The paper describes a heterobimetallic mixed-ligand hexanuclear precursor NaMnsub.2(thd)sub.4(OAc)sub.2 (1) (thd = 2,2,6,6-tetramethyl-3,5-heptadionate; OAc = acetate) that was designed based on its ...lithium homoleptic analogue, LiMnsub.2(thd)sub.5, by replacing one of the thd ligands with an acetate group in order to accommodate 5-coordinated sodium instead of tetrahedral lithium ion. The complex, which is highly volatile and soluble in a variety of common solvents, has been synthesized by both the solid-state and solution methods. The unique “dimer-of-trimers” heterometallic structure consists of two trinuclear NaMnsup.II sub.2(thd)sub.4sup.+ units firmly bridged by two acetate ligands. X-ray diffraction techniques, DART mass spectrometry, ICP-OES analysis, and IR spectroscopy have been employed to confirm the structure and composition of the hexanuclear complex. Similar to the Li counterpart forming LiMnsub.2Osub.4 spinel material upon thermal decomposition, the title Na:Mn = 1:2 compound was utilized as the first single-source precursor for the low-temperature preparation of Nasub.4Mnsub.9Osub.18 tunnel oxide. Importantly, four Mn sites in the hexanuclear molecule can be potentially partially substituted by other transition metals, leading to heterotri- and tetrametallic precursors for the advanced quaternary and quinary Na-ion oxide cathode materials.
Lithium-manganese-oxide (Li-Mn-O) spinel is among the promising and economically viable, high-energy density cathode materials for enhancing the performance of lithium-ion batteries. However, its ...commercialization is hindered by its poor cyclic performance. In computational modelling, pivotal in-depth understanding of material behaviour and properties is sizably propelled by advancements in computational methods. Hence, the current work compares traditional DFT (CASTEP) and linear-scaling DFT (ONETEP) in a LiMnsub.2Osub.4 electronic property study to pave way for large-scale DFT calculations in a quest to improve its electrochemical properties. The metallic behaviour of Lisub.xMnsub.2Osub.4 (0.25 ≤ x ≤ 1) and Lisub.2Mnsub.2Osub.4 was correctly determined by both CASTEP and ONETEP code in line with experiments. Furthermore, OCV during the discharge cycle deduced by both codes is in good accordance and is between 5 V and 2.5 V in the composition range of 0 ≤ x ≤ 1. Moreover, the scaling of the ONETEP code was performed at South Africa's CHPC to provide guidelines on more productive large-scale ONETEP runs. Substantial total computing time can be saved by systematically adding the number of processors with the growing structure size. The study also substantiates that true linear scaling of the ONETEP code is achieved by a systematic truncation of the density kernel.
This review aims to summarize more than 100 years of research on spinel compounds, mainly focusing on the progress in understanding their magnetic, electronic, and polar properties during the last ...two decades. Over the years, more than 200 different spinels, with the general formula AB2X4, were identified or synthesized in polycrystalline or single-crystalline form. Many spinel compounds are magnetic insulators or semiconductors; however, a number of spinel-type metals exists including superconductors and some rare examples of d-derived heavy-fermion compounds. In the early days, they gained importance as ferrimagnetic or even ferromagnetic insulators with relatively high saturation magnetization and high ordering temperatures, with magnetite being the first magnetic mineral known to mankind. From a technological point of view, spinel-type ferrites with the combination of high electrical resistance, large magnetization, and high magnetic ordering temperature made them promising candidates for many applications. However, spinels are also known as beautiful gemstones, with the famous “Black Prince’s Ruby” in the front centre of the Imperial State Crown. In addition, spinels are important for the earth tectonics, and the detection of magnetite in a Martian meteorite even led to the speculation of life on Mars. However, most importantly in the perspective of this review, spinels played an outstanding role in the development of concepts of magnetism, in testing and verifying the fundamentals of magnetic exchange, in understanding orbital-ordering and charge-ordering phenomena including metal-to-insulator transitions, in developing the concepts of magnetic frustration, in establishing the importance of spin–lattice coupling, and in many other aspects. The still mysterious Verwey transition in magnetite was one of the very first illuminating examples of this complexity, which results from the fact that some ions can exist in different valence states in spinels, even at a given sublattice. In addition, the A-site as well as the B-site cations in the spinel structure form lattices prone to strong frustration effects resulting in exotic ground-state properties. The A-site ions are arranged in a diamond lattice. This bipartite lattice shows highly unusual ground states due to bond-order frustration, with a strength depending on the ratio of inter- to intra-sublattice exchange interactions of the two interpenetrating face-centred cubic lattices. The occurrence of a spiral spin-liquid state in some spinels is an enlightening example. Very recently, even a meron (half-skyrmion) spin structure was identified in MnSc 2S4 at moderate external magnetic fields. In case the A-site cation is Jahn–Teller active, additional entanglements of spin and orbital degrees of freedom appear, which can give rise to a spin–orbital liquid or an orbital glass state. In systems with such a strong entanglement, the occurrence of a new class of excitations – spin–orbitons – has been reported. The B-site cations form a pyrochlore lattice, one of the strongest contenders of frustration in three dimensions. A highly degenerate ground state with residual zero-point entropy and short-range spin ordering according to the ice rules is one of the fascinating consequences, which is known already for more than 50 years. At low temperatures, in B-site spinels the occurrence of spin molecules has been reported, strongly coupled spin entities, e.g., hexamers, with accompanying exotic excitations. A spin-driven Jahn–Teller effect is a further possibility to release magnetic frustration. This phenomenon has been tested in detail in a variety of spinel compounds. In addition, in spinels with both cation lattices carrying magnetic moments, competing magnetic exchange interactions become important, yielding ground states like the time-honoured triangular Yafet–Kittel structure. Very recently, it was found that under external magnetic fields this triangular structure evolves into very complex spin orders, which can be mapped on spin super-liquid and spin super-solid phases. In addition, due to magnetic frustration, competing interactions, and coupling to the lattice, very robust magnetization plateaus appear in a variety of spinel compounds as function of an external magnetic field. Furthermore, spinels gained considerable importance in elucidating the complex physics driven by the interplay of spin, charge, orbital, and lattice degrees of freedom in materials with partly filled d shells. This entanglement of the internal degrees of freedom supports an exceptionally rich variety of phase transitions and complex ground states, in many cases with emerging functionalities. It also makes these materials extremely susceptible to temperature, pressure, or external magnetic and electric fields, an important prerequisite to realize technological applications. Finally, yet importantly, there exists a long-standing dispute about the possibility of a polar ground state in spinels, despite their reported overall cubic symmetry. Indeed, recently a number of multiferroic spinels were identified, including multiferroic spin super-liquid and spin super-solid phases. The spinels also belong to the rare examples of multiferroics, where vector chirality alone drives long-range ferroelectric order. In addition, a variety of spinel compounds were investigated up to very high pressures up to 40 GPa and in high magnetic fields up to 100 T, revealing complex (p,T) and (H,T)-phase diagrams.
•Ground-breaking concepts in magnetism were developed conceptually using spinel compounds.•Spinels are prototypical examples of strongly frustrated magnets.•Ferroelectricity and multiferroicity in spinels result from very different mechanisms.•Spinels represent outstanding examples of metal-to-insulator transitions and charge order.•Examples of well-established d-derived heavy-fermion behaviour.
•Key features of magnetic spinel ferrites (SFs) and their derivative composites were reviewed.•Application of SFs as adsorbents for water treatment was assessed.•Factors affecting the adsorption ...performance of SFs were discussed.•Fabrication, regeneration, stability, and safety of SFs were summarized.
Spinel ferrite (SF) magnetic materials are an important class of composite metal oxides containing ferric ions and having the general structural formula M2+Fe23+O4 (where M=Mg2+, Co2+, Ni2+, Zn2+, Fe2+, Mn2+, etc.). SFs possess unique physicochemical properties including excellent magnetic characteristics, high specific surface area, surface active sites, high chemical stability, tunable shape and size, and the ease with which they can be modified or functionalized. As a result of their multifunctional properties, affordability, and magnetic separation capability, SF adsorbents are a top choice for water purification applications that require high adsorption efficiencies and rapid kinetics. In this review, we discuss adsorption performance and possible applications of SFs and their derivatives for treating a wide range of aqueous pollutants such as metal ions, dyes, and pharmaceuticals. Key parameters influencing the sorption performance such as particle size, shape, annealing temperature, functionalization, and metal ion doping have been comprehensively discussed. In addition, adsorbate–adsorbent interactions, desorption, regeneration, and utilization of spent adsorbent have also been summarized. The review also covered, how SFs are prepared from industrial waste using green synthetic routes and general remarks about toxicological effects.
COsub.2 resourceful utilization contributes to the goal of carbon neutrality. Chemical Looping Dry Reforming (CLDR) has attracted significant attention as a method for converting COsub.2 to CO. ...NiFesub.2Osub.4 oxygen carrier (OC) is found to be a potential material for CLDR. However, the migration process of lattice oxygen, which are critical for the conversion of COsub.2 to CO, was not extensively investigated. In this study, the reduction and oxidation degrees of the NiFesub.2Osub.4 were finely modulated in a thermogravimetric analyzer. The lattice oxygen migration mechanism of the NiFesub.2Osub.4 in redox cycles was characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and in-situ Raman. The novelty of this paper is clarifying the release-uptake paths of lattice oxygen during COsub.2 resourceful utilization. The result indicates that the concentration gradient between the surface and the bulk drives the diffusion of lattice oxygen. The stabilization of surface lattice oxygen content is attributed to the rapid migration of O anion, which is closely associated with the movement process of Ni particles inward and outward through the spinel bulk. In addition, a highly reactive chemical reaction interface consisting of lattice oxygen and the corresponding metal atoms is always present on the surface of the oxygen carrier and is confirmed by an in-situ Raman and XPS during the whole process of CLDR. The results of this paper offer reference and basis for further development and design of CLDR using spinel OC.
Pristine Li-rich layered cathodes, such as Li1.2Ni0.2Mn0.6O2 and Li1.2Ni0.1Mn0.525Co0.175O2, were identified to exist in two different structures: LiMO2 R3̅m and Li2MO3 C2/m phases. Upon 300 cycles ...of charge/discharge, both phases gradually transform to the spinel structure. The transition from LiMO2 R3̅m to spinel is accomplished through the migration of transition metal ions to the Li site without breaking down the lattice, leading to the formation of mosaic structured spinel grains within the parent particle. In contrast, transition from Li2MO3 C2/m to spinel involves removal of Li+ and O2‑, which produces large lattice strain and leads to the breakdown of the parent lattice. The newly formed spinel grains show random orientation within the same particle. Cracks and pores were also noticed within some layered nanoparticles after cycling, which is believed to be the consequence of the lattice breakdown and vacancy condensation upon removal of lithium ions. The AlF3-coating can partially relieve the spinel formation in the layered structure during cycling, resulting in a slower capacity decay. However, the AlF3-coating on the layered structure cannot ultimately stop the spinel formation. The observation of structure transition characteristics discussed in this paper provides direct explanation for the observed gradual capacity loss and poor rate performance of the layered composite. It also provides clues about how to improve the materials structure in order to improve electrochemical performance.
Composite materials with the composition (100-x)Cu1.3Mn1.7O4/xLaNi0.6Fe0.4O3 (CMxLNF, where: x = 5, 10, 15, 20, 30, 40 and 50 wt%) were evaluated as possible coating materials for SOFC/SOEC metallic ...interconnects. The introduction of LaNi0.6Fe0.4O3 into spinel matrices improved electrical conductivity compared to pure Cu1.3Mn1.7O4. The measured Seebeck coefficient values indicate that carrier concentration increased with LaNi0.6Fe0.4O3 content, yet the activation energy of the carriers' mobility decreased. Extensive structural and microstructural studies of selected CM/LNF systems showed that composite material components exhibited high stability in relation to one another. Additionally, LaNi0.6Fe0.4O3 significantly improved the chemical stability of Cu1.3Mn1.7O4 in the presence of chromia, as tested after 150 h of exposure to air at 800 °C. Finally, coatings based on selected composite powders were deposited electrophoretically on the steel and were oxidized for 1500 h in air at 750 °C, confirming that CM10LNF can be applied as effective protective-conducting coatings on low-chromium ferritic steel.
•Cu1.3Mn1.7O4 (CM) spinel / LaNi0.6Fe0.4O3 (LNF) perovskite composites were prepared.•CM/LNF components are mutually stable after sintering.•Adding LNF to CM matrix improves electrical conductivity.•CM/LNF has better chemical stability vs chromia than reference CM.•CM10LNF coating on FSS substrate is more corrosion-resistant than CM.
The active sites on oxygen electrocatalyst and the number of inherent active species are important factors affecting the performance of Zn-air battery. Constructing multiphase interfaces is an ...effective strategy to increase the number of active species for oxygen electrocatalysts. In this work, the number of intrinsic active species of spinel oxygen electrocatalyst was increased and its catalytic activity was enhanced by the synergistic action of bimetallic center three interfaces and heteroatom-doped carbon nanostructures. The resulting NiCo2O4/NCNTs/NiCo as catalyst exhibits superior activity toward ORR (E1/2 = 0.83 V, JL = −5.38 mA cm−2) and OER (Ej10 = 1.58 V). Further, the obtained catalyst work as a cathode assembles as Zn-air battery with a high open-circuit potential of 1.51 V and excellent cycle stability (586 h). Theoretical results indicate that the desorption of *OH species is the rate-determining step for ORR, the multiphase interfaces in the NiCo2O4/NCNTs/NiCo will provide additional electrons due to the upward shift of antibonding orbitals relative to the Fermi level. Consequently, it boosts the oxygen adsorption and charge transfer and accelerate the reaction kinetics. This work emphasizes the synergistic effect between multiphase interfaces in transition metal composite catalysts and opens up a promising way for the preparation of efficient and stable transition metal electrocatalysts.
•A unique triphasic interfacial structure catalyst has been synthesized by a simple strategy.•The ultra-thin carbon layer catalyzed by NiCo alloy facilitates interfacial charge transfer.•The obtained catalyst exhibits excellent oxygen catalytic performance for zinc-air battery.
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