spin liquids are anticipated to exhibit superconductivity upon suitable doping, the influence of doping on the electrical conductivity of TbInO3 remains an open question. Chapter 6 will unveil the outcomes of our investigations into the doping effect in hexagonal TbInO3. The dissertation will wrap up with a concise exploration of future directions in Chapter 7.

The transmission electron microscope (TEM) provides multiple modes to study structure-property relationships in materials. In particular, quantum materials, whose properties may be affected by subtle alterations to atomic configuration, require such a valuable instrument for illuminating these relationships. Scanning transmission electron microscopy (STEM) and convergent beam electron diffraction (CBED) are two techniques within the TEM that may be utilized to examine the atomic structure and crystal symmetries of materials. Cadmium arsenide (Cd3As2) is classified as a topological semimetal due to its linear band crossings in the bulk electronic band structure and surface states. Different publications in the past have proposed competing centrosymmetric and non-centrosymmetric crystal structures for Cd3As2, an important distinction that has major implications on the expected band structure and underlying physics. Due to the complicated crystal structure of Cd3As2, as well as the limitations in x-ray diffraction techniques in identifying an inversion center, particularly in thin films, we used CBED to determine the exact point group symmetries of epitaxially grown Cd3As2, finding it to possess an inversion center. Because Cd3As2 possesses fourfold rotational symmetry and an inversion center, it can be classified as a Dirac semimetal. We sought to topologically tune Cd3As2 by introducing biaxial epitaxial stress, producing tensile and compressively stressed films. Using CBED, we found that fourfold rotational symmetry was lost in compressively strained, (112)-oriented Cd3As2, promoting the opening of a band gap and a topological insulator state. We also found that inversion symmetry was broken in tensile strained, (001)-oriented Cd3As2, which may lead to band splitting away from the node and suggest previous studies proposing a non-centrosymmetric crystal structure may have been influenced by residual strain.Strontium titanate (SrTiO3) is an incipient ferroelectric material in which a ferroelectric state may be realized through strain. Of interest in this thesis is realization of the ferroelectric state by inducing compressive epitaxial stress. SrTiO3 also undergoes a superconducting transition at low temperatures, though the mechanism by which this occurs is not completely understood, nor is the relationship between superconductivity and ferroelectricity in this material. Past studies have intimated a relationship between nano-sized polar domains that form at room temperature and these phenomena. Namely, the size of the polar nanodomains may be considered a predictor of certain superconducting parameters. In this study, the SrTiO3 film thickness was varied, and STEM was used to identify Ti-site displacements and to map out the polar domains. We found that polar nanodomains existed in only the thickest film (70 nm), but were screened in all others. The two thickest films (70 nm and 40 nm) underwent ferroelectric and superconducting transitions, while thinner films did not. A critical thickness is predicted to exist between 25 nm and 40 nm for these phenomena in compressively strained SrTiO3 films.

Although superconductivity in strontium titanate (SrTiO3) was discovered in the 1960s, the origin of superconductivity in SrTiO3 has not been completely understood. Moreover, its unusual superconducting characteristics have made SrTiO3 a fascinating playground to potentially study multiple aspects of unconventional superconductivity. For example, SrTiO3 is one of the most dilute superconductors where standard Bardeen-Cooper-Schrieffer (BCS theory) is believed to be no longer valid. Furthermore, the presence of superconductivity and ferroelectricity and their interplay possibly leads to an unconventional pairing mechanism in SrTiO3. In this dissertation, high-quality, doped, ferroelectric, compressively strained SrTiO3 thin films were grown using hybrid molecular beam epitaxy (MBE) to unveil unique superconducting properties. First, we present unconventional characteristics of relatively thick (160-180 nm) strained SrTiO3 films. In some strained films, we show signatures of unusual superconducting states such as in-plane critical fields far above the Pauli limiting field and second harmonic resistances. The study of superconductivity in ultra-thin SrTiO3 films is hindered by strong surface carrier depletion, which makes thin films insulating. We solve this issue by introducing thin (~10 nm) EuTiO3 capping layers. We show that the EuTiO3 capping layer effectively prevents strong carrier depletion in SrTiO3 films and that a capped 40 nm-thick SrTiO3 film becomes superconducting. We utilize the EuTiO3 capping layer to elucidate the connection between superconductivity and ferroelectricity in SrTiO3 films. Systematic film thickness study shows that superconductivity and ferroelectricity are suppressed simultaneously in the SrTiO3 films of thicknesses less than 40 nm. We speculate that this result implies that broken inversion symmetry (spin-orbit coupling) plays an important role in the superconductivity of strained SrTiO3 films.

The central material of interest for this thesis is strontium titanate (SrTiO3). Doped SrTiO3 exhibits a wide range of remarkable properties. The focus of this work is to investigate its ferroelectric and superconducting behavior. We utilize scanning transmission electron microscopy (STEM) to understand the role of the nanoscale SrTiO3 structure in these properties. SrTiO3 is an incipient ferroelectric in unstrained, pure form, but easily becomes ferroelectric when subjected to small perturbations. Understanding the nature of the ferroelectric transition in SrTiO3 is essential as it can play an important role in other properties. A hallmark of order-disorder transitions is the formation of polar domains above the Curie temperature. These polar regions percolate below the Curie temperature to form a long-range ordered ferroelectric state. Using high-angle annular dark-field STEM and analyzing off-centering of Ti columns, we show local polar regions at room temperature in compressively strained SrTiO3 films, highlighting the order-disorder nature of the ferroelectric transition in this material. Next, we focus on understanding the competition between mobile carriers and polar crystal distortions. Elucidating the nature of this competition is of great interest for polar superconductors, which have attracted significant interest for their potential to host unconventional superconducting states. We observe a systematic suppression of the nanodomains and ferroelectricity with increasing amount of Sm dopant atoms. The itinerant electrons screen polar distortions and disrupt the nanodomains above the Curie temperature. The results provide direct evidence that long-range Coulomb interactions, already present in the paraelectric phase, are driving the ferroelectric transition and are becoming increasingly short-ranged with doping. Moreover, we show that the disorder caused by the dopant atoms themselves presents a second contribution to the destabilization of the ferroelectric state. This disorder destroys the nanodomains at high dopant concentrations.The focus of the second part of this thesis concerns the superconductivity of SrTiO3. Doped SrTiO3 is a superconductor whose pairing mechanism is still not fully understood. Superconductivity in SrTiO3 occurs in the vicinity of a ferroelectric instability, and recent theories suggest a link between two orders. Here, by using atomic structure imaging, we find that the superconducting critical temperature correlates with the length scale of polar order. The superconducting transition temperature is enhanced when polar nanodomains are sufficiently large. In these cases, the Cooper pairs reside in a non-centrosymmetric environment with strong spin-orbit coupling. The findings point to the length scale of polar nanodomains and spin-orbit coupling as important parameters controlling the superconductivity of SrTiO3. We investigate the robustness of superconductivity to magnetic and nonmagnetic disorder and discuss the importance of findings in explaining the unconventional nature of superconductivity in this material.

Sub-10 nm ferroelectric and multiferroic materials are attracting increased scientific and technological interest, owing to their exciting physical phenomena and prospects in miniaturized electronic devices, neuromorphic computing, and ultra-compact data storage. The Bi6Ti2.9Fe1.5Mn0.6O18 (B6TFMO) Aurivillius system is a rare example of a multiferroic that operates at room temperature. Since the formation of magnetic impurity phases can complicate attempts to measure ferromagnetic signal intrinsic to the B6TFMO multiferroic phase and thus limits its use, herein we minimize this by utilizing relatively large (49%) bismuth excess to counteract its volatility during sub-10 nm growth. X-ray diffraction, electron microscopy, and atomic force microscopy show sample crystallinity and purity are substantially improved on increasing bismuth excess from 5 to 49%, with the volume fraction of surface impurities decreasing from 2.95–3.97 vol% down to 0.02–0.31 vol%. Piezoresponse force microscopy reveals 8 nm B6TFMO films are ferroelectric, with an isotropic random distribution of stable in-plane domains and weaker out-of-plane piezoresponse. By reducing the volume fraction of magnetic impurities, this work demonstrates the recent progress in the optimization of ultra-thin B6TFMO for future multiferroic technologies. We show how the orientation of the ferroelectric polarization can be switched in 8 nm B6TFMO and arrays can be “written” and “read” to express states permitting anti-parallel information storage.

Ferroelectric HfO2-based materials hold great potential for widespread integration of ferroelectricity into modern electronics due to their robust ferroelectric properties at the nanoscale and compatibility with the existing Si technology. Earlier work indicated that the nanometer crystal grain size was crucial for stabilization of the ferroelectric phase of hafnia. This constraint caused high density of unavoidable structural defects of the HfO2-based ferroelectrics, obscuring the intrinsic ferroelectricity inherited from the crystal space group of bulk HfO2. Here, we demonstrate the intrinsic ferroelectricity in Y-doped HfO2 films of high crystallinity. Contrary to the common expectation, we show that in the 5% Y-doped HfO2 epitaxial thin films, high crystallinity enhances the spontaneous polarization up to a record-high 50 µC/cm2 value at room temperature. The high spontaneous polarization persists at reduced temperature, with polarization values consistent with our theoretical predictions, indicating the dominant contribution from the intrinsic ferroelectricity. The crystal structure of these films reveals the Pca21 orthorhombic phase with a small rhombohedral distortion, underlining the role of the anisotropic stress and strain. These results open a pathway to controlling the intrinsic ferroelectricity in the HfO2-based materials and optimizing their performance in applications.

To upgrade the electric properties of lead-free piezoceramics, (1–x)(Ba0.98Ca0.02Ti0.94Sn0.04Zr0.02)O3-xY2O3 (abbreviated as (1–x)BCTSZ-xY, x = 0 mol%, 0.02 mol%, 0.04 mol%, 0.06 mol%, 0.08 mol% and 0.1 mol%) ceramics were successfully synthesized by traditional solid-state sintering method. The phase structure and microstructure of ceramics were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and piezoresponse force microscopyeramics (PFM). The electric properties of ceramics were researched through piezoelectric, dielectric and ferroelectric test instruments. The results show that all samples have pure perovskite structure and favorable electric properties. The optimal electric properties which especially include superior ferroelectric properties are gained when Y2O3 content is 0.06 mol% (d33 = 419 pC/N, kp = 52%, Tc = 89.5 °C, er = 26900, tanδ = 2.86%, Pr = 14.41 μC/cm2, Ec = 1.8 kV/cm). Moreover, the temperature-dependent dielectricity of samples shows apparent relaxor behavior under different frequencies. The Curie–Weiss law further proves that all samples are typical relaxor ferroelectrics, and the relaxor degree of samples decreases with increase of Y2O3 content. In conclusion, Y2O3 plays a significant role in enhancing electric properties of BCTSZ ceramics.

Ferroelectric polymer poly(vinylidene fluoride) (PVDF) shows excellent electro-activity and is promising for flexible electronic devices. However, the processing of PVDF into the favourable ferroelectric structure (β-phase) presents difficulties, while its copolymer with trifluoroethylene (PVDF-TrFE) can directly crystallize into β-phase, but shows limited thermal stability and high-cost processing. As a result, an easily implementable method, pressing-and-folding (P&F), was used to produce highly compatible blended films of PVDF and PVDF-TrFE without using any hazardous solvent or complex polymer processing equipment. Hot-pressed PVDF (molecular weight: 530 kg/mol) and PVDF-TrFE (molar ratio: 51/49) films were firstly stacked before undergoing P&F treatment. Compared to extrusion-blended films before and after P&F, the P&F stacked films showed isotropic crystalline structure of β-phase, as confirmed using X-ray diffraction and infrared spectroscopy. The ferroelectric remnant polarization of the P&F stacked films is 0.068 C/m2, surpassing pure PVDF-TrFE (0.062 C/m2) and the simulated value of remnant polarization of pure PVDF (～0.065 C/m2). The above findings promise to provide inspirations for new processing strategy on PVDF based functional polymers.

Off-stoichiometry of perovskite structural Bi0.5Na0.5TiO3 (BNT) ferroelectrics can give rise to considerable oxide-ion conductivity. The inherent structural characteristics are urgent to be resolved due to its particular sensitivity of the conduction mechanism to the nominal composition and synthesis process. Herein, a thorough study of the temperature-dependent neutron, X-ray diffraction and Raman spectrum is carried out on a series of equivalently substituted A-site deficient non-stoichiometric and pristine BNT. Phase transition and defect association are systemically investigated in these dominated rhombohedral phases at room temperature, associated with well saturated ferroelectric states. Significant structural evolution identified by Rietveld refinements and the origin of the electrical performance are clarified at elevated temperatures, focusing on the subtle distortions of ionic displacements, oxygen octahedral tilts and local chemical environments for oxygen vacancies. The ion migration ability mediated by oxygen vacancies that are not energetically favorable in BNT mainly depends on the external substitutional disorder, and is strongly affected by the dopant concentration. Together with the lone pair substitution concept, superior oxide ionic conductivity is achieved, and an alternative strategy is provided in designing BNT based oxide ion conductors.

NaNbO3-based ceramics usually show ferroelectric-like P-E loops at room temperature due to the irreversible transformation of the antiferroelectric orthorhombic phase to ferroelectric orthorhombic phase, which is not conducive to energy storage applications. Our previous work found that incorporating CaHfO3 into NaNbO3 can stabilize its antiferroelectric phase by reducing the tolerance factor (t), as indicated by the appearance of characteristic double P-E loops. Furthermore, a small amount of MnO2 addition effectively regulate the phase structure and tolerance factor of 0.94NaNbO3-0.06CaHfO3 (0.94NN-0.06CH), which can further improve the stability of antiferroelectricity. The XRD and XPS results reveal that the Mn ions preferentially replace A-sites and then B-sites as increasing MnO2. The antiferroelectric orthorhombic phase first increases and then decreases, while the t shows the reversed trend, thus an enhanced antiferroelectricity and the energy storage density Wrec of 1.69 J/cm3 at 240 kV/cm are obtained for 0.94NN-0.06CH-0.5%MnO2(in mass fraction). With the increase of Mn content to 1.0% from 0.5%, the efficiency increases to 81% from 45%, although the energy storage density decreases to 1.31 J/cm3 due to both increased tolerance factor and non-polar phase.

Nowadays, photovoltaic effect has been widely studied in various ferroelectric materials due to its applications as optoelectronic devices. In this work, with BiFeO3 (BFO) films as the photovoltaic materials, we report the effects of Eu3+ doping content on the phase structure, ferroelectric and optical properties of BFO films grown on Ca0.96Ce0.04MnO3/YAlO3 (001) substrate. We found that a small doping level of 0.05 could induce a phase change of BFO from tetragonal to rhombohedral, due to the shrinking of the lattice upon Eu3+ doping and the breaking of surface terrace structure induced by Ca0.96Ce0.04MnO3 layer. This results in a sharp band gap reduction from 3.30 eV to 2.60 eV, and a decrease in the coercivity of ferroelectric polarization switching. Based on these findings, we investigate the photovoltaic effects of ITO/EuxBi1-xFeO3/Ca0.96Ce0.04MnO3 vertical capacitors. It is found that the short-circuit current density (Jsc) decreases with increasing Eu3+ doping, whereas the open-circuit voltage (Voc) first increases to a level of 0.1 V and then decreases with further Eu3+ doping. This could be explained by the combined effect of Schottky-junction and depolarization field on the photovoltaic process. Our research suggests that a moderate Eu3+ doping is helpful for enhancing the photovoltaic effect of BFO thin film devices.

As a high-k material, hafnium oxide (HfO2) has been used in gate dielectrics for decades. Since the discovery of polar phase in Si-doped HfO2 films, chemical doping has been widely demonstrated as an effective approach to stabilize the ferroelectric phase in HfO2 based thin films. However, the extra capping layer deposition, post-growth annealing and wake-up effect are usually required to arouse the ferroelectricity in HfO2 based thin films, resulting in the increase of complexity for sample synthesis and the impediment of device application. In this study, the ferroelectricity is observed in non-capped dopant-free HfO2 thin films prepared by pulsed laser deposition (PLD) without post-growth annealing. By adjusting the deposited temperature, oxygen pressure and thickness, the maximum polarization up to 14.7 μC/cm2 was obtained in 7.4 nm-thick film. The fraction of orthorhombic phase, concentrations of defects and size effects are considered as possible mechanisms for the influences of ferroelectric properties. This study indicates that PLD is an effective technique to fabricate high-quality ferroelectric HfO2 thin films in the absence of chemical doping, capping layer deposition and post-growth annealing, which may boost the process of nonvolatile memory device application.

Owing to the complex composition architecture of these solid solutions, some fundamental issues of the classical (1−x)Bi1/2Na1/2TiO-xBi1/2K1/2TiO3 (BNT-xBKT) binary system, such as details of phase evolution and optimal Na/K ratio associated with the highest strain responses, remain unresolved. In this work, we systematically investigated the phase evolution of the BNT-xBKT binary solid solution with x ranging from 0.12 to 0.24 using not only routine X-ray diffraction and weak-signal dielectric characterization, but also temperature-dependent polarization versus electric field (P-E) and current versus electric field (I-E) curves. Our results indicate an optimal Na/K ratio of 81/19 based on high-field polarization and electrostrain characterizations. As the temperature increased above 100 °C, the x = 0.19 composition produces ultrahigh electrostrains (> 0.5%) with high thermal stability. The ultrahigh and stable electrostrains were primarily due to the combined effect of electric-field-induced relaxor-to-ferroelectric phase transition and ferroelectric-to-relaxor diffuse phase transition during heating. More specifically, we revealed the relationship between phase evolution and electrostrain responses based on the characteristic temperatures determined by both weak-field dielectric and high-field ferroelectric/electromechanical property characterizations. This work not only clarifies the phase evolution in BNT-xBKT binary solid solution, but also paves the way for future strain enhancement through doping strategies. © 2021 The Chinese Ceramic Society. This work was supported by the National Natural Science Foundation of China (Grant Nos. 51772239 and 51761145024 ) and the Fundamental Research Funds for the Central Universities ( XJTU ). The SEM work was done at International Center for Dielectric Research (ICDR), Xi'an Jiaotong University, Xi'an, China.

Herein, Ag1-3xSmxNbO3 (0 ≤ x ≤ 0.025) antiferroelectric ceramics were successfully synthesized by solid state methods. The effect of Sm3+ doping on the structure, property and energy storage performance were studied. With the increasing Sm3+ concentrations, the average grain size decreased. Meanwhile, the stability of high temperature M phases (i.e., the structure between Tf and T3) was expanded, which led to low loss for energy storage. Both of structure analysis and ferroelectric tests revealed the existence of weakly polar/AFE-like phase below Tf. The Sm3+ doping tended to suppress the ferroelectric behavior and expand the stability of antiferroelectricity. Consequently, a significantly enhanced energy storage performance (Wrec = 3.8 J/cm3, η = 73 %) could be achieved in Ag0.97Sm0.01NbO3 ceramic, which was almost 1.5 times larger than that in non-doped AgNbO3 (Wrec = 2.4 J/cm3, η = 45 %) under the similar applied field of 1705 kV/cm ± . In particular, the performance of the ceramic showed great temperature stability with variation of ± 5 % from 25 °C to 125 °C. These results indicated that the Ag0.97Sm0.01NbO3 ceramic could be an ideal lead-free candidate used in the energy storage field.

Na1/2Bi1/2TiO3-based materials exhibit potential for applications in high-power ultrasonics. The composites of Na1/2Bi1/2TiO3-yBaTiO3 (NBTyBT; y denotes mole%) with ZnO inclusions were demonstrated to stabilize a ferroelectric equilibrium that led to enhanced thermal depolarization temperature (Td) and increased mechanical quality factor (Qm). This work addresses the influence of the matrix NBTyBT phase by investigating two limiting choices based on symmetry (tetragonal/rhombohedral) and polar (relaxor/ferroelectric) nature. While the composites constituting the tetragonal NBT9BT (non-ergodic relaxor at room temperature) matrix phase exhibit improved Td, the critical temperatures in the composites with rhombohedral NBT3BT (displaying spontaneous ferroelectric order at room temperature) exhibit only marginal changes. Further, NBT3BT composites feature a 45% increase in Qm, while the corresponding increase is roughly three-fold for the NBT9BT composites. A 3-D Finite Element Method is used to simulate the electric field gradient at the matrix/inclusion interface, with the effective field distribution estimated to be higher than the applied field for highly conducting ZnO inclusions. The electrical properties indicate that, while the deviatoric stress at the matrix/inclusion interface stabilizes the ferroelectric equilibrium for the relaxor matrix phase, the stresses disrupt the long-range order for the ferroelectric matrix phase. These results establish the volume-limit of the second phase to stabilize a ferroelectric equilibrium, in addition to substantiating the role of residual stress evidenced by changes in the polar nature. Finally, a comparison of the composites with different NBTyBT phases is presented, with NBT6BT:ZnO composites demonstrating an optimal increase in both Td and Qm.

BiFe1-2xZnxMnxO3 (BFZMO, with x = 0–0.05) thin films were synthesized via sol–gel method. Effects of (Zn, Mn) co-doping on the structure, ferroelectric, dielectric, and optical properties of BiFeO3 (BFO) films were investigated. BFZMO thin films exhibit rhombohedral structure. Scanning electron microscopy (SEM) images indicate that co-doping leads to a decrease in grain size and number of defects. Leakage current density (4.60×10−6 A/cm2) of BFZMO film with x = 0.02 was found to be two orders of magnitude lower than that of pristine BFO film. Owing to decreased leakage current density, saturated P–E curves were obtained. Maximum double remnant polarization of 413.2 μC/cm2 was observed for BFZMO thin film with x = 0.02, while that for the BFO film was found to be 199.68 μC/cm2. The reason for improved ferroelectric properties is partial substitution of Fe ions with Zn and Mn ions, which resulted in a reduction in the effect of oxygen vacancy defects. In addition, co-doping was found to decrease optical bandgap of BFO film, opening several possible routes for novel applications of these (Zn, Mn) co-doped BFO thin films.

Advances in flexible electronics are driving dielectric capacitors with high energy storage density toward flexibility and miniaturization. In the present work, an all-inorganic thin film dielectric capacitor with the coexistence of ferroelectric (FE) and antiferroelectric (AFE) phases based on Pb0.96La0.04(Zr0.95Ti0.05)O3 (PLZT) was prepared on a 2D fluorophlogopite mica substrate via a simple one-step process. The flexible capacitor exhibits a high recoverable energy density (Urec) of ≈ 44.2 J/cm3, a large electric breakdown strength (EBDS) of 3011 kV/cm, excellent frequency stability (500 Hz-20 kHz) and high thermal stability over 30–190 °C. It also demonstrates an outstanding bending endurance, which can maintain a high energy storage performance under various bending radii (R = 2–10 mm) or 103 repeated bends at 4 mm. The FE phase is stable near the film surface and the interface with the bottom electrode. The AFE phase with multi-domains has incommensurate modulation structures with super-periodicity of 6.5, 6.9 and 5.2. It indicates that the PLZT/LNO/F-Mica capacitor has high potential for energy storage application and may provide great opportunities for exploring new energy storage materials.

In this work, we synthesized a novel and eco-friendly relaxor ferroelectric system, i.e., (1−x)(0.8Bi0.5Na0.4K0.1TiO3-0.2SrTiO3)-xNaNbO3 (BS-xNN) ceramics with x = 0.1–0.4 based on a solid state reaction technique. The structural, microstructural, dielectric as well as ferroelectric characteristics of BS-xNN ceramics were comprehensively examined. According to our findings, the Ti–O coupling reduces as the NN content increases, while the Nb–O coupling strengthens, leading in a nano-scale polarization mismatch-reestablishment process that enhances energy storage performance. Because of the reduction in thickness and porosity, the viscous polymer process significantly increases the breakdown strength of ceramic samples. More crucially, at 260 kV/cm, 0.6BS-0.4NN ceramics have an ultrahigh recoverable energy storage density Wrec of 4.44 J/cm3 and a high energy storage efficiency η of 81.8%. Furthermore, thermal stability of energy storage performance is also identified across a wide temperature range of 30 °C from 140 °C at 215 kV/cm. The higher performance of energy storage not only indicates the feasibility of our technique, but also serves as a model for the manufacturing of high-quality dielectric ceramics with a wide range of industrial applications.

Piezoelectric materials are widely used in electromechanical energy conversion, such as in sensors, transducers, and self-powered materials. In this paper, the influence of the Sm doping content on the microstructure and ferroelectric, piezoelectric, dielectric, and field-induced strain properties of 0.70Pb(Mg1/3Nb2/3)O3-0.30PbTiO3 (PMN-PT) ceramics was investigated. Sm-doped PMN-PT ceramics with both high piezoelectric properties (d33∼1406 pC/N) and a large electromechanical coupling coefficient (kp∼0.69) were synthesized. Based on their piezoelectric effect, a maximum output voltage of 31 V was achieved under external forces. The output voltages showed satisfactory stability, repeatability, and sensitivity under periodic external forces; hence, Sm-doped PMN-PT piezoelectric ceramics are potential candidates for energy conversion and signal monitoring.

This paper reports the improvement of electrical, ferroelectric and endurance of Hf0.5Zr0.5O2 (HZO) thin-film capacitors by implementing W electrode. The W/HZO/W capacitor shows excellent pristine 2Pr values of 45.1 μC/cm2 at ±6 V, which are much higher than those of TiN/HZO/W (34.4 μC/cm2) and W/HZO/TiN (26.9 μC/cm2) capacitors. Notably, the maximum initial 2Pr value of W/HZO/W capacitor can reach as high as 57.9 μC/cm2 at ±7.5 V. These strong ferroelectric polarization effects are ascribed to the W electrode with a fairly low thermal expansion coefficient which provides a larger in-plane tensile strain compared with TiN electrode, allowing for enhancement of o-phase formation. Moreover, the W/HZO/W capacitor also exhibits higher endurance, smaller wake-up effect (10.1%) and superior fatigue properties up to 1.5 × 1010cycles compared to the TiN/HZO/W and W/HZO/TiN capacitors. Such improvements of W/HZO/W capacitor are mainly due to the decreased leakage current by more than an order of magnitude compared to the W/HZO/TiN capacitor. These results demonstrate that capping electrode material plays an important role in the enhancement of o-phase formation, reduces oxygen vacancies, mitigates wake-up effect and improves reliability.

BiFeO3–BaTiO3–based solid solutions are promising candidates for high–temperature piezoelectric devices because of their high Curie temperature (TC) and considerable electrical properties. Here, we reported an optimum composition of 0.7Bi(Fe0.999Mn0.001)O3–0.3BaTiO3 ceramic with a large piezoelectric constant (d33) of 230 pC/N and a high TC of 505 °C, which was attributed to the intentional introducing of the ceramic with MnO and MnO2 mixture. Furthermore, an in situ d33 measurement was carried out, demonstrating excellent thermal stability for the 0.7Bi(Fe0.999Mn0.001)O3–0.3BaTiO3 specimen. The d33 remained above 200 pC/N in the temperature range of 25 °C–400 °C and its fluctuation was less than ± 15 %. It was determined that the high d33 in the 0.7BiFe0.999Mn0.001)O3–0.3BaTiO3 ceramic originated from a synergistic effect of rhombohedral distortion, intrinsic response, and ferroelectric order. The findings establish a solid correlation between electrical properties and phase/domain structure, and provide a novel approach to improve the piezoelectric properties for BiFeO3–BaTiO3–based ceramics.

BaTiO3 ceramics doped with double perovskite Sr2KMoO6 (BT-SKM) are fabricated via solid-state reaction technology. The effects of SKM dopants on the structure, band gap and electrical/magnetic properties of BT are systematically studied. XRD and Raman spectra analysis show polycrystalline perovskite structure of the samples, which confirms the structural changes. With the addition of SKM dopants, the grain size of the samples decreases significantly. The band gaps of doped BT samples reduce, and the minimum band gap of BT-SCM is 1.77 eV, which is apparently reduced compared with the band gap of pure BT of 3.22 eV. However, the ferroelectric properties are weakened in samples doped with SKM. This ascribes to the introduction of more oxygen vacancies by dopants, which impedes the switching of domains, resulting in deterioration of ferroelectric properties. Furthermore, ferromagnetism of BT-SNM is observed, which may be attributed to the long-range exchange interaction between Ni2+ ions and oxygen vacancies. These results reveal the potential applications of these perovskite oxides in photovoltaic and memory devices.

Recently, the anomalous photovoltaic effect of ferroelectric materials has attracted considerable attention in the construction of efficient solar cells owing to the above-bandgap photovoltage of these materials. In this study, we investigate the anomalous photovoltaic effect of relaxor-based ferroelectric Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 (PIMN-PT) ceramics with large remnant polarization and a narrow optical bandgap. Excellent photovoltaic performance with an ultrahigh open-circuit voltage of 23 V (575 V/cm) is achieved, which is higher than the open-circuit voltages of all reported polycrystalline materials with similar thickness. The phase structure, microstructure morphology, domain structure, ferroelectric and optical characteristics are analyzed, which could provide clues to the origin of the ultrahigh open-circuit voltage of PIMN-PT ceramics. The results suggest that the relaxor-based ferroelectric PIMN-PT system is a potential candidate for photovoltaic solar energy conversion devices.

Recently, there is a surge of research interest in configurable ferroelectric conductive domain walls which have been considered as possible fundamental building blocks for future electronic devices. In this work, by using piezoresponse force microscopy and conductive atomic force microscopy, we demonstrated the controlled manipulation of various conductive domain walls in epitaxial BiFeO3 thin films, e.g. neutral domain walls (NDW) and charged domain walls (CDWs). More interestingly, a specific type of nanoscale domains was also identified, which are surrounded by highly conductive circular CWDs. Similar nanoscale domains can also be controlled created and erasured by applying local field via conductive probe, which allow nondestructive current readout of different domain states with a large on/off resistance ratio up to 102. The results indicate the potential to design and develop high-density non-volatile ferroelectric memories by utilizing these programable conductive nanoscale domain walls.

As microelectronic devices move toward integration and miniaturization, the thin film capacitors with high energy density and charge/discharge efficiency have attracted immense interests in modern electrical energy storage systems. Despite morphotropic phase boundary (Na0.8K0.2)0.5Bi0.5TiO3-based lead-free materials with outstanding ferroelectric and piezoelectric properties, while large ferroelectric hysteresis with high remanent polarization (Pr) hinder to improve energy storage capability. Here, novel lead-free relaxor-ferroelectric (RFE) thin film capacitors with high energy density are successfully prepared in (1-x) (Na0.8K0.2)0.5Bi0.5TiO3-xBa0.3Sr0.7TiO3 [(1-x)NKBT-xBST] systems. Introducing BST into the NKBT systems is expected to reduce remanent polarization (Pr) on account of coupling reestablishment of the polar nano-regions (PNRs) and improving the relaxation behavior. As a result, 0.6NKBT-0.4BST thin film exhibits high energy density (Wrec ∼ 54.79 J/cm3) together with satisfactory efficiency (η ∼ 76.42%) at 3846 kV/cm. The stable energy storage performances are achieved within the scope of operating temperatures (20–200 °C) and fatigue cycles (1-107 cycles). This work furnishes a new technological way for the design of high energy-density thin film capacitors.

With the development of energy-storage technology and power electronics industry, dielectric capacitors with high energy density are in high demand owing to their high power density. Especially for ferroic dielectrics (including ferroelectrics and antiferroelectrics) showing dipole moments in the perovskite unit cells, large dielectric response under electric fields makes them own outstanding potentials for achieving high energy-storage density. Particularly, destroying long-range ferroelectric/antiferroelectric ordering by enhancing compositional disorder is found to be effective for extremely improving energy-storage properties (both high recoverable energy-storage density and efficiency), because these macroscopic-nonpolar nanodomains are highly polarizable under strong external electric fields. Moreover, owing to the heterogeneous structure induced dielectric relaxation characteristics, excellent temperature stability can be achieved simultaneously. The recent developments of (anti)polar nanoregions with different possible local symmetries in various lead-free perovskite-structured dielectrics have been summarized in this review, together with the achieved energy-storage properties based on them. This review would provide a guidance for preparing high-performance energy-storage capacitors by local-structure engineering for next-generation pulse power applications.

Lead-free ceramics with both high piezoelectric response and good temperature stability are urgently demanded for electromechanical conversion devices. Unfortunately, owing to coexistence of polymorphic phases near room temperature (RT), enhanced piezoelectric properties were usually achieved with occurrence of strong temperature dependence in modified BaTiO3 (BT)-based ceramics. In this work, we demonstrate that tailoring grain orientations of tetragonal BT-based ceramics can effectively produce substantially enhanced and thermally stabilized piezoelectric response. Both c- and c-oriented tetragonal (Ba0.85Ca0.15) (Zr0.05Ti0.95)O3 (BCZT) ceramics with texture degrees F>90% were synthesized via templated grain growth. Interestingly, the ceramics textured along the c polar axis show much higher microscopic and macroscopic piezoelectric properties than those with nonpolar c texture, indicating an “extender” ferroelectricity nature. Compared with randomly oriented samples, the c-oriented ceramics exhibit simultaneously ∼1.6 times higher piezoelectric strain d33∗ (∼760 pm/V), 4.4 times higher piezoelectric figure of merit d33×g33 (8.8 × 10−12 m2/N), and better temperature stability (strain variation ≤5% between RT and 110 °C). Such thermally stabilized strain response can be mainly attributed to wide temperature range of tetragonal phase and stable domain structure. This work provides a promising route for further developing lead-free piezoceramics with high and temperature-insensitive performance, which can greatly broaden their application areas.

Piezophotonics is a great interesting field of physics that has led to a number of important technologies, such as light source, smart sensors, and mechatronics. In this work, we reported Pr-doped (Bi0·5Na0.5)TiO3-based lead-free ceramics with strong red photoluminescence emission and large strain response (d33∗ = 460 pm/V, S = 0.32 %). The PL emission can be quenched by decreasing the intensity by 93 % after electrical polarization (E = 50 kV/cm). The local structure and electric field-induced structural changes were systematically investigated to reveal the significant distinction in photoluminescence properties caused by electrical polarization. The results indicated that polarization treatment eliminates the structural inhomogeneities and establishes a long-range ferroelectric tetragonal and rhombohedral distortion. The crystal structure transformed irreversibly from a non-ergodic to a normal ferroelectric state. PL quenching originated from the decreased distortion of octahedral due to the transition from a non-ergodic state to a highly ordered symmetrical structure. Meanwhile, the enlarged domain structure contributed to the photoluminescence quenching effect. Our findings demonstrate that an electric field can be a robust tool for adjusting the photoluminescence property and provide insights into the relationship between the structure and PL properties of BNT-based ceramics under an external stimulus.