Topotactic Redox Research Articles

Graphene Quantum Dots as an Oxygen Reservoir for Topotactic Phase Transition‐Based Memristive Devices

AbstractA novel class of transition metal oxides, capable of reversible topotactic phase transition between the oxygen‐deficient brownmillerite and oxygen‐rich perovskite, has emerged as a promising material for memristive and magnetoelectric devices. However, the absence of a local oxygen source in the device structure necessitates an oxygen exchange process between the surrounding atmosphere and the switching layer during operation, which can lead to unreliable device performance. In this study, graphene quantum dots (GQDs) are introduced into a SrFe0.5Co0.5Ox memristive device as an oxygen reservoir for the nanoscale topotactic redox process. The SrFe0.5Co0.5Ox memristive devices with GQDs exhibit reliable resistive switching performance compared to SrFe0.5Co0.5Ox devices without GQDs. To understand the effect of GQDs on the device structure, a pulse endurance test is carried out in a high vacuum. The devices with GQDs show rather good endurance behavior, while devices without GQDs exhibit endurance failure. These results provide a deeper understanding of the potential use of GQDs in enhancing the performance of SrFe0.5Co0.5Ox memristive devices, with implications for tuning nanoscale topotactic phase transition for multi‐functional properties.

Electrochemical Control Ionic Defects Modulation Induced Phase Transition in SrCoOx: Progress and Prospect

Transition metal oxides (TMOs), as one of the advanced materials, have been widely studied due to their unique electronic, magnetic, optical, and thermal transport properties. Among them, SrCoOx (SCOx) is known as an oxygen sponge, where the ordered one-dimensional oxygen vacancy channels in the structure can work as the pathway for hydrogen/oxygen ions migration, facilitating the modulation of oxygen stoichiometry through the topotactic redox reaction. In this way, a “multi-state” phases transition with tunable physical properties can be realized. In this review, we summarize recent research progress in the utilization of H+ and O2 ions to induce distinct phase transitions in SCOx, which result in obvious physical property changes. The ability to control the properties of SCOx over a wide range through the incorporation of ionic defects provides a promising route for the development of advanced functional devices.

Topotactic redox cycling in SrFeO2.5+δ explored by 3D electron diffraction in different gas atmospheres

Using in situ 3D electron diffraction on SrFeOx nanocrystals during the solid–gas redox reaction, we uncovered oxygen-vacancy ordering and domain formation not observable by in situ solid–gas bulk diffraction techniques.

The evolution of local structure of Mo6S8 during Li+ electrochemical storage studied by in-situ tender X-ray absorption spectroscopy

Chevrel phase Mo6S8 has attracted great interest because of its capability to accommodate different types of small cations through reversible topotactic redox reactions. However, the intercalation mechanism of Mo6S8 is still far from being fully understood owing to the complexity of the crystal structure and limitation of the probe tools. In our study, the reaction mechanism of Li ​+ ​intercalation into Mo6S8 was studied as a model by in-situ tender X-ray absorption spectroscopy (itXAS) in the viewpoint of local structure. According to the isobestic points of the 1st derivative, the discharge process is divided into five regions while the charge process is divided into four regions, where most of the regions are assigned to the two-phase transition reactions. The intensity change of the two spectroscopic features of the 1st derivative is found to be correlated, implying that the intercluster bond length directly affects the localization of the unoccupied states of the intracluster bond. The asymmetric evolution of the intensity of a’ and b’ as well as the asymmetric partition of reaction regions demonstrate the asymmetry of the reaction path in discharge and charge process. The results help to clarify the remaining debates on the phase transitions. Besides, itXAS is demonstrated to be a powerful tool to study the reaction mechanism of Mo6S8, which is complementary to NMR, XRD and so on.

Optimization of the multi-mem response of topotactic redox La1/2Sr1/2Mn1/2Co1/2O3−x

Memristive systems emerge as strong candidates for the implementation of resistive random access memories and neuromorphic computing devices, as they can mimic the electrical analog behavior or biological synapses. In addition, complementary functionalities, such as memcapacitance, could significantly improve the performance of bio-inspired devices in key issues, such as energy consumption. However, the physics of mem systems is not fully understood so far, hampering their large-scale implementation in devices. Perovskites that undergo topotactic transitions and redox reactions show improved performance as mem systems, compared to standard perovskites. In this paper, we analyze different strategies to optimize the multi-mem behavior (memristive and memcapacitive) of topotactic redox La1/2Sr1/2Mn1/2Co1/2O3−x (LSMCO) films grown on Nb:SrTiO3. We explored devices with different crystallinities (from amorphous to epitaxial LSMCO), out-of-plane orientation [(001) and (110)], and stimulated either with voltage or current pulses. We found that an optimum memory response is found for epitaxial (110) LSMCO stimulated with current pulses. Under these conditions, the system efficiently exchanges oxygen with the environment minimizing, at the same time, self-heating effects that trigger nanostructural and chemical changes that could affect the device integrity and performance. Our work contributes to pave the way for the integration of multi-mem topotactic redox oxide-based interfaces in multiple device architectures, in order to exploit their memristive and memcapacitive properties for data storage or neuromorphic computation.

Large memcapacitance and memristance at Nb:SrTiO3/La0.5Sr0.5Mn0.5Co0.5O3-δ topotactic redox interface

The possibility to develop neuromorphic computing devices able to mimic the extraordinary data processing capabilities of biological systems spurs the research on memristive systems. Memristors with additional functionalities such as robust memcapacitance can outperform standard devices in key aspects such as power consumption or miniaturization possibilities. In this work, we demonstrate a large memcapacitive response of a perovskite memristive interface, using the topotactic redox ability of La0.5Sr0.5Mn0.5Co0.5O3-δ (LSMCO, 0 ≤ δ ≤ 0.62). We demonstrate that the multi-mem behavior originates at the switchable n-p diode formed at the Nb:SrTiO3/LSMCO interface. We found for our Nb:SrTiO3/LSMCO/Pt devices a memcapacitive effect CHIGH/CLOW ∼ 100 at 150 kHz. The proof-of-concept interface reported here opens a promising venue to use topotactic redox materials for disruptive nanoelectronics, with straightforward applications in neuromorphic computing technology.

Order/Disorder and in Situ Oxide Defect Control in the Bixbyite Phase YPrO3+δ (0 ≤ δ < 0.5)

The YPrO3+δ system is a nearly ideal model system for the investigation of oxide defect creation and annihilation in oxide ion conductor related phases with potential applications as solid state electrolytes in solid oxide fuel cells. The formation, structure, high temperature reactivity, and magnetic susceptibility of phase pure YPrO3+δ (0 ≤ δ ≤ 0.46) are reported. The topotactic reduction and oxidation of the YPrO3+δ system was investigated by powder X-ray in situ diffraction experiments and revealed bixbyite structures (space group: Ia3̅) throughout the series. Combined neutron and X-ray data clearly show oxygen uptake and removal. The research provides a detailed picture of the Y(3+)/Pr(3+)/Pr(4+) sublattice evolution in response to the redox chemistry. Upon oxidation, cation site splitting is observed where the cation in the ((1)/4, (1)/4, (1)/4) position migrates along the body diagonal to the (x, x, x) position. Any oxygen in excess of YPrO3.0 is located in the additional 16c site without depopulating the original 48e site. The in situ X-ray diffraction data and thermal gravimetric analysis have revealed the reversible topotactic redox reactivity at low temperatures (below 425 °C) for all compositions from YPrO3 to YPrO3.46. Magnetic susceptibility studies were utilized in order to further confirm praseodymium oxidation states. The linear relation between the cubic unit cell parameter and oxygen content allows for the straightforward determination of oxygen stoichiometry from X-ray diffraction data. The different synthesis strategies reported here are rationalized with the structural details and the reactivity of YPrO3+δ phases and provide guidelines for the targeted synthesis of these functional materials.

Elucidation of the electrochromic mechanism of nanostructured iron oxides films

Nanostructured hematite thin films were electrochemically cycled in an aqueous solution of LiOH. Through optical, structural, morphological, and magnetic measurements, the coloration mechanism of electrochromic iron oxide thin films was elucidated. The conditions for double or single electrochromic behavior are given in this work. During the electrochemical cycling, it was found that topotactic transformations of hexagonal crystal structures are favored; i.e. α-Fe 2O 3 to Fe(OH) 2 and subsequently to δ-FeOOH. These topotactic redox reactions are responsible for color changes of iron oxide films.

ChemInform Abstract: Topotactic Redox Chemistry of NaFeAs in Water and Air and Superconducting Behavior with Stoichiometry Change.

AbstractThe redox chemistry and transformations of NaFeAs in water and air are characterized by single crystal and powder XRD, magnetization measurements, TEM, and Moessbauer spectroscopy.

Topotactic Redox Chemistry of NaFeAs in Water and Air and Superconducting Behavior with Stoichiometry Change

We report experimental evidence that shows superconductivity in NaFeAs occurs when it is Na deficient. The oxidation of NaFeAs progresses differently in water and in air. In water the material oxidizes slowly and slightly retaining the original anti-PbFCl structure. In air NaFeAs oxidizes topotactically quickly and extensively transforming to the ThCr2Si2 structure type. Water acts as a mild oxidizing agent on the FeAs layer by extracting electrons and Na+ cations from the structure, while oxidation in air is more extensive and leads to change in structure type from NaFeAs to NaFe2As2. The superconducting transition temperature moves dramatically during the oxidation process. Exposed to water for an extended time period NaFeAs shows a substantial increase in Tc up to 25 K with contraction of unit cell volume. NaFe2As2, the air oxidized product, shows Tc of 12 K. We report detailed characterization of the redox chemistry and transformation of NaFeAs in water and air using single crystal and powder X-ray di...

Lattice Dynamics To Trigger Low Temperature Oxygen Mobility in Solid Oxide Ion Conductors

SrFeO(2.5) and SrCoO(2.5) are able to intercalate oxygen in a reversible topotactic redox reaction already at room temperature to form the cubic perovskites Sr(Fe,Co)O(3), while CaFeO(2.5) can only be oxidized under extreme conditions. To explain this significant difference in low temperature oxygen mobility, we investigated the homologous SrFeO(2.5) and CaFeO(2.5) by temperature dependent oxygen isotope exchange as well as by inelastic neutron scattering (INS) studies, combined with ab initio (DFT) molecular dynamical calculations. From (18)O/(16)O isotope exchange experiments we proved free oxygen mobility to be realized in SrFeO(x) already below 600 K. We have also evidence that low temperature oxygen mobility relies on the existence of specific, low energy lattice modes, which trigger and amplify oxygen mobility in solids. We interpret the INS data together with the DFT-based molecular dynamical simulation results on SrFeO(2.5) and CaFeO(2.5) in terms of an enhanced, phonon-assisted, low temperature oxygen diffusion for SrFeO(3-x) as a result of the strongly reduced Fe-O-Fe bond strength of the apical oxygen atoms in the FeO(6) octahedra along the stacking axis. This dynamically triggered phenomenon leads to an easy migration of the oxide ions into the open vacancy channels and vice versa. The decisive impact of lattice dynamics, giving rise to structural instabilities in oxygen deficient perovskites, especially with brownmillerite-type structure, is demonstrated, opening new concepts for the design and tailoring of low temperature oxygen ion conductors.

Synthesis, Crystal Structures and Properties of Na2ReO3 and of a Second Modification of Na5ReO6

AbstractSingle crystals of Na5ReO6 have been obtained from the reaction of Na2O, rhenium metal and CdO in Ag containers at 550 °C. At 700 °C Na2ReO3 is formed possibly via a topotactic redox reaction of Na5ReO6 with rhenium metal. Here we report the crystal structures of Na2ReO3 and a second modification of Na5ReO6, ex‐situ X‐ray diffraction studies of the reactivity within the system Na / Re / O, selected spectroscopic investigations of the two compounds and Extended‐Hückel calculations of the [Re2O6]4‐ anion, containing a rhenium‐rhenium triple‐bond.

Topotactic redox reactions of copper(ii) and iron(iii) salts within VOx nanotubes

Novel Cu-VOx and Fe-VOx multiwalled nanotubes have been obtained by exchange of the protonated template in the parent dodecylamine-VOx nanotubes with Cu(II) or Fe(III) salts. The intercalation of the transition metal species within the multiwalled structure of the tubes does not significantly affect the overall tubular morphology, but leads to a significant reduction of the inter-layer distance in the multiwalls from 3.02 to 1.09–1.05 nm. Electron microscopies, XRD, FTIR and XPS were used to study the nature of the host–guest interactions before and after the intercalation of the transition metal species. For the first time, it was observed that the substitution of dodecylamine by the transition metal salts induces a partial speciation of the intercalated species to their lower oxidation states, while an overall change in the structure of the VOx layers occurs in the case of the Cu-VOx nanotubes. The high structural flexibility of the VOx nanotubes was evidenced at each step of this study. The controlled thermolysis of the Cu-containing VOx nanotubes at 650 °C under N2 leads to the formation of single-walled mesoporous V2O3 nanotubes and Cu0 nanoparticles.

Evidences for the Formation of Chromium in the Unusual Oxidation State Cr(IV): III. Experimental Electronic Structure of the Channel Compounds TlxCr5Se8 (x=1 and 0.2)

The X-ray photoemission core level spectra of TlCr5Se8 and Tl0.2Cr5Se8 definitely exclude the oxidation of Tl(I) to Tl(III) when Tl is removed using a topotactic redox reaction between TlCr5Se8 and bromine in acetonitrile. Alterations are only observed for the multiplett structure of the Cr 3s core level giving evidences for a partial oxidation of Cr(III) to Cr(IV). Significant differences are observed in the valence band regions that were investigated with He I and He II radiation. For the Tl-poor sample an increased electron density at the Fermi level is observed in accordance with the higher electrical conductivity of this sample. The variations of the emissions in the region down to about 3 eV below EF can nicely be explained on the basis of the reduced Tl content and a partial oxidation of Cr(III) to Cr(IV).

Evidence for the formation of valence band holes due to topotactical Tl removal in the ternary channel compound TlCr 3S 5: chemical reactivity and experimental electronic structure

Tl-poor samples Tl 3 Cr 3S 5 (0.5 ≤ × ≤ 1) are prepared via a topolactical redox reaction between TlCr 3S 5 and Br 2 in CH 3CN. Unexpectedly, the reaction stops at a composition Tl 1Cr 3S 5 with x ≈ 0.50(5). Single crystals with a reduced Tl content are very brittle, indicating a high mechanical strain. X-ray photoemission investigations performed on TlCr 3S 5 and on Tl 0.5Cr 3S 5 demonstrate that the Cr 2p and Tl 4f core levels are identical for both samples. As a consequence, the possibility of an oxidation of Cr(III) or Tl(I) during the redox reaction is excluded. In contrast, significant changes are observed for the S 2p core level, suggesting a partial oxidation of the S 2− anions. Ultraviolet photoemission experiments conducted with He I and He II radiation reveal that TlCr 3S 5 and Tl 0.5Cr 3S 5 are semiconductors with a slightly smaller band gap in the Tl-poorer sample. The valence band spectra show pronounced differences between the two samples. The detailed analysis of both the core level spectra and the valence band spectra strongly suggests that the topotactic redox reaction leads to the formation of valence band holes, i.e. a partial oxidation of the sulfur anions. The results of all experiments indicate that the termination of the chemical redox reaction at a composition Tl 0.5Cr 3S 5 is caused by lattice strain and not due to the loss of the electrical conductivity of the host lattice.

Evidences for the Formation of Chromium in the Unusual Oxidation State Cr(IV): I. Chemical Reactivity, Microhomogeneity, and Crystal Structures of the Nonstoichiometric Channel Compounds TlxCr5Se8(0 ≤x≤ 1)

The topotactic redox reaction between TlCr5Se8and bromine in acetonitrile leads to the formation of metastable samples with 0 ≤x< 1 of TlxCr5Se8. Preparation at elevated temperatures yields for the first time a ternary transition metal chalcogenide crystallizing in the TlV5S8type structure with a Tl contentx< 0.33. As expected the topotactic redox reaction does not follow simple kinetics but is rather explained on the basis of the superposition of at least three different fundamental steps. EDAX investigations conducted on selected single crystals reveal that dependent on the deintercalation temperature, Tl is laterally inhomogeneous distributed along the needle axis which coincides with the crystallographicbaxis. A pronounced maximum is observed at the middle of the crystals. In stoichiometric TlCr5Se8the detailed analysis of the anisotropic displacement parameters of the Tl atoms reveal that the Tl atoms are displaced from the central position by about 0.26 Å. As a consequence the Tl atoms are coordinated by seven Se atoms in an irregular coordination polyhedron. Since the positionsx0zare related by a center of symmetry they cannot be occupied simultaneously. Hence it must be assumed that TlCr5Se8has a domain structure with local symmetryCm. In the Tl poorer phases the refinement of the Tl atoms at the central position leads to unusual highU22components. The observed microinhomogenity as well as an enhanced mobility and/or static disorder of the Tl atom within the channel may be responsible for this. The value forU11decreases with decreasing Tl content whereasU33is not affected. This observation is indicative for a different displacement of the Tl atoms from the central position. With respect to the possible reaction mechanisms, according to our structural investigations, the oxidation of monovalent Tl to trivalent Tl can be excluded. With decreasing Tl content the lattice parameters exhibit very anisotropic behavior, which is a direct consequence of the large changes of the interatomic Cr–Cr distances. The average 〈Cr–Se〉 as well as the nonbonding Se–Se distances are only slightly affected. These results suggest an oxidation of the formally trivalent Cr to the unusual tetravalent state Cr(IV), rather than the formation of valence band holes.

Chemical reactivity of K xTi 6Se 8 and the crystal structures of (H 2O) 1.5Ti 6Se 8 and Ti 3Se 4-channel compounds with the hexagonal Nb 3S 4 structure

The topotactic redox reaction of K 0.52Ti 6Se 8 with molecular iodine in acetonitrile allows the preparation of the binary titanium selenide Ti 3Se 4 with the hexagonal Nb 3S 4-type structure. The species formed during the redox reaction was identified as the complex KI⋅I 2. The reaction of Ti 3Se 4 with water leads to the formation of the new ternary compound (H 2O) 1.5Ti 6Se 8. Simu ltaneous thermogravimetric analysis and mass spectrometry shows the emission of water over a wide temperature range and a weight loss which is in accordance with the content determined by single crystal X-ray work. The shape of the emission peak is indicative for a diffusion-limited reaction. The crystal structures of Ti 3Se 4 and (H 2O) 1.5Ti 6Se 8 are presented. In the water containing sample the oxygen atoms are displaced from the center of the hexagonal channels towards the TiSe 6 octahedra. This displacement leads to the formation of weak TiO and SeO interactions. The average OO distances within the channels are indicative for the formation of hydrogen bonds. The intercalated water exhibits only limited influence on the crystal structure of the host material.

Catalytic oxidation of sulfide ions over NiPS 3

Sulfide ion oxidation to elementary sulfur over NiPS 3, which is an efficient Catalyst for that process, has been studied by means of electrochemical measurements, electron spin resonance spectroscopy and X-ray photoelectron spectroscopy. Based on the results obtained by these physical methods and on reaction kinetics, a mechanism of the process is suggested. Reduction and oxidation steps of the Catalytic cycle are modelled by the topotactic redox reaction of intercalation-deintercalation of hydrated alkali ions.

Mechanism of formation of halogen-containing high-temperature superconductors based on YBa 2Cu 3O 7− x

There are two points of view in the literature on the mechanism of formation of high-temperature superconductors (HTSC - X, where X =Cl, Br, I). One of these supposes that a topotactic redox intercalation reaction proceeds, the other assumes that the isomorphous substitution of oxygen by halogens takes place. It has been shown by tensometric measurements, the anomalous scattering effect, EXAFS, and Mo¨ssbauer spectroscopy that the interaction of halogens with the tetragonal modification of YB a 2Cu 3O 7− x proceeds by intercalation of halogens into the anion vacant position of the Cu(1) layer. The charge transfer from the Cu(2) layers to the halogens results in the formation of electron holes in the Cu O pyramids, which determines HTSC-properties of the intercalate compounds obtained.

Intercalation of cations into niobyl-vanadyl phosphate by redox reactions

Mixed niobyl-vanadyl phosphate, Nb0.86V0.14OPO4•2.7H2O, intercalates various alkali, NH4+ alkaline earth, Zn2+, Cd2+, transition metal, and H+ cations by topotactic redox reactions. All the intercalates thus obtained are tetragonal (a = b = 6.44 Å, c lower than in the precursor solid). The degree of hydrolysis of the mixed phosphate in the intercalate in the redox reactions is always lower than 1%. Upon reduction, the cation exchange capacity (CEC) of the intercalates is 71 mequiv. per 100 g of solid calcined at 1000 °C. Keywords: mixed niobyl-vanadyl phosphate, layered compounds, redox intercalation.