Tetrahedral Sites Research Articles

Synthesis of porous Mg2MnO4 spinel microspheres and enhanced lithium storage properties

Porous Mg2MnO4 spinel microspheres were synthesized by a facile solvothermal method. Various techniques such as X-ray powder diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, were used to characterize the phase composition, morphology and electronic structure of materials. The results show that Mg2MnO4 spinel material was obtained at the temperature of 600–800 °C and has a microsphere structure. The microspheres were assembled by nanoparticles, and the particle size increases with the increase of annealing temperature. Analyses of XPS spectra reveal that Mn exists in multiple valence states (+2, +3, +4) in Mg2MnO4 materials, and all the materials are mixed spinel with Mg2+ and Mn2+/3+/4+ ions occupy both tetrahedral and octahedral sites of spinel structure, and the degree of inversion decreases with increasing annealing temperature. Mg2MnO4 microspheres exhibit excellent lithium storage performance, the first discharge specific capacity of M2MO-8 is 835.3 mAh g−1 under a voltage window of 0–3 V and with a current density of 100 mA g−1, and after 100 cycles the discharge capacity of the material is as high as 406.5 mAh g−1. In addition, this material shows high cyclic reversibility. The enhanced lithium storage properties of the material can be attributed to its porous structure, which promote the transport of lithium ions and reduce the damage to the structure caused by the volume change during the charge and discharge process.

The role of phase transformation (tetragonal – spinel) on the structural and mechanical properties of Ni doped CuFe2O4

Nano-ferrites of Cu1–xNixFe2O4 (x = 0 to 1 with step 0.2) system was synthesized utilizing the flash auto combustion process annealed at 600oC for 3 h. The structural characterization for synthesized samples was carried out using x-ray Diffraction (XRD), Fourier transition infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The hardness of the prepared material was measured using micro-indentation creep technology. XRD pattern verified the creation of a single-phase cubic spinel structure. The undesired CuO phase forms around \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:2\ heta\\:={50}^{^\\circ\\:}\\:$$\\end{document}for pure Copper ferrite and decreases with increasing Ni content. The average crystalline size decreases from 27.92 nm to 13.28 nm by doping process from x = 0.2 to x = 1 which retards the growth of crystalline size. FTIR spectra are distinguished by the presence of two prominent absorption bands, ν2 for the octahedral site and ν1 for the tetrahedral site, in the range of approximately 593 and 471 cm− 1, respectively. FTIR analysis verified the formation of the ferrite system’s spinel structure. The TEM images show a nanocrystalline nature with some agglomeration and the crystallites are spherical in shape which their sizes are agrees well with that obtained from XRD measurements. The hardness decreases as the dwell time increases. The hardness and yield strength (Y) values were significantly improved due to the decrease in the crystallite size after Ni doping. The stress exponent (n) value increases by increasing Ni content which means that the mechanical properties improved due to increment of resistance.

Impact of substituting Cu2+/Ce3+ cations on the structural, magnetic and electrical properties of cobalt nano ferrites

The nano ferrite compounds Co1-xCuxFe2-yCeyO4 (with x values of 0.0, 0.25, 0.5, and 0.75, and y values of 0.0, 0.03, 0.06, and 0.09) were synthesized through the sol-gel auto-combustion method. The characteristics of spinel ferrites were tailored by creating nano ferrites with desirable properties, which were then sintered at 1150 °C for 2 h in the presence of rare earth (Ce3+) and transition metals (Cu2+). They were investigated using dielectric, FTIR, FESEM, XRD, VSM, and DC electrical resistivity experiments. XRD examination verified the samples' cubic spinel structure by measuring the average crystallite size, x-ray density, and lattice constant. FESEM images revealed grain sizes ranging from 41.07 to 156 nm. FTIR spectra in the range of 415 cm−1 to 430 cm−1 further supported the substitution of Cu2+/Ce3+ ions in the tetrahedral sites, indicated by an increase in the lattice parameter. Measurements of the remanence ratio, saturation magnetization, anisotropy constant, coercivity, and magnetic moment, among other magnetic characteristics, were made. Higher concentrations of Cu2+/Ce3+ ions were found to considerably reduce magnetic saturation (Ms), coercivity (Hc), and remanence (Mr). These materials were semiconducting because their activation energy varied between 0.52 and 0.62 eV, and their DC resistivity increased with increasing Cu2+/Ce3+ concentration. Above 1 MHz, the dielectric properties became frequency-independent and decreased with increasing frequency. These ferrites seem highly potential for devices working at high frequency.

Structure-optical properties of CdO glass matrix with low levels of cobalt impurities

The impacted role of cadmium oxide (CdO) on the structure-optical properties of current borate glasses with low cobalt oxide impurities was studied. The study was performed on a glass system of the formula [x CdO – (80-x) B2O3 – 0.4 CoO – 19.6 Na2O]; x = 0, 6, 12, 18 and 24 mol%, and produced by melt quenching method. Structure and optical absorption spectroscopy were done to explore the electronic transitions of Co cations inside the glass matrix. The obtained data indicated that structure and optical properties were significantly changed following CdO additives. Bands of optical absorption within both visible and near-infrared ranges showed signatures from Co2+ cations in the tetrahedral and octahedral sites, while Co3+ cations in the octahedral sites within the glass network. Optical absorption also changed significantly for the CdO-doped glasses which exhibited red shifts in visible absorption bands and blue shifts in the NIR absorption bands. The obtained absorption bands in both visible and NIR were employed to obtain the crystal field splitting parameter (Δt) showing augmented records from 3364 cm−1 to 3377 cm−1 with more CdO addition. The Tauc gap energy values were then attained showing decreased values from 3.50 to 3.27 eV when further CdO is added. Further, the impacted role of CdO addition on the nonlinearity optical properties of the present glasses was finally estimated. Potential increases in the linear and nonlinear optical are discussed based on polarizability and basicity increments inside the glass matrix.

Effect of Alloying on the Hydrogen Sorption in Ti–Zr–Mn-Based Alloys. Pt. 1: C14-Type Laves-Phase-Based Alloys

The alloys of the Ti–Zr–Mn system based on the C14-type Laves phase are considered as ones of the most promising materials for safe storage and transportation of hydrogen. These alloys have appropriate parameters for activating the processes of absorption and release of hydrogen, a low cost, and a fairly high cyclic stability. In this work, the microstructure and phase composition of the starting alloys and the crystal structure of the hydrides synthesized from them are studied. Possible ways to reduce the cost of the final products are shown. The fact that changing the method of the alloy fabrication does not significantly affect its hydrogen absorption properties is shown. On the example of the considered alloys, it is shown that, as expected, alloying with an element with a larger atomic radius that forms a stable chemical compound with hydrogen results in an increase in the hydrogen capacity. This is explained by both the increased radius of the tetrahedral interstitial sites, where hydrogen atoms are located after dissolution, and the higher total amount of the element interacting with hydrogen.

Growth, magnetic, and electronic properties of Ni-Zn ferrites thin films

Abstract Thin films of Ni-Zn ferrite grown on MgO(111) single crystal substrate were prepared using radiofrequency magnetron sputtering, with a target of nominal composition Ni0.5Zn0.5Fe2O4. Subsequently, x-ray diffraction (XRD) was performed, which revealed characteristic reflections of a Ni-Zn ferrite structure, confirming the unique formation of the ferrite. X-ray photoelectron spectroscopy (XPS) revealed the presence of metal ions in their appropriate valence states within the crystalline structure of the Ni-Zn ferrite. The variation in binding energy observed in the thin film is attributed to changes in the environment of Fe3+ and Zn2+ or Ni2+ ions, resulting from the non-equilibrium distribution of cations in tetrahedral and octahedral sites. The saturation magnetization and the coercivity field were 7.05 μ B / cell and 513 ± 32 Oe, respectively. In addition, ferromagnetic resonance studies were made using broad-band FMR spectroscopy based on a coplanar waveguide (CPW) spectrometer.

Glass-ceramics and molybdenum doping synergistic approach for Nasicon-type solid-state electrolytes

Advancing energy density, enabling lithium metal anodes, and ensuring unparalleled safety and operational reliability in lithium batteries hinge on advancing inorganic solid-state electrolytes. To overcome current impediments, we present an innovative approach that integrates glass-ceramics with a pioneering new Nasicon strategy involving molybdenum doping. In the conducted study, a series of 14Li2O-9Al2O3-38TiO2-(39-x)P2O5-xMoO3 glasses, denoted as LATPMox, along with their corresponding glass-ceramics (LATPMox-GC), have exhibited a promising characteristic as solid electrolytes. X-ray diffraction (XRD) analysis confirms the formation of the novel Mo-doped Nasicon phases in the glass-ceramics, as validated by Rietveld refinement. Examination of the crystallization kinetic behavior of the glasses reveals a three-dimensional nucleation process with spherical particle growth, featuring an activation energy of 165 kJ mol−1. Transmission Electron Microscopy TEM characterization aligns crystallization behavior with crystallite and distribution within the glass matrix, resulting in a compact and dense microstructure. The structural properties of the resultant phases are examined through FT-IR, Raman spectroscopy, and TEM-SEAD analysis. Vickers indentation tests were employed to assess the microscopic fracture toughness, and both the glass and glass-ceramics materials demonstrated favorable mechanical performance. Optical characterization using UV–visible absorption highlights the reduction of Mo6+ to Mo5+, likely occupying tetrahedral sites within the crystalline lattice. Impedance spectroscopy measurement showcases the effective promotion of ionic conductivity following Mo doping, reaching a total conductivity value of 5.50 × 10−5 Ω−1 cm−1 along with a high lithium transference number of 0.99 at room temperature for LATPMo2.6-GC glass-ceramic. This value is larger than that of many other glass-ceramics as well as that of the well-known lithium phosphorous oxy-nitride LiPON solid electrolyte whose ionic conductivity at RT is around 2 × 10−6 Ω−1 cm−1.

Crystal Chemistry of Synthetic Mg(Si1−xGex)O3 Pyroxenes: A Single-Crystal X-ray Diffraction Study

Germanate-pyroxenes often are used as model systems to study the stability and phase relationships of analog silicate systems. Based on such analyses, it is assumed that silicates and germanates behave ideally in terms of mixing. A systematic study was performed to monitor in detail the changes introduced by a Si4+ through Ge4+ replacement in the important rock-forming pyroxene enstatite MgSiO3. Well-shaped, idiomorphic singe crystals of a MgSi1−xGexO3 pyroxene solid solution were grown at ambient pressure from a high-temperature flux-assisted synthesis. Structural analysis using single-crystal X-ray diffraction methods revealed orthorhombic symmetry, Pbca, Z = 8, for the complete solid-solution series. Long-term storage over a period of 8 years at ambient conditions or annealing at 525 °C over a period of 10 weeks did not change the symmetry of the proposed thermodynamically stable monoclinic polymorph. Within the solid-solution series, lattice parameters increased almost linearly with increasing Si4+ by Ge4+ substitution. The main changes occurred on the tetrahedral sites, which showed an almost linear increase in individual and average bond lengths but also in distortion parameters. The refined site occupancy of Si4+ and Ge4+ showed a distinct preference of Ge4+ for the TB site. The altered topology and kinking state in the tetrahedral chains also imposed significant changes to the bonding topology and geometry of the neighboring M1 and M2 sites.

Engineering Co vacancy at tetrahedral site in spinel Co3O4 to enhance the oxygen evolution reaction performance in alkaline and neutral electrolyte

Spinel Co3O4 which possesses two types of Co sites such as Co2+ at tetrahedral site and Co3+ at octahedral site, has been regarded as a promising non-noble metal oxygen evolution reaction (OER) catalyst to replace the state-of-the-art IrO2 catalyst. Constructing cationic vacancy is an effective method to regulate the electronic structure and optimize the catalytic performance of the active site. Therefore, the spinel Co3O4 with Co vacancy at tetrahedral site (d-Zn0.3-Co3O4) or at octahedral site (d-Al0.3-Co3O4) were fabricated in this paper by alkali etching the corresponding Zn doped Co3O4 or Al doped Co3O4. Density of state (DFT) calculations show that the Co vacancy can enhance the d band center close to Fermi level. While Co vacancy at tetrahedral site can greatly enhance the catalytic activity of neighbour Co2+ site, which is even better than the traditional octahedral Co3+ site. Therefore, d-Zn0.3-Co3O4 shows the superior OER activity gives a small 293 mV (1 M KOH) and 364 mV (1 M PBS) overpotential at 10 mA cm−2 and admirable long-term stability.

Fabrication and characterization of blue nano ceramic-inks: Utilization of cobalt aluminate pigments prepared by solid state method

The current study focuses on the development and fabrication of ceramic nano inks with improved properties. Firstly, two types of pigments were prepared through a solid state process using fine gibbsite (Al(OH)3) or aluminum chloride (AlCl3) with cobalt chloride hexahydrate (CoCl2.6H2O). The stoichiometric precursors were well-mixed and fired at 1000oC; then milled by a high energy ball mill to obtain nano pigments. The prepared pigments were investigated for their phase composition and crystallite size by X-ray diffraction, and further confirmed by Fourier Transform Infrared (FTIR). The morphology and particle size were examined using a scanning electron microscope. The optical properties in terms of UV–Vis, photoluminence emission spectra and CIE chromaticity diagram were also tested. The chemical-state of cobalt and surface elemental composition of prepared Co-aluminate pigments were tested by X-ray photoelectron spectroscopy (XPS). Four groups of ceramic-inks (twelve formulations) were prepared using different percentages of cobalt aluminate pigments and additives. The fabricated inks were investigated for their morphology, rheology, contact-angle and sedimentation behavior. The results revealed that two cobalt aluminate pigments with spinel structure, nano sized particles and blue color, were successfully prepared by the proposed method. The results of optical properties indicated that the cobalt ions are located mainly in tetrahedral sites with a small amount in octahedral sites. The prepared pigments were successfully used in the fabrication of ceramic nano inks with improved properties. The viscosity of most fabricated inks was in the range needed for the industry (4–40 mPa s).

Cr-doped Li-based [formula omitted]-(Beta) type hexaferrites: XRD, FTIR, XPS and dielectric evaluations for high-frequency applications

The rapid growth of electronic technology and communications have led to increase demand for high performance microwave absorbing materials. Ferrites are frequently used to absorb microwaves and protect against electromagnetic contamination. In the present study, β-type hexaferrite samples with different chemical compositions of LiFe11-xCrxO17 (x = 0.0, 0.1, 0.3, and 0.5) have been manufactured through the sol-gel auto-combustion technique. The effect of substituting Cr3+ on its structural, electrical and dielectric characteristics was examined. The XRD evaluation helped to develop a single-phase structure for all samples. The substitution of Cr3+ has changed the structural characteristics. FTIR analysis was used to investigate the vibrational properties of Cr-doped β-type hexaferrite. Two absorption bands in the infrared spectroscopy were used to indicate the tetrahedral and octahedral sites. XPS results revealed the incorporation of every metal ion related to their particular electronic states. The dielectric studies were determined in terms of frequency (1–6 GHz) and every sample behavior was described using the Maxwel-Wagner and Koop theories. Impedance analysis was utilized to study the effects of relaxation time and grain boundaries on the electrical features of the produced nanomaterials. Impedance Cole-Cole plots prove that all samples exhibit non-Debye-type multiple relaxation processes. It was observed that among all the synthesized samples, LiFe10.9Cr0.1O17 has a minimum reflection loss of −37.80 dB at 1.22 GHz. Cr-doped β-type hexaferrite results suggested using and designing hexaferrite-based nanocomposite for high-frequency absorbers in the microwave range.

Improved activity and significant O2 resistance of Cs doped Co3O4 catalyst for N2O decomposition

Csx-Co3O4 catalysts with excellent low temperature activity and oxygen resistance for N2O decomposition were prepared using a citric acid complexation method. The mechanism of Cs doping on Co3O4 was systematically investigated by characterization techniques such as XRD, Raman, N2 adsorption, HR-TEM, XPS, H2-TPR, O2-TPD, and combined with DFT calculation methods. The results indicated that the presence of Cs inhibited the growth of Co3O4 grains and increased the specific surface area. Cs doping significantly increased the number of exposed oxygen vacancies, which tended to form around CoO4 (Co2+) tetrahedral sites replaced by Cs, following the trend Evac(1 1 1) <Evac(1 1 0) <Evac(1 0 0). Furthermore, the interaction between Cs and Co enhanced the electron donating ability of Co, promoted the reduction of Co3+ to Co2+ and weakened the Co-O bond. Among all the samples, the one with a Cs/Co molar ratio of 0.1 exhibited the highest activity. At 275 °C, the Rs of Cs0.1-Co3O4 was 39.5 times that of pure Co3O4. More importantly, Cs0.1-Co3O4 exhibited excellent oxygen resistance, achieving a high N2O conversion rate of 94 % under conditions of 300 °C and 20 % O2.

High Entropy Protected Sharp Magnetic Transitions in Highly Disordered Spinel Ferrites.

How disorder affects magnetic ordering is always an intriguing question, and it becomes even more interesting in the recently rising high entropy oxides due to the extremely high disorder density. However, due to the lack of high-quality single crystal samples, the strong compositional disorder effect on magnetic transition has not been deeply investigated. In this work, we have successfully synthesized high-quality single crystalline high entropy spinel ferrites (Mg0.2Mn0.2Fe0.2Co0.2Ni0.2)xFe3-xO4. Our findings from high-temperature magnetization and neutron diffraction experiments showed ferrimagnetic transitions at 748, 694, and 674 K for x values of 1, 1.5, and 1.8, respectively. Notably, the magnetic transition almost showed no broadening for x values of 1 and 1.5, compared to Fe3O4. Extended X-ray absorption fine structure measurements provided insights into the elemental distribution among the octahedral and tetrahedral sites. The random distribution of elements across these sites reduced the formation of local clusters and short-range orders, enhancing sample homogeneity and preserving the sharpness of the magnetic transition, despite bond length variation. Our study not only marks the first successful synthesis of an HEO bulk single crystal exhibiting long-range magnetic order but also sheds light on the interaction between high configurational entropy and magnetic orderings. This opens new avenues for future research and applications of magnetic high entropy oxides.

Pressure-Induced YbFe2O4-Type to Spinel Structural Change of InGaMgO4

Spinel-type InGaMgO4 with a = 8.56615(3) Å was prepared by treating layered YbFe2O4-type InGaMgO4 at 6 GPa and 1473 K. DFT calculation and Rietveld analysis of synchrotron X-ray powder diffraction data revealed the inverse spinel structure with In3+:Ga3+/Mg2+ = 0.726:0.274 in the tetrahedral site and 0.137:0.863 in the octahedral site. InGaMgO4 spinel is an insulator with an experimental band gap of 2.80 eV, and the attempt at hole doping by post-annealing in a reducing atmosphere to introduce an oxygen defect was unsuccessful. This is the first report of the bulk synthesis of AB2O4 compounds with both YbFe2O4 and spinel polymorphs.

First‐Principle Calculation of the Hydrogen Adsorption and Diffusion Characteristics at an Ni3Al/Ni Interface

ABSTRACTThe occurrence of hydrogen embrittlement often leads to significant fracturing in materials. Thus, a comprehensive understanding of the interactions of hydrogen and its effects on material properties is essential. In this study, first‐principle calculations based on density functional theory are conducted to investigate the adsorption and diffusion characteristics of hydrogen at the Ni/Ni3Al(γ/γ') interface of a Ni‐based single‐crystal superalloy. The results show that hydrogen can stably adsorb onto the γ/γ′ interface, nearby octahedral interstitial sites, and tetrahedral interstitial sites in the γ‐phase. Analysis of the density of states and charge density confirms that the hydrogen atoms formed covalent bonds and ionic bonds with nearby Al atoms. A transition state structure search based on linear synchronous transit (LST) and quadratic synchronous transit (QST) models is utilized to calculate the diffusion paths and energy barriers of the hydrogen atoms between the stable adsorption sites near the interface. In addition, the permeation characteristics of hydrogen atoms at the interface are analyzed. Furthermore, it is confirmed that hydrogen atoms entering the matrix from the interface preferentially occupy the positions of the regular octahedral interstitial sites, diffuse towards adjacent regular 6Ni octahedral interstitial sites through the inclined octahedral interstitial sites between the atomic layers, and converge at the interface.

Study on the structural, magnetic, and magnetodielectric properties of M-type BaFe12O19 and SrFe12O19 hexaferrite nanoparticles

This study addresses the synthesis and characterization of hexagonal barium hexaferrite (BaFe12O19) and strontium hexaferrite (SrFe12O19) nanoparticles, aiming to enhance their structural and magnetic properties for potential applications. Using the sol–gel method, nanoparticles were synthesized and calcinated at 1000 °C, 1100 °C, and 1200 °C for 5 h. A comprehensive analysis was conducted on the prepared nanoparticles' structural, functional, morphological, and magnetic properties. X-ray diffraction confirmed the hexagonal structure and single-crystal phase for BaFe12O19 and SrFe12O19 across all calcination temperatures. The Rietveld analysis of XRD patterns of all the samples have been carried. Fourier transform infrared spectroscopy identified fundamental vibrations at octahedral and tetrahedral sites, verifying the presence of characteristic functional groups. Morphological studies revealed that at an annealing temperature of 1100 °C, the average nanoparticle diameters were 335 nm for BaFe12O19 and 302 nm for SrFe12O19. The elemental analysis and distribution of elements of hexaferrites were studies using Energy Dispersive X-ray Analysis with color mapping method. Magnetic properties were extensively analyzed, including saturation magnetization (Ms), coercivity (Hc), retentivity (Mr), anisotropy field (Ha), squareness ratio (Mr/Ms), and magnetocrystalline anisotropy (K1). At 1100 °C, the magnetic properties of BaFe12O19 are Ms = 55.96 emu/g, Hc = 4308 G, Mr/Ms = 0.51, Ha = 31.30 kG, and K1 = 5.65 × 105 erg/cm3. For SrFe12O19, these values are Ms = 50.80 emu/g, Hc = 3507 G, Mr/Ms = 0.52, Ha = 21.20 kG, and K1 = 5.65 × 105 erg/cm3. Additionally, the study explored the magnetodielectric properties of BaFe12O19 and SrFe12O19 nanoparticles. At a magnetic field of 6000 G, the magnetocapacitance percentage (MC%) and magnetotangent loss percentage (ML%) for BaFe12O19 are −6.23 % and −8.85 %, respectively, while for SrFe12O19, these values are −4.38 % and −1.0 %. The findings indicate that the synthesized hexaferrite BaFe12O19 and SrFe12O19 nanoparticles exhibit significant potential for applications in sensors, permanent magnets, and data recording, with optimized structural and magnetic properties achieved at specific calcination temperatures.

Solution and solubility of H atoms at the W/Cu interface.

Metallic interfaces are locations where hydrogen (H) is expected to segregate and lead to the formation and stabilization of defects. This work focuses on the tungsten/copper (W/Cu) interface built according to theWbcc(001)/Cuhcp(112-0)orientation. H behavior is subsequently determined at the interface and in its vicinity with electronic structure calculations based on the density functional theory (DFT). The electronic and vibrational properties determined in this way followed a thermodynamic treatment to deliver the solubility of H as a function of the temperature and chemical potential. The 96 interstitial positions we investigated reveal that H predominantly occupies the octahedral (Oh) sites in the copper network. Reversely, H exclusively occupies the tetrahedral (Td) sites in the tungsten network. The solubility of H is higher in the interface plane where both octahedral and tetrahedral sites are occupied. Despite this work is a first step toward kinetic modeling of hydrogen transport across the W/Cu interface, we conclude that theWbcc(001)/Cuhcp(112-0)would behave like a sink where hydrogen isotopes could accumulate.

Visualization of Tetrahedral Li in the Alkali Layers of Li-Rich Layered Metal Oxides.

Understanding Li+ ion diffusion pathways in Li-rich layered transition metal (TM) oxides is crucial for understanding the sluggish kinetics in anionic O2- redox. Although Li diffusion within the alkali layers undergoes a low-barrier octahedral-tetrahedral-octahedral pathway, it is less clear how Li diffuses in and out of the TM layers, particularly given the complex structural rearrangements that take place during the oxidation of O2-. Here, we develop simultaneous electron ptychography and annular dark field imaging methods to unlock the Li migration pathways in Li1.2Ni0.13Mn0.54Co0.13O2 associated with structural changes in the charge-discharge cycle. At the end of TM oxidation and before the high-voltage O oxidation plateau, we show that the Li migrating out of the TM layers occupies the alkali-layer tetrahedral sites on opposite sides of the TM layers, forming Li-Li dumbbell configurations, consistent with the density functional theory calculations. Also occurring are the TM migration and phase transition from O3 to O1 stacking, leading to unstable tetrahedral Li and the absence of Li contrast in imaging. Upon further Li deintercalation to 4.8 V, most of the tetrahedral Li are removed. After discharging to 2 V, we did not identify the reformation of tetrahedral Li but observed permanently migrated TMs at the alkali-layer sites, disfavoring the Li occupying the tetrahedral sites for diffusion. Our findings suggest a landscape of Li diffusion pathways in Li-rich layered oxides and strategies for minimizing the disruption of Li diffusion.

Dopant site occupancies and iron valence states in SrZnxFe18−xO27 W-type hexaferrites using site-selective X-ray/electron spectroscopy

Zn-doped W-type Sr hexaferrite (SrZnxFe18−xO27; SrZnx-WHF) is an anticipated rare-earth-free hard magnetic material with strong magnetocrystalline anisotropy and saturation magnetization. To examine the origin of its high magnetic performance, the site distribution of Zn over seven crystallographically inequivalent Fe sites and the site-dependent valence states of Fe were investigated using site-selective elemental/chemical analysis techniques. The dominant occupation of Zn2+ at the 4etet(S) and 4ftet(S) tetrahedral sites was determined quantitatively using high-angular-resolution electron-channeling X-ray spectroscopy and statistical data analysis. The combined application of high-angular-resolution electron channeling electron spectroscopy and atomic-column resolution scanning transmission electron microscopy–electron energy-loss spectroscopy helped unambiguously clarify that most of the Fe2+ ions existed at the 6goct(S-S) octahedral site. Density functional theory calculations have shown that Zn doping leads to a redistribution of the charges toward Fe ions at this site. This redistribution preserves the local charge balance and amplifies the total number of parallel spins. Hence, the enhanced magnetocrystalline anisotropy and saturation magnetization in SrZnx-WHF can be inferred to originate from the charge redistribution at the 6goct(S-S) site, which is influenced by the presence of nonmagnetic Zn.

Effect of substitution of aluminium and neodymium on structural and magnetic properties of yttrium iron garnet studied using 57Fe internal field NMR

The structural and magnetic properties of non-magnetic aluminium and magnetic neodymium substituted yttrium iron garnet samples (Y3-xAlxFe5O12 and Y3-nNdnFe5O12) were investigated. Powder samples were synthesized using auto combustion route. The Rietveld refinement analysis of the samples confirmed the presence of the garnet phase and showed presence of minimal impurity phases, specifically perovskite and hematite. Substitution on Y3Fe5O12 with Al resulted in a linear decrease in saturation magnetization, while Nd substitution showed a non-linear change. In accordance with this the Internal Field Nuclear Magnetic Resonance (IFNMR) spectrum exhibited a downward shift in peak frequency when Al was substituted, and a significant reduction in intensity with Nd substitution in yttrium iron garnet samples. Aluminium ions take up either tetrahedral or octahedral positions in the lattice, apart from the dodecahedral site, because of their smaller ionic radii. This causes a decrease in the hyperfine field that originates from Fe3+ ions in tetrahedral and octahedral sites. On the other hand, neodymium ions occupy dodecahedral sites and are expected not to affect Fe ions. Concurrently, an observable peak shift was not observed in the IFNMR spectrum for Nd substitution. However, Nd substitution introduced strain in the lattice due to their larger ionic radii compared to yttrium, which they replace. This strain causes disorder in tetrahedral and octahedral sites and affects 57Fe signal intensity. The magnetic properties observed from VSM analysis is also interpreted in view of observed changes in the 57Fe IFNMR spectrum. X-ray Photoelectron spectroscopy (XPS) was employed mainly to explore spin orbit splitting of Fe 2p state in the samples.