Universal symmetry property of point defects in crystals

Defect dipole stretching enables ultrahigh electrostrain

Piezoelectric actuators have been extensively utilized as micro-displacement devices because of their advantages of large output displacement, high sensitivity, and immunity to electromagnetic interference. Here, we propose a straightforward approach to design <110>-oriented defect dipoles by introducing A-site vacancies and oxygen vacancies in (K0.48Na0.52)0.99NbO2.995 ceramics. As a result, we achieve ultrahigh electrostrains of 0.7% at 20 kV cm-1 (with an effective piezoelectric strain coefficient d33* = 3500 pm V-1), outperforming the performance of existing piezoelectric ceramics at the same driving field. The exceptional electrostrain is primarily attributed to the large stretching of defect dipoles when subjected to an applied electric field, a phenomenon that has been experimentally confirmed. Moreover, the strong interaction between these defect dipoles and <110> spontaneous polarizations plays a critical role in minimizing hysteresis and ensuring excellent fatigue resistance. Our findings present a practical and effective strategy for developing high-performance piezoelectric materials tailored for advanced actuator applications.

Phase Structure and Electrical Properties of Sm-Doped BiFe0.98Mn0.02O3 Thin Films

Bi_1−xSm_xFe_0.98Mn_0.02O₃ (x = 0, 0.02, 0.04, 0.06; named BSFMx) (BSFM) films were prepared by the sol-gel method on indium tin oxide (ITO)/glass substrate. The effects of different Sm content on the crystal structure, phase composition, oxygen vacancy content, ferroelectric property, dielectric property, leakage property, leakage mechanism, and aging property of the BSFM films were systematically analyzed. X-ray diffraction (XRD) and Raman spectral analyses revealed that the sample had both R3c and Pnma phases. Through additional XRD fitting of the films, the content of the two phases of the sample was analyzed in detail, and it was found that the Pnma phase in the BSFMx = 0 film had the lowest abundance. X-ray photoelectron spectroscopy (XPS) analysis showed that the BSFMx = 0.04 film had the lowest oxygen vacancy content, which was conducive to a decrease in leakage current density and an improvement in dielectric properties. The diffraction peak of (110) exhibited the maximum intensity when the doping amount was 4 mol%, and the minimum leakage current density and a large remanent polarization intensity were also observed at room temperature (2Pr = 91.859 μC/cm²). By doping Sm at an appropriate amount, the leakage property of the BSFM films was reduced, the dielectric property was improved, and the aging process was delayed. The performance changes in the BSFM films were further explained from different perspectives, such as phase composition and oxygen vacancy content.

Hydrogen Impurities in ZnO: Shallow Donors in ZnO Semiconductors and Active Sites for Hydrogenation of Carbon Species

ZnO, as a low-cost yet significant semiconductor, has been widely used in solar energy conversion and optoelectronic devices. In addition, Cu/ZnO-based catalysts can convert syngas (H₂, CO, and CO₂) into methanol. However, the main concern about the intrinsic connection between the physical and chemical properties and the structure of ZnO still remains. In this work, efforts are made to decipher the physical and chemical information encoded into the structure. Through using NMR-IR techniques, we, for the first time, report a new ZnO model with three H⁺ cations incorporated into one Zn vacancy. ¹H magic-angle spinning NMR and IR spectra demonstrate that Ga³⁺ cations are introduced into the Zn vacancies of the ZnO lattice, which replace the H⁺ cation, and thus further confirm the feasibility of our proposed model. The exchange between the H⁺ cation in Zn vacancies and the D₂ gas phase shows that ZnO can activate H₂ because of the quantized three H⁺ cations in the defect site.

Experimental Observation of Vacancy-assisted Martensitic Transformation Shift in Ni-Fe-Ga Alloys

Positron annihilation lifetime spectroscopy is used to experimentally demonstrate the direct relationship between vacancies and the shift of the martensitic transformation temperature in a Ni_{55}Fe_{17}Ga_{28} alloy. The evolution of vacancies assisting the ordering enables shifts of the martensitic transformation up to 50 K. Our results confirm the role that both vacancy concentration and different vacancy dynamics play in samples quenched from the L2_{1} and B2 phases, which dictate the martensitic transformation temperature and its subsequent evolution. Finally, by electron-positron density functional calculations V_{Ni} is identified as the most probable vacancy present in Ni_{55}Fe_{17}Ga_{28}. This work evidences the capability of vacancies for the fine-tuning of the martensitic transformation temperature, paving the way for defect engineering of multifunctional properties.

Internal Electric Field in Co-Doped BaTiO3 With Co2+/3+, Nb5+, Li+, and F-: Impact on Functional Properties and Charge Compensation With Niobium and Fluorine Ions

Dense barium titanate (BaTiO₃) ceramics ( [Formula: see text]) with a microscale grain size are obtained at 800 °C-1100 °C by a solid-state ceramic process. BaTiO₃ (BT) doped with Co^2+/3+ leads to a significant improvement in the properties ( pC/N). Soft and hard characteristics of the piezoceramics are observed depending on the dopant ions. The Co/Li acceptor dopants lead to hard piezoceramics and aging phenomena. Aged BT:Co, Li exhibits double loops and a distorted hysteresis cycle for nonpoled and poled ceramics, respectively. Ceramics poled by the increasing field process at room temperature and the field cooling process present different poled and aged states, which are dependent on the thermal history and poling process. The distorted hysteresis loops for BT:Co, Li indicate an increased internal bias field with aging time. Insertion of donor dopants, such as Nb⁵⁺ ions, significantly reduces the internal field. These behaviors are related to the presence of defect dipoles ( [Formula: see text]"- [Formula: see text] due to the insertion of acceptor dopants in the B-sites following the oxygen vacancies to equilibrate charge compensation. BT:Co sintered with LiF leads to a quasi-symmetric hysteresis loop, indicating that F^- may insert into an oxygen site and counteract the formation of oxygen vacancies. Dielectric drift of BT:Co, Li shows resilience to an ac electric field, which is related to the increased internal field. BT doped with 0.75 mol% Co^2+/3+ and 1 mol% Li₂CO₃ presents hard piezoelectric behavior with a Rayleigh coefficient α = 2.53 10^-7 m/V and the capability to handle high electrical stress of up to 400 [Formula: see text]/mm.

On the piezoelectric efficiency of rotator and extender ferroelectrics in random polycrystalline aggregates with various engineered domain states

Phase transition between polar polymorphs may result in piezoelectric enhancement.

Improving the Functional Control of Aged Ferroelectrics Using Insights from Atomistic Modeling

We provide a fundamental insight into the microscopic mechanisms of the aging processes. Using large-scale molecular dynamics simulations of the prototypical ferroelectric material PbTiO_{3}, we demonstrate that the experimentally observed aging phenomena can be reproduced from intrinsic interactions of defect dipoles related to dopant-vacancy associates, even in the absence of extrinsic effects. We show that variation of the dopant concentration modifies the material's hysteretic response. We identify a universal method to reduce loss and tune the electromechanical properties of inexpensive ceramics for efficient technologies.

Impact of the volume change on the ageing effects in Cu-Al-Ni martensite: experiment and theory

The time evolution of the physical properties of martensite during martensite ageing is traditionally explained by the symmetry-conforming short-range order (SC-SRO) principle, which requires the spatial configuration of crystal defects to follow the symmetry change of the host lattice. In the present study, we show that the volume change of the host lattice also contributes to the ageing effects in Cu-Al-Ni shape memory alloy besides the symmetry change. To substantiate this statement the gradual increase of the storage modulus with time at constant temperature was measured by dynamic mechanical analysis (DMA) and the experimental results were quantitatively described in the framework of the symmetry-conforming Landau theory of martensitic transformations in a crystal with defects. The comparison of experimental and theoretical results confirmed that the time dependence of the storage modulus is caused by two different physical mechanisms. Evaluations showing that the first mechanism is driven by the spontaneous symmetry change and the second mechanism is caused by the volume change after the martensitic transformation was carried out.

Slow aging dynamics and avalanches in a gold-cadmium alloy investigated by x-ray photon correlation spectroscopy

Results of a x-ray photon correlation spectroscopy experiment on the very weakly first order martensitic transformation of a Au50.5Cd49.5 single crystal are presented. Slow non-equilibrium-dynamics are observed in a narrow temperature interval in the direct vicinity of the otherwise athermal phase transformation. These dynamics are associated with the martensite-aging effect. The dynamical aging is accompanied by an avalanchelike behavior which is identified with an incubation-time phenomenon.

In situ observation of thermally activated domain memory and polarization memory in an aged K⁺-doped (Ba, Sr)TiO₃ single crystal

Different ferroelectric domains are degenerate states of the same ferroelectric phase; thus they are energetically equivalent and, in principle there exists no preference for a particular domain pattern. However, the existence of point defects is considered to stabilize certain preferential domain states. In order to study the temperature violation on such stabilized domains, we performed in situ observation on an aged K⁺-doped (Ba, Sr)TiO₃ single crystal and found that both the domain configuration and polarization state can be memorized after experiencing a thermally activated ferro-para-ferro transition cycle, as manifested by a reappearance of the same domain pattern and double P-E hysteresis loop. In contrast, after the sample was aged in the paraelectric state (>10 min), these memory effects disappeared. The above memory effects are considered to originate from the interaction between point defects and the crystal symmetry driven by a symmetry-conforming tendency of point defects. Such a mechanism suggests that the memory effects are relevant to the existence of acceptor dopant and associated mobile oxygen vacancies, and they are not restricted to a particular dopant. Thus similar memory effects are expected to exist in a wide range of ferroelectric materials with acceptor doping.

The Symmetry-Conforming Theory of Martensite Aging

The Landau theory has been developed for the description of martensite aging. The characteristic features of the theory are: i) the multicomponent non-scalar character of the order parameter describing the slow reconfiguration of lattice defects after martensitic transformation (MT); ii) the complete agreement with Symmetry-Conforming Short-Range-Order principle formulated by X. Ren and K. Otsuka; iii) the applicability to the different MT-s and various defects related to aging phenomena. The physical values interpreted as the components of internal stress, which stabilizes certain variant of martensitic phase, have been composed of the components of slow non-scalar order parameter. An applicability of the developed theory to the description of influence of aging on the MT temperature and yield stress was demonstrated.

Shape memory effect and superelasticity in a strain glass alloy

The shape memory effect and superelasticity are usually found in alloys exhibiting spontaneous martensitic transformation. Thus it is hard to imagine that such interesting effects can appear in a system without a martensitic transformation. In this Letter we show shape memory and the superelasticity effect in a nonmartensitic Ti48.5Ni51.5 alloy, which has no martensitic transformation but undergoes a "strain glass" transition. In situ x-ray diffraction experiment showed that the shape memory and superelasticity in strain glass stem from a stress-induced strain glass to martensite transformation and its reverse transformation. The new shape memory and superelasticity in strain glass extends the regime of the shape memory effect and superelasticity and may lead to novel applications.

Large electric-field-induced strain in ferroelectric crystals by point-defect-mediated reversible domain switching

Ferroelectric crystals are characterized by their asymmetric or polar structures. In an electric field, ions undergo asymmetric displacement and result in a small change in crystal dimension, which is proportional to the applied field. Such electric-field-induced strain (or piezoelectricity) has found extensive applications in actuators and sensors. However, the effect is generally very small and thus limits its usefulness. Here I show that with a different mechanism, an aged BaTiO(3) single crystal can generate a large recoverable nonlinear strain of 0.75% at a low field of 200 V mm(-1). At the same field this value is about 40 times higher than piezoelectric Pb(Zr, Ti)O(3) (PZT) ceramics and more than 10 times higher than the high-strain Pb(Zn(1/3)Nb(2/3))O(3)-PbTiO(3) (PZN-PT) single crystals. This large electro-strain stems from an unusual reversible domain switching (most importantly the switching of non-180 degrees domains) in which the restoring force is provided by a general symmetry-conforming property of point defects. This mechanism provides a general method to achieve large electro-strain effect in a wide range of ferroelectric systems and the effect may lead to novel applications in ultra-large stroke and nonlinear actuators.