Topotactically transformable antiphase boundaries with enhanced ionic conductivity

Engineering lattice defects have emerged as a promising approach to effectively modulate the functionality of devices. Particularly, antiphase boundaries (APBs) as planar defects have been considered major obstacles to optimizing the ionic conductivity of mixed ionic-electronic conductors (MIECs) in solid oxide fuel applications. Here our study identifies topotactically transformable APBs (tt-APBs) at the atomic level and demonstrates that they exhibit higher ionic conductivity at elevated temperatures as compared to perfect domains. In-situ observation at the atomic scale tracks dynamic oxygen migration across these tt-APBs, where the abundant interstitial sites between tetrahedrons facilitate the ionic migration. Furthermore, annealing in an oxidized atmosphere can lead to the formation of interstitial oxygen at these APBs. These pieces of evidence clearly clarify that the tt-APBs can contribute to oxygen conductivity as anion diffusion channels, while the topotactically non-transformable APBs cannot. The topotactic transformability opens the way of defect engineering strategies for improving ionic transportation in MIECs.

Research Progress on the Application of Topological Phase Transition Materials in the Field of Memristor and Neuromorphic Computing

Topological phase transition materials have strong coupling between their charge, spin orbitals, and lattice structure, which makes them have good electrical and magnetic properties, leading to promising applications in the fields of memristive devices. The smaller Gibbs free energy difference between the topological phases, the stable oxygen vacancy ordered structure, and the reversible topological phase transition promote the memristive effect, which is more conducive to its application in information storage, information processing, information calculation, and other related fields. In particular, extracting the current resistance or conductance of the two-terminal memristor to convert to the weight of the synapse in the neural network can simulate the behavior of biological synapses in their structure and function. In addition, in order to improve the performance of memristors and better apply them to neuromorphic computing, methods such as ion doping, electrode selection, interface modulation, and preparation process control have been demonstrated in memristors based on topological phase transition materials. At present, it is considered an effective method to obtain a unique resistive switching behavior by improving the process of preparing functional layers, regulating the crystal phase of topological phase transition materials, and constructing interface barrier-dependent devices. In this review, we systematically expound the resistance switching mechanism, resistance switching performance regulation, and neuromorphic computing of topological phase transition memristors, and provide some suggestions for the challenges faced by the development of the next generation of non-volatile memory and brain-like neuromorphic devices based on topological phase transition materials.

Control of Oxygen Vacancy Ordering in Brownmillerite Thin Films via Ionic Liquid Gating

Oxygen defects and their atomic arrangements play a significant role in the physical properties of many transition metal oxides. The exemplary perovskite SrCoO_3-δ (P-SCO) is metallic and ferromagnetic. However, its daughter phase, the brownmillerite SrCoO_2.5 (BM-SCO), is insulating and an antiferromagnet. Moreover, BM-SCO exhibits oxygen vacancy channels (OVCs) that in thin films can be oriented either horizontally (H-SCO) or vertically (V-SCO) to the film's surface. To date, the orientation of these OVCs has been manipulated by control of the thin film deposition parameters or by using a substrate-induced strain. Here, we present a method to electrically control the OVC ordering in thin layers via ionic liquid gating (ILG). We show that H-SCO (antiferromagnetic insulator, AFI) can be converted to P-SCO (ferromagnetic metal, FM) and subsequently to V-SCO (AFI) by the insertion and subtraction of oxygen throughout thick films via ILG. Moreover, these processes are independent of substrate-induced strain which favors formation of H-SCO in the as-deposited film. The electric-field control of the OVC channels is a path toward the creation of oxitronic devices.

Carbon dioxide and water incorporation mechanisms in SrFeO3-δ phases: a computational study

With a higher propensity for low temperature synthesis routes along with a move toward lower solid oxide fuel cell operating temperatures, water and carbon dioxide incorporation in strontium ferrite is of importance. Despite this, the mechanisms are not well understood. In this work, classical-potential-based computational techniques are used to determine the favourability of water and CO₂ incorporation mechanisms in both SrFeO_3-δ and SrFeO_2.5. Our studies suggest that intrinsic Frenkel and Schottky type defects are unlikely to form, but that water and carbon dioxide incorporation are favourable in both phases. Water incorporation is likely for both the cubic and brownmillerite phases, with hydroxyl ions preferring to sit on octahedral oxygen sites in both structures, causing slight tilting of the shared octahedra. Interstitial hydroxyl ions are only likely for the brownmillerite phase, where the hydroxyl ions are most stable between adjacent FeO₄ tetrahedral chains. Carbon dioxide incorporation via carbonate defects is most favourable when a carbonate molecule exists on an iron site, preferring the iron site with lower oxygen coordination. This involves formation of multiple oxygen vacancies surrounding the iron site, and thus we conclude that carbonate can trap oxygen vacancies.

Structural, Magnetic, and Dielectric properties of Sr4Fe6O13 ferrite prepared of small crystallites

A stable Sr₄Fe₆O₁₃ was prepared as small crystallites by auto-combustion of a sol-gel in air followed by annealing the later at pertinent temperatures. A green sample, as annealed at elevated temperatures, yields a single Sr₄Fe₆O₁₃ phase of tailored magnetic properties. The structural, morphological, magnetic and electrical properties were investigated by X-ray diffraction, transmission electron microscopy, vibrating sample magnetometer, and broadband dielectric spectrometer. Hard magnetic Sr₄Fe₆O₁₃ properties arise with saturation magnetization M_s = 12.4 emu/g, coercivity H_c = 3956.7 Oe and squareness 0.512. Studies made at low temperatures reveals M_s decreasing on increasing temperature from 17.5 emu/g at 85 K down to 12.4 emu/g at 305 K, while H_c rises from 1483 Oe at 85 K to 1944 Oe at 305 K. The ac-conductivity follows the Jonscher relation. The dc-conductivity at high temperatures/low frequencies exhibits a plateau and it depends linearly on a characteristic frequency according to the Barton-Nakajima-Namikawa) relation.

Oxygen storage capacity and structural flexibility of LuFe2O4+x (0≤x≤0.5)

Combining functionalities in devices with high performances is a great challenge that rests on the discovery and optimization of materials. In this framework, layered oxides are attractive for numerous purposes, from energy conversion and storage to magnetic and electric properties. We demonstrate here the oxygen storage ability of ferroelectric LuFe2O4+x within a large x range (from 0 to 0.5) and its cycling possibility. The combination of thermogravimetric analyses, X-ray diffraction and transmission electron microscopy evidences a complex oxygen intercalation/de-intercalation process with several intermediate metastable states. This topotactic mechanism is mainly governed by nanoscale structures involving a shift of the cationic layers. The ferrite is highly promising because absorption begins at a low temperature (~=200 °C), occurs in a low oxygen pressure and the uptake of oxygen is reversible without altering the quality of the crystals. The storage/release of oxygen coupled to the transport and magnetic properties of LnFe2O4 opens the door to new tunable multifunctional applications.

Direct evidence of stacking disorder in the mixed ionic-electronic conductor Sr4Fe6O12+δ

Determining the structure-to-property relationship of materials becomes particularly challenging when the material under investigation is dominated by defects and structural disorder. Knowledge on the exact atomic arrangement at the defective structure is required to understand its influence on the functional properties. However, standard diffraction techniques deliver structural information that is averaged over many unit cells. In particular, information about defects and order-disorder phenomena is contained in the coherent diffuse scattering intensity which often is difficult to uniquely interpret. Thus, the examination of the local disorder in materials requires a direct method to study their structure on the atomic level with chemical sensitivity. Using aberration-corrected scanning transmission electron microscopy in combination with atomic-resolution electron energy-loss spectroscopy, we show that the controversial structural arrangement of the Fe2O2+δ layers in the mixed ionic-electronic conducting Sr4Fe6O12+δ perovskite can be unambiguously resolved. Our results provide direct experimental evidence for the presence of a nanomixture of "ordered" and "disordered" domains in an epitaxial Sr4Fe6O12+δ thin film. The most favorable arrangement is the disordered structure and is interpreted as a randomly occurring but well-defined local shift of the Fe-O chains in the Fe2O2+δ layers. By analyzing the electron energy-loss near-edge structure of the different building blocks in the Sr4Fe6O12+δ unit cell we find that the mobile holes in this mixed ionic-electronic conducting oxide are highly localized in the Fe2O2+δ layers, which are responsible for the oxide-ion conductivity. A possible link between disorder and oxygen-ion transport along the Fe2O2+δ layers is proposed by arguing that the disorder can effectively break the oxygen diffusion pathways.

Anisotropic oxygen diffusion at low temperature in perovskite-structure iron oxides

Oxygen-ion conduction in transition-metal oxides is exploited in, for example, electrolytes in solid-oxide fuel cells and oxygen-separation membranes, which currently work at high temperatures. Conduction at low temperature is a key to developing further utilization, and an understanding of the structures that enable conduction is also important to gain insight into oxygen-diffusion pathways. Here we report the structural changes observed when single-crystalline, epitaxial CaFeO₂.₅ thin films were changed into CaFeO₂ by low-temperature reductions with CaH₂. During the reduction process from the brownmillerite CaFeO₂.₅ into the infinite-layer structure of CaFeO₂, some of the oxygen atoms are released from and others are rearranged within the perovskite-structure framework. We evaluated these changes and the reaction time they required, and found two oxygen diffusion pathways and the related kinetics at low temperature. The results demonstrate that oxygen diffusion in the brownmillerite is highly anisotropic, significantly higher along the lateral direction of the tetrahedral and octahedral layers.