Bismuth Ferrite-Based Lead-Free High-Entropy Piezoelectric Ceramics.

Piezoelectric ceramics, as essential components of actuators and transducers, have captured significant attention in both industrial and scientific research. The "entropy engineering" approach has been demonstrated to achieve excellent performance in lead-based materials. In this study, the "entropy engineering" approach was employed to introduce the morphotropic phase boundary (MPB) into the bismuth ferrite (BF)-based lead-free system. By employing this strategy, a serial of novel "medium to high entropy" lead-free piezoelectric ceramics were successfully synthesized, namely (1-x)BiFeO3-x(Ba0.2Sr0.2Ca0.2Bi0.2Na0.2)TiO3 (BF-xBSCBNT, x = 0.15-0.5). Our investigation systematically examined the phase structure, domain configuration, and ferroelectric/piezoelectric properties as a function of conformational entropy. Remarkable performances with a largest strain of 0.50% at 100 kV/cm, remanent polarization ∼40.07 µC/cm2, coercive field ∼74.72 kV/cm, piezoelectric coefficient ∼80 pC/N, and d33* ∼500 pm/V were achieved in BF-0.4BSCBNT ceramics. This exceptional performance can be attributed to the presence of MPB, coexisting rhombohedral and cubic phases, along with localized nanodomains. The concept of high-entropy lead-free piezoelectric ceramics in this study provides a promising strategy for the exploration and development of the next generation of lead-free piezoelectric materials.

In-gap states and strain-tuned band convergence in layered structure trivalent iridate K0.75Na0.25IrO2.

Iridium oxides (iridates) provide a good platform to study the delicate interplay between spin-orbit coupling (SOC) interactions, electron correlation effects, Hund's coupling and lattice degrees of freedom. An overwhelming number of investigations primarily focus on tetravalent (Ir4+, 5d5) and pentavalent (Ir5+, 5d4) iridates, and far less attention has been paid to iridates with other valence states. Here, we pay our attention to a less-explored trivalent (Ir3+, 5d6) iridate, K0.75Na0.25IrO2, crystallizing in a triangular lattice with edge-sharing IrO6 octahedra and alkali metal ion intercalated [IrO2]- layers, offering a good platform to explore the interplay between different degrees of freedom. We theoretically determine the preferred occupied positions of the alkali metal ions from energetic viewpoints and reproduce the experimentally observed semiconducting behavior and nonmagnetic (NM) properties of K0.75Na0.25IrO2. The SOC interactions play a critical role in the band dispersion, resulting in NM Jeff = 0 states. More intriguingly, our electronic structure not only uncovers the presence of intrinsic in-gap states and nearly free electron character for the conduction band minimum, but also explains the abnormally low activation energy in K0.75Na0.25IrO2. Particularly, the band edge can be effectively modulated by mechanical strain, and the in-gap states feature enhanced band-convergence characteristics by 6% compressive strain, which will greatly enhance the electrical conductivity of K0.75Na0.25IrO2. The present work sheds new light on the unconventional electronic structures of trivalent iridates, indicating their promising application as a nanoelectronic and thermoelectric material, which will attract extensive interest and stimulate experimental works to further understand the unprecedented electronic structures and exploit potential applications of the triangular trivalent iridate.

Adjusting the Energy-Storage Characteristics of 0.95NaNbO3-0.05Bi(Mg0.5Sn0.5)O3 Ceramics by Doping Linear Perovskite Materials.

Passive electronic components are an indispensable part of integrated circuits, which are key to the miniaturization and integration of electronic components. As an important branch of passive devices, the relatively low energy-storage capacity of ceramic capacitors limits their miniaturization. To solve this problem, this study adopts the strategy of doping linear materials, specifically CT, into 0.95NaNbO3-0.05Bi(Mg0.5Sn0.5)O3 (0.95NN-0.05BMS) ceramics to increase the disorder of the system through the nonequivalent substitution of A and B sites to achieve the sintering temperature and the residual polarization. Meanwhile, the breakdown electric field strength (Eb) is improved by adjusting the activation energy of the material and the relative density of the sample. Thus, an ultrahigh Wrec of 6.35 J/cm3 and a η of 80% are obtained at an Eb of 646 kV/cm. Additionally, through the analysis of the dielectric temperature spectrum, it is found that the 0.88(0.95NN-0.05BMS)-0.12CT sample can satisfy the technical standards of general ceramic Z5U and patch ceramic X6R. The performance of the ceramics also remains stable within a temperature range of 20-200 °C, a frequency range of 1-100 Hz, and 104 cycles. The charge and discharge tests of the ceramics show that the t0.9 of the sample floats between 1.02 and 1.04 µs, which illustrates its potential application in the field of pulsed power components.

Ferroelastic lattice rotation and band-gap engineering in quasi 2D layered-structure PdSe2 under uniaxial stress.

Transition metal dichalcogenides (TMDCs) have attracted extensive attention in recent years for their novel physical and chemical properties as well as promising applications in the future. In the present paper, based on first-principles simulations, we focused on the bulk of the TMDC material PdSe2 and provided new insights into its unique structural properties and electronic structures under uniaxial stress. For the first time, we revealed that this orthorhombic PdSe2 is an intrinsic ferroelastic material with stress-driven 90° lattice rotation in the layer stacking direction. Strikingly, the ferroelastic phase transition originated from the bond reconstructions in the unusual square-planar (PdSe4)2- structural units. Specifically, low switching barriers and strong ferroelastic signals rendered room-temperature shape memory accessible. Moreover, the ferroelastic phase transition was accompanied with semiconductor-to-metal-to-semiconductor transitions under uniaxial compressive stress, which could be applied in electronic switching devices. In addition, the band gap was closely associated to the interlayer spacing, which could be engineered by the uniaxial tensile stress. These outstanding stress-engineered properties suggest that orthorhombic PdSe2 is a promising material for potential applications in microelectromechanical and nanoelectronic devices.

New Insights into the Electronic Structure and Photoelectrochemical Properties of Nitrogen-Doped HNb3O8 via a Combined in Situ Experimental and DFT Investigation.

The nitrogen-doping approach has been intensively adopted to improve various properties of metal oxides, especially for adjusting the energy band structure and extending the photoresponse range of oxide photocatalysts. However, the nitrogen doping behavior is still unintelligible and complex due to the diversity of compositions and crystal structures. In this work, new insights into the electronic structure and photoelectrochemical (PEC) properties of nitrogen-doped HNb3O8 were presented. On the one hand, we utilized an in situ experimental strategy to ascertain the effect of nitrogen doping on the energy band and photoelectrochemical (PEC) properties of HNb3O8 and nitrogen-doped HNb3O8 (N-HNb3O8). Their energy band level, donor densities, and interfacial charge transfer properties were studied by Mott-Schottky plots and electrochemical impedance spectroscopy. After nitrogen doping, the conduction band position is unusually descended by 0.23 eV, the valance band position is raised by 0.51 eV, the donor density (Nd) is increased from 3.71 × 1021 to 6.46 × 1021 cm-3, and interfacial charge transfer efficiency is reduced, though. On the other hand, density functional theoretical calculations were also conducted, so as to understand the electronic structures of HNb3O8 and N-HNb3O8. After nitrogen doping, the electronic structure is modified due to the upshift of the valance band edge consisting of hybrid N 2p and O 2p orbitals and the downshift of the conduction band edge consisting of the H 1s and Nb 4d orbitals. Furthermore, these insights into the behavior of nitrogen-doped semiconductors have important guiding significance toward their potential applications.

Facile preparation of protonated hexaniobate nanosheets and its enhanced photocatalytic activity.

Exfoliated hexaniobate nanosheets E-H2K2Nb6O17-x (E-HKNO) with broad light absorption (up to 850 nm) and high adsorption properties were prepared via ion exchange and transient annealing processes with micron-size K4Nb6O17 powders as the precursor. The as-prepared E-HKNO nanosheets show excellent visible light photodegradation performances when compared to degussa P25, which was evaluated in terms of degradation of Rhodamine B (Rh B). High adsorption and broad light absorption characteristics could be attributed to the exfoliation behavior and the reduction of surface Nb5+ to Nb4+, which was confirmed by x-ray photoelectron spectroscopy (XPS) and Raman spectra. From the Mott-Schottky analysis, the E-HKNO is an n-type semiconductor and has a higher flat band voltage (-0.46 V versus RHE at pH = 7), compared with K4Nb6O17. In addition, the electrochemical impedance spectroscopy (EIS) indicates that the E-HKNO nanosheets have an increased semiconductor-electrolyte charge transfer resistance, which is not conducive to the separation of photogenerated carriers (e--h+). Accordingly, a small amount of holes scavenger (EDTA) was added to improve the photodegradation performance of the E-HKNO, since the holes scavenger can inhibit the recombination of the photogenerated carriers. This work provides not only a facile method for the preparation of an efficient E-HKNO nanosheets photocatalyst, but also new insights for further enhancing the photodegradation performance by adding trace scavenger.