Tetrahedral 8a Sites Research Articles

Preferential spin canting in nanosize zinc ferrite

Zinc ferrite nanoparticles powder with average size of 10.0±0.5nm was synthesized by the citrate precursor route. We studied the structural and magnetic properties using X-ray diffraction, vibrating sample magnetometry and Mössbauer spectroscopy. X-ray diffraction patterns show that the synthesized zinc ferrite possesses good spinel structure. Both Mössbauer and magnetization data indicate superparamagnetic ferrimagnetic particles at room temperature. The magnetic behavior is determined by a considerable degree of cation inversion with FeIII in tetrahedral A-sites. Mössbauer spectroscopy at low temperature and in high applied magnetic field reveals that A-site spins are aligned antiparallel to the applied field with some possible angular scatter whereas practically all octahedral B-site spins are canted contrasting some earlier reported partial B-site spin canting in nanosize zinc ferrite. Deviations from the antiferromagnetic arrangement of B-site spins are supposed to be caused by magnetic frustration effects.

Effects of Co-substitution on the structural and magnetic properties of NiCoxFe2−xO4 ferrite nanoparticles

The sol–gel auto-combustion method was used to prepare nanocrystalline powders of Co-substituted nickel ferrite with the general formula NiCoxFe2−xO4 (x=0.0, 0.1, 0.25, and 0.5). The effects of Co-doping on the structural, morphological, and magnetic properties of the samples were subsequently evaluated by X-ray diffraction (XRD), Fourier transform infrared (FTIR), field emission scanning electron microscopy (FE-SEM), and vibrating sample magnetometer (VSM). Using the MAUD program, the full pattern fitting of Rietveld method was employed to determine the exact coordinate of the atoms, unit cell dimensions, and ion occupancy. X-ray diffraction measurements by Rietveld refinement confirmed the crystalline structure and phase purity of all the ferrites prepared. FTIR results also confirmed the formation of a spinel phase and FE-SEM images showed that the particles were in the nanosize range. Moreover, Rietveld analysis and saturation magnetization (Ms) revealed that Co3+ replaced Fe3+ in the tetrahedral A-sites up to x=0.1. then, it replaced Fe3+ in both A- and B-sites for x≥0.25. Finally, VSM results demonstrated that while Ms remained nearly constant with increasing Co3+ substitution, coercivity (Hc) increased significantly. It may be suggested that the larger magneto-crystalline anisotropy of Co3+ ions is responsible for the increased Hc observed in the Co-doped Ni ferrite samples.

Characterization, structural and magnetic properties of the as-prepared Mg-substituted Cu-nanoferrites

The as-prepared Cu1−xMgxFe2O4 nanoferrites, 0⩽x⩽1, were synthesized using the co-precipitation method. The samples were characterized using the X-ray diffraction, transmission electron microscopy, infrared (IR) and Mössbauer spectroscopy and vibrating sample magnetometer. This study proved that the samples have single-phase cubic spinel structure, nanometric size and superparamagnetic behavior. The nanoparticle and crystallite size (R) showed a dependence on the Mg ion content x. The Mössbauer spectra were analyzed and assigned to two magnetic subpatterns and/or two quadrupole doublets due to Fe ions among the tetrahedral A-sites and octahedral B-sites. Six IR absorption bands were observed and assigned to the corresponding vibration modes. The saturation magnetization showed decrease against x and proved dependence on R. The lattice constant, hopping lengths, strain, site characteristic bands and B-site force constant showed x dependence, whereas oxygen parameter and A-site force constant did not. The site ionic radii and edges, density, porosity, Debye temperature, stiffness constant, hyperfine interaction and magnetic parameters were deduced and discussed as functions of x and the cation distributions were estimated.

Structural and magnetic properties of nickel–zinc ferrite nanocrystalline magnetic particles prepared by microwave combustion method

Magnetic, nano-crystalline samples of zinc substituted nickel ferrite, (ZnxNi1-xFe2O4 for x = 0.0-0.9 in step of 0.2), are synthesized using microwave combustion synthesis technique. The structural properties of Ni-Zn are deter- mined using X-ray powder diffraction, transmission electron microscopy; Fourier transforms infrared spectroscopy and neutron diffraction techniques. Average crystalline size obtained from X-ray diffraction and neutron diffraction is in the range of 30-60 nm. The cation distribution obtained from X-ray diffraction and neutron diffraction show that Zn occupies only tetrahedral A-site in the spinel lattice. The values of magnetic moment derived from magnetization measurements and neutron diffraction agrees nearly 97 % to that of bulk at 300 K. This methodology can be used to prepare large quantities (about 10 g) of sample at one time using kitchen microwave oven working at 1,200 W power.

Synthesis and Crystal Structure of Lithium Iron Titanium Oxides Li1/2-+1/2xFe5/2-3/2xTixO4 with 0 ≤ x ≤ 1.666

Li[Li1/3Ti5/3]O4 (LTO) is an ideal negative electrode material for long-cycle life lithium-ion battery (LIB), because of its “zero-strain” character [1]. In the LTO crystal lattice with Fd-3m space group, Li+ ions occupy both tetrahedral 8a and octahedral 16d sites, while Ti4+ ions sit the 16d site. We, thus, expect a solid solution compound Li1/2+1/2x Fe5/2–3/2x Ti x O4 (LFTO) between LTO and Fe[Li1/2Fe3/2]O4 via an intermediate spinel compound Li1/2Fe1/2[Li1/2Fe1/2Ti]O4. Fe[Li1/2Fe3/2]O4 is known to be one of the ferrites, which exhibit a soft ferromagnetic behavior. Here we report the results of powder X-ray diffraction (XRD) and electron diffraction (ED) measurements, and Raman spectroscopy analyses on the LFTO samples with 0 ≤ x ≤ 1.666. According to XRD measurements, all the LFTO samples were assigned as a spinel-framework structure. For the x = 0 sample, the extra diffraction lines were clearly observed in addition to the normal diffraction lines in the Fd-3m. The extra diffraction lines are attributed to the superlattice diffraction lines due to the ordering between Li+ and Fe3+ ions at the octahedral site. As x increases from 0.25, such superlattice diffraction lines disappear, but, surprisingly, superlattice diffraction lines appear again at x > 1.2. The crystal structure at x £ 0.25 and 1.2 < x < 1.6 are assigned as the P4332 space group. Combining with the result of x dependence of cubic lattice parameter (a c), the crystal structure of LFTO is classified into the four regions; (I) regular spinel structure of Fd3-m with x > 1.55, (II) superlattice spinel structure of P4332 with 1.2 ≤ x ≤ 1.55, (III) regular spinel structure of Fd3-m with 0.25 < x < 1.2, and (IV) superlattice spinel structure of P4332 with x ≤ 0.25.AcknowledgementThis work was supported by Grant-in-Aid for Scientific Research (C), 25410207, MEXT, Japan.

Structural and magnetic properties correlated with cation distribution of Mo-substituted cobalt ferrite nanoparticles

Mo-substituted cobalt ferrite nanoparticles; CoFe2−2xMoxO4 (0.0≤x≤0.3) were prepared by a one-step solution combustion synthesis technique. The reactants were metal nitrates and glycine as a fuel. The samples were characterized using an X-ray diffraction (XRD), a transmission electron microscope (TEM) and a vibrating sample magnetometer (VSM). XRD analysis revealed a pure single phase of cubic spinel ferrites for all samples with x up to 0.3. The lattice parameter decreases with Mo6+ substitution linearly up to x=0.15, then nonlinearly for x≥0.2. Rietveld analysis and saturation magnetization (Ms) revealed that Mo6+ replaced Fe3+ in the tetrahedral A-sites up to x=0.15, then it replaced Fe3+ in both A-sites and B-sites for x≥0.2. The saturation magnetization (Ms) increases with increasing Mo6+ substitution up to x=0.15 then decreases. The crystallite size decreased while the microstrain increased with increasing Mo6+ substitution. Inserting Mo6+ produces large residents of defects and cation vacancies.

Solubility limits in Mn–Mg ferrites system under hydrothermal conditions

In the present investigation, we successfully synthesized a pure MnFe2O4 ferrite by the hydrothermal method. Moreover, the effect of Mg ion content on the formation of Mn1−xMgxFe2O4 particles (with x varying from 0.1 to 1.0) was also investigated using XRD, SEM, TEM and Mossbauer Spectroscopy.Phases formed in the system Mn1−xMgxFe2O4; 0.0≤x≤1.0 were investigated under hydrothermal conditions at 453K.The produced phases were characterized by X-ray diffraction, Scanning, transmission microscopy and Mossbauer spectroscopy. The information of composition, cation distribution in the spinel structure and the particle size of the products were obtained. The spinel ferrites; Mn1−xMgxFe2O4 were formed in the range 0.0≤x≤0.3. However, sample with x>0.3 showed semi-crystalline magnesium hydroxide (Mg(OH)2) and hematite (Fe2O3) beside the ferrite phase. For x=1.0, only magnesium hydroxide and hematite are formed without any ferrites.Particles of uniform size around 10–20nm were obtained in the spinel structure of Mn1−xMgxFe2O4 with x=0.0 and 0.1. The corresponding average crystallite size for each sample was 40.3nm and 39.2nm respectively. In addition, the Mossbauer spectra were analyzed into two subspectra, one for the tetrahedral A-site and the other for the octahedral B-site. The Mossbauer parameters were determined and discussed for the studied system. The cation distribution was estimated from the analysis of the Mossbauer spectra as well as the X-ray diffraction patterns. The results showed that Mg ions occupy mainly B-site while both Mn and Fe ions are distributed between A- and B-sites.

Beyond One Li+ Storage in Spinel-Type LiMTiO4 (M = Fe, Mn) and Understanding of Structural Mechanisms

Electrochemical energy storage using rechargeable Li-ion batteries is one of the most competitive technologies for future stationary and automotive applications. High capacity cathode materials (> 250 mA h g-1) for Li-ion batteries are of great interest to meet the future demands. A majority of the existing cathode materials deliver capacity based on a single electron or even less than one electron (per transition metal) reaction with limited practical capacity. A feasible strategy to obtain higher capacity is to use cathode materials that enable reversible more than one-electron reaction. In this contribution, we demonstrate that spinel-type lithium transition metal titanates, LiMTiO4 (M = Fe, Mn), can deliver a reversible discharge capacity of about 250 mA h g-1. Partial Li+ can be extracted out of the spinel host upon charging. Unoccupied crystallographic sites in the spinel host could accommodate guest Li+ into the host framework. Thus, an overall up to 1.6 Li+ storage capability has been achieved. Detailed structural characterizations were performed to understand the encouraging electrochemical Li+ storage performance of the spinel LiMTiO4. Nanosized LiFeTiO4 and LiMnTiO4 powders were synthesized by a sol-gel route.1 LiFeTiO4 with a native carbon coating was obtained by sintering the xerogels under argon atmosphere, whereas LiMnTiO4 with a carbon-free bare surface was obtained by air-sintering. Both materials crystallize in disordered cubic spinel phase (Fd-3m space group), as observed from the X-ray diffraction analysis (Fig. 1). In LiFeTiO4 the occupancy of Li/Fe at tetrahedral 8a sites is about 2:1, whereas in LiMnTiO4 8a sites are mainly occupied by Li. The charge-discharge curves and cyclability for both materials are shown in Fig. 2. A discharged capacity of around 250 mA h g-1 has been achieved at a C/40 rate for LiFeTiO4/C and at a C/20 rate for LiMnTiO4. The observed average discharge voltage is 2.5 V and 3.1 V for LiFeTiO4/C and LiMnTiO4, respectively. Superior capacity retention was observed for both materials.2,3 57Fe Mössbauer spectroscopy confirmed that the initial Fe3+ in LiFeTiO4 can be partially oxidized to Fe4+ upon charging to 4.8 V and then the Fe3+/Fe4+ ions can be reduced to Fe2+ after discharging to 1.5 V. Upon long-term cycling, Li x FeTiO4 (0.4 < x < 2) maintains the spinel phase.2 Unlike the sloping charge/discharge profiles for LiFeTiO4/C, LiMnTiO4 shows initially a two-plateau charge/discharge profile similar to that of classic spinel LiMn2O4. Electrochemical cyclic voltammetry confirmed that the two-plateau regions are associated with Mn2+/Mn3+ and Mn3+/Mn4+ redox couples. After a few cycles, the charge/discharge profile evolves progressively into a sloping profile. Structural characterization of the cycled LiMnTiO4 shows a phase transition from spinel to rock-salt and tetragonal Li1+x MnTiO4 phases after first discharge. Upon further charging, the rock-salt phase converts reversibly back to the spinel phase. The reversible spinel/rock-salt phase transition occurs with the cubic framework remaining intact. The presence of tetragonal phase indicates that the Mn3+ Jahn-Teller distortion is partially involved during lithiation. However, unlike LiMn2O4, the cubic/tetragonal two-phase region located at 3.1/2.8 V is related to Mn3+ reduction to Mn2+ (non Jahn-Teller distortion ions) leading to a final composition of Li2Mn2+Ti4+O4. Over further cycling, the content of tetragonal phase (~40%) does not change much. In addition, the lattice of the tetragonal phase shows reversible change during charge/discharge. This distinct feature enables LiMnTiO4 to be cycled over a broad voltage range (1.5–4.8 V) and makes LiMnTiO4promising as high-capacity cathode material. To summarize, spinel type Li-transition metal titanates exhibit the capability for up to 1.6 Li+ storage, durable cycling performance and fast Li+kinetics. The abundance and nontoxicity of Fe, Mn and Ti are ideal for the fabrication of electrode materials and this class of materials has the potential to become an alternative for future large-scale applications. Acknowledgement:This work was financially supported by the BMBF and the “Helmholtz Initiative for Mobile/Stationary Energy Storage Systems”.

Crystallographic and Magnetic Properties of Nano-sized Nickel Substituted Cobalt Ferrites Synthesized by the Sol-gel Method

Nano-sized nickel substituted cobalt ferrite powders, <TEX>$Ni_xCo_{1-x}Fe_2O_4$</TEX> (<TEX>$0.0{\leq}x{\leq}1.0$</TEX>), were fabricated by the sol-gel method, and their crystallographic and magnetic properties were studied. All the ferrite powders showed a single spinel structure, and behaved ferrimagnetically. When the nickel substitution was increased, the lattice constants and the sizes of particles of the ferrite powders decreased. The M<TEX>$\ddot{o}$</TEX>ssbauer absorption spectra of <TEX>$Ni_xCo_{1-x}Fe_2O_4$</TEX> ferrite powders could be fitted with two six-line subspectra, which were assigned to a tetrahedral A-site and octahedral B-sites of a typical spinel crystal structure. The increase in values of the magnetic hyperfine fields indicated that the superexchange interaction was stronger, with the increased nickel concentration in <TEX>$Ni_xCo_{1-x}Fe_2O_4$</TEX>. This could be explained using the cation distribution, which can be written as, <TEX>$(Co_{0.28-0.28x}Fe_{0.72+0.28x})[Ni_xCo_{0.72-0.72x}Fe_{1.28-0.28x</TEX><TEX>}</TEX><TEX>]</TEX><TEX>O_4$</TEX>. The two values of the saturation magnetization and the coercivity decreased, as the rate of nickel substitution was increased. These decreases could be explained using the cation distribution, the magnetic moment, and the magneto crystalline anisotropy constant of the substituted ions.

Effect of cation and anion doping on microstructure and electrochemical properties of the LiMn1.5Ni0.5O4−δ spinel

A detailed investigation of the structural and electrochemical impact of lithium and fluorine substitutions on the 4.7 V LiMn1.5Ni0.5O4−δ (LMNO) positive electrode for Li-ion batteries was accomplished. Impact of subsequent transition metal substitution on the tetrahedral 8a sites was also investigated relative to rate and elevated temperature cycling stability. Significantly improved cycling in excess of 600 cycles was established for an anion modified spinel relative to an optimized benchmark material at 60 °C.

Magnetic Properties of Irradiated Nickel Ferrite Thermoplastic Natural Rubber Nanocomposite

The effect of electron beam (EB) irradiation at different doses on the magnetic, microstructure, morphological and thermal properties of NiFe2O4/Thermoplastic Natural Rubber (TPNR) nanocomposite was investigated. The NiFe2O4/TPNR nanocomposite samples were prepared by using a Haake mixer in weight ratio of 12:88. The TPNR matrix consists of natural rubber (NR), liquid natural rubber (LNR) and high density polyethylene (HDPE) in weight ratio of 20:10:70. The samples were irradiated using a 2 MeV EB machine in doses from 0 - 200 kGy. Magnetic properties studied by using the vibrating sample magnetometer (VSM) at room temperature showed that the values of saturation magnetization (MS), remanence magnetization (MR) and the coercivity (HC) value increased with increasing doses of irradiation. The increase in MSand MRvalues is attributed to the increase in concentration of Fe3+ions at octahedral B-site and decrease of concentration at the tetrahedral A-site in the NiFe2O4cubic structure. X-ray diffraction (XRD) analysis of the samples showed that peak intensities decreased and the width of the peaks increased with increasing doses of irradiation. Scanning electron microscope (SEM) image of the nanocomposite cross section showed the presence of defects which is more visible with increasing doses of irradiation. In the case of thermal properties, differential scanning calorimetry (DSC) analysis showed that the crystallization temperature (Tc) and the degree of crystallinity (Xc) of the nanocomposite samples decreased with increasing doses of irradiation due to crosslinking of polymeric chains which hinders the growth of crystals.

Variation in the structural and magnetic properties of heterovalent Mn2+–Si4+ substituted MnCrFeO nanoparticles

We have synthesized heterovalent Mn2+–Si4+ substituted MnCrFeO nanoparticles with a nominal composition Mn1+xSixCrFe1−2xO4 (x = 0.0–0.3) via sol–gel auto-combustion method. X-ray diffractometer, transmission electron microscopy, magnetization measurements were used to study the effects of Mn2+–Si4+ heterovalent ions on the structural and magnetic properties of MnCrFeO. As a result, it was found that the Mn2+–Si4+ ions affect the crystalline structures and magnetic properties of MnCrFeO. X-ray diffraction pattern showed that the samples have the single phase cubic spinel structure of which the lattice constant slightly increased upon Mn2+–Si4+ substitution. The mean crystallite size of the samples was in the range of 21–27 nm as deduced from the XRD line broadening. Cation distribution was estimated using XRD data and it shows that Mn2+ and Si4+ ions prefer tetrahedral A-site. Magnetic measurement shows that saturation magnetization and magneton number decreased with Mn2+–Si4+ substitution with the formation of a collinear spin arrangement.

Cation distribution and Mössbauer spectral studies of Mg0.2Mn0.5Ni0.3InxFe2−xO4 ferrites (x = 0.0, 0.05 and 0.10)

The effect of substitution of diamagnetic indium ions in Mg–Mn–Ni ferrite with the composition Mg0.2Mn0.5Ni0.3InxFe2−xO4 (x=0.0, 0.05 and 0.10) synthesized by citrate precursor technique has been investigated. Crystallographic and magnetic properties have been investigated using X-ray diffraction, TEM, VSM and Mössbauer spectroscopy. The single phase cubic spinel structure of these samples has been confirmed from X-ray diffraction analyses. In3+ ions initially prefer tetrahedral A-site up to x=0.05 due to which saturation magnetization and magnetic moment increases, followed by subsequent decrease when indium ions begin to occupy octahedral B-site for higher concentration of x. Magnetization measurements exhibit Neel’s collinear ferrimagnetic structure for samples up to x=0.05. Mössbauer spectra of these samples studied at 300K show two characteristic ferrimagnetic Zeeman sextets for x=0.0, followed by relaxation phenomenon corresponding to x=0.05 and 0.10. Mössbauer measurements also show dependence of Zeeman spectral lines on smaller particle size which is indicative of their superparamagnetic nature. The dependence of Mössbauer parameters such as isomer shift, quadrupole splitting, linewidth and hyperfine magnetic field on In3+ ions concentration have been discussed. The variation of hyperfine magnetic field with increasing In3+ ions content has been explained by Neel’s molecular field theory.

Structural Changes in the LiCrMnO4 Cathode Material during Electrochemical Li Extraction and Insertion

Structural features of Li-extraction from the LiCrMnO4 cathode material in Li-electrochemical cells were investigated by in situ synchrotron powder diffraction and completed by ex situ neutron powder diffraction and magnetization measurements on delithiated products. The observed diffraction patterns reveal that both Li-extraction during the cell charge up to 5.15 V and the following Li-insertion upon the discharge down to 3.2 V vs. Li+/Li are one-phase solid-solution processes. According to the Rietveld structure refinement Li atoms were completely removed from the tetrahedral 8a site during the cell charge with the C/10 rate. However, only about 40% Li atoms were inserted during the cell discharge with the same rate. The unit cell parameter a and the average metal−oxygen distance (Cr,Mn-O) show non-linear dependence on the lithium content upon extraction, with the slope changing at 4.9 V and manifesting a change in the mechanism of redox behavior. While the first step between 3 V and 4.9 V is presumably accompanied by the oxidation of Cr3+ to Cr4+, the charging above 4.9 V requires a further oxidation of Cr4+ toward Cr6+. The paramagnetic effective moment of LixCrMnO4 compounds changes non-monotonously with decreasing Li-content.

Effect of Conductive Additives on the Structural and Electrochemical Properties of Li4Ti5O12Spinel

The effect of a conductive agent on the structural and electrochemical properties of <TEX>$Li_4Ti_5O_{12}$</TEX>(LTO) spinel was investigated through neutron diffraction during Li intercalation and electrochemical measurements. The charging process of LTO is known as transformation of the white <TEX>$(Li_3)_{8a}[LiTi_5]_{16d}O_{12}$</TEX> into a dark-colored <TEX>$(Li_{3-X})_{8a}[Li_{X+Y}]_{16c}[LiTi_5]_{16d}O_{12}$</TEX> by incorporating the inserted Li into octahedral 16c sites, and the Li in tetrahedral 8a sites shifted to 16c sites. The occupancy of the tetrahedral 8a site varied with the existence of carbon in the electrode. Without carbon, the lattice parameter and cell volume of LTO decreased more notably than in the carbon-containing LTO electrode during Li insertion process. These phenomena might be attributed that the Li occupancy of the tetrahedral 8a of the LTO electrode without carbon was less than that of the carbon-containing LTO electrode.

Cation Distribution Dependence on Thermoelectric Properties of Doped Spinel M&lt;sub&gt;0.6&lt;/sub&gt;Fe&lt;sub&gt;2.4&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt;

The electrical conductivity, Seebeck coefficient, and thermal conductivity of polycrystalline M0.6Fe2.4O4 (M = Ni, Ni0.5Mg0.5, Ni0.5Zn0.5, Zn) were measured to elucidate cation distribution-dependent changes. Preferential occupation by the doped cation in the iron spinel has been noted: Zn2+ ions prefer to occupy the tetrahedral A-site, while Ni2+ and Mg2+ prefer to occupy the octahedral B-site. While the electrical conductivity and Seebeck coefficient are almost cation distribution-independent, the thermal conductivity at room temperature is sensitive to the cation distributions. The lowest thermal conductivity of 2.0Wm11 K11 at room temperature is observed for Zn0.6Fe2.4O4. The value is about one third of that of Ni0.6Fe2.4O4. The thermal transport of MxFe31xO4 is mainly affected by cation distribution at the A-site, while the electrical transport is affected by the B-site, which is discussed in terms of the point defects at the Aand B-sites. Due to the disordering at the Aand Bsites, the thermal conductivity of MxFe31xO4 could be reduced without decreasing the electrical conductivity. Doped spinel-ferrite MxFe31xO4 would be a kind of “phonon-glass electron-crystal” material. [doi:10.2320/matertrans.M2012023]

Orbital moment determination in (MnxFe1−x)3O4 nanoparticles

Nanoparticles of (MnxFe1−x)3O4 with a concentration ranging from x = 0 to 1 and a crystallite size of 14–15 nm were measured using X-ray absorption spectroscopy and X-ray magnetic circular dichroism to determine the ratio of the orbital moment to the spin moment for Mn and Fe. At low Mn concentrations, the Mn substitutes into the host Fe3O4 spinel structure as Mn2+ in the tetrahedral A-site. The net Fe moment, as identified by the X-ray dichrosim intensity, is found to increase at the lowest Mn concentrations then rapidly decrease until no dichroism is observed at 20% Mn. The average Fe orbit/spin moment ratio is determined to initially be negative and small for pure Fe3O4 nanoparticles and quickly go to 0 by 5%–10% Mn addition. The average Mn moment is anti-aligned to the Fe moment with an orbit/spin moment ratio of 0.12 which gradually decreases with Mn concentration.

Enhanced magnetism of Fe3O4 nanoparticles with Ga doping

Magnetic (GaxFe1−x)3O4 nanoparticles with 5%–33% gallium doping (x = 0.05–0.33) were measured using x-ray absorption spectroscopy and x-ray magnetic circular dichroism to determine that the Ga dopant is substituting for Fe3+ as Ga3+ in the tetrahedral A-site of the spinel structure, resulting in an overall increase in the total moment of the material. Frequency-dependent alternating-current magnetic susceptibility measurements showed these particles to be weakly interacting with a reduction of the cubic anisotropy energy term with Ga concentration. The element-specific dichroism spectra show that the average Fe moment is observed to increase with Ga concentration, a result consistent with the replacement of A-site Fe by Ga.

Spectral studies of Co substituted Ni–Zn ferrites

The spinel ferrites Zn 0.35Ni 0.65− x Co x Fe 2O 4, 0≤ x≤1, have been prepared using the standard ceramic technique. Room temperature Mössbauer, X-ray and infrared IR spectra were used for carrying out this study. X-ray patterns reveal that all the samples have single-phase cubic spinel structure. The Mössbauer spectra of the samples show a paramagnetic phase for x=0 and a six-line magnetic pattern and a central paramagnetic phase for x≥0.1. They are analyzed and attributed to two magnetic subpatterns and two quadrupole doublets due to Fe 3+ ions at the tetrahedral A-sites and octahedral B-sites. Four absorption bands are observed in IR spectra. They confirm the spinel structure of the samples and existence of Fe 3+ ions in the sample sublattices. The deduced hyperfine interactions, lattice parameters, absorption band positions and intensities and force constant are found to be dependent on the substitution factor x, where the cation distribution is estimated. The hyperfine magnetic fields, magnetization and lattice resonant frequency are found to be dependent on the interionic distance.

Influence of Li addition on charge/discharge behavior of spinel lithium titanate

This article provides proof of an efficient approach to adjust the behavior of Li + intercalation into Li 4Ti 5O 12 spinel structures prepared by spray-drying followed by solid-state calcination. The spherical Li 4Ti 5O 12 powders consist of a number of grains with an average size of 300–500 nm, showing high rate capability at various rates of 0.1–20 C. The addition of Li-ions greatly modifies the Li 4Ti 5O 12 crystallites with different relative intensity ratios of (3 1 1)/(4 0 0), R value, determined from X-ray diffraction results. The overall reversible capacity in the voltage range of 2.5–0.01 V vs. Li/Li + is limited by the vacant tetrahedral (8a) and octahedral (16c) sites. The decrease of capacity ratio of second plateau (1.0–0.01 V) to first plateau (2.5–1.0 V) with the R value proves that the ratio of site vacancy greatly affects the capacity contribution. At high C rates, the tetrahedral (8a) sites exhibit a better accessibility to accommodate Li-ions during the intercalation process, compared with the accessibility of octahedral (16c) sites. This extra capacity of Li 4Ti 5O 12, originating from the Li + insertion at low potential, facilitates not only an enhanced reversible capacity but also a wider working voltage, leading to a higher energy density for rechargeable Li-ion batteries.