Fe-substituted Samples Research Articles

Neutron-Scattering Study of Fe-Substitution Effect on the Static Spin Correlation in La2-xSrxNiO4

We have performed elastic neutron scattering measurements on the Fe-doped La 2- x Sr x NiO 4 (LSNO) with 0.12 ≤ x ≤0.50 to study the magnetic impurity effects on the static spin correlations. In the pristine LSNO system, it is well known that both spin and charge stripe orders are stabilized at low temperatures in the wide hole concentration range from 0.12 to 0.70. The incommensurability (e) increases upon Sr-doping and tends to saturate with e max ∼0.47 (r.l.u.) for x > 0.5. We found that 4% Fe-substitution for Ni ion reduces e at the fixed Sr concentration for the all measured samples with 0.12 ≤ x ≤0.5. The e in the Fe-substituted samples with x = 0.25, 0.33, 0.37, and 0.50 was 0.02–0.04 (r.l.u.) smaller than that in the pristine samples. Furthermore, we observed commensurate peak in the Fe-doped x = 0.12 sample. No well-defined incommensurate magnetic peaks were detected, unlike to the result for the pristine x = 0.12 sample. The effect of Fe-substitution on the magnetic ordering temperature ( T spin...

Magnetic Properties of Ru-rich Ru2−x Fe x CrZ (Z=Si, Ge)

The properties of Heusler compounds Ru2−xFexCrGe are investigated and compared with those of Ru1.9Fe0.1CrSi. Ru2CrGe is confirmed to exhibit an antiferromagnetic transition with Neel temperature 13 K by magnetic susceptibility and specific heat measurements. When Fe is substituted for Ru, a peak in the magnetic susceptibility is observed, and in the lower temperature region irreversibility between temperature dependences of magnetization for zero-field-cooling and field-cooling conditions is found. Nevertheless, in specific heat of Ru1.9Fe0.1CrGe, there is no anomaly to indicate phase transition. The specific heat is almost identical to that for Ru1.9Fe0.1CrSi. The above results demonstrate that the magnetic states in the low temperature region of Fe-substituted samples are spin-glass-like states. Slight substitution of Fe for Ru destroys the long-range-order and lead to peculiar spin-glass-like states.

Fe-substituted nanometric La 0.9K 0.1Co 1− xFe xO 3− δ perovskite catalysts used for soot combustion, NO x storage and simultaneous catalytic removal of soot and NO x

A series of nanometric Fe-substituted La 0.9K 0.1Co 1− x Fe x O 3− δ ( x = 0, 0.05, 0.1, 0.2, 0.3) perovskite catalysts were prepared by citric acid complexation, which show novel activity for diesel soot oxidation, NO x storage, and simultaneous NO x–soot removal. Their structures and physical–chemical properties were examined by XRD, BET, SEM, XPS and EXAFS techniques. In La 0.9K 0.1Co 1− x Fe x O 3− δ catalysts, the partial substitution of Co 3+ by Fe 3+ significantly enhances the catalytic activity towards soot oxidation or NO x storage and reduction. The catalyst La 0.9K 0.1Co 0.9Fe 0.1O 3− δ shows the highest activity for simultaneous NO x–soot removal, over which the maximal soot oxidation rate is achieved at only 362 °C ( T m), the NO x storage capacity reaches 213 μmol g −1, and the percentage for NO x reduction by soot is 12.5%. Compared with the unsubstituted one (La 0.9K 0.1CoO 3− δ ), the activity enhancement of Fe-substituted samples results from the formation of high valence ion (Fe 4+) at B-site, relative high content of surface lattice oxygen and the high concentration of NO 2. Different from the widely accepted viewpoint that surface adsorbed oxygen species are responsible for soot combustion over perovskite catalysts, the surface lattice oxygen is identified as active oxygen species for soot combustion.

Understanding the Improved Electrochemical Performances of Fe-Substituted 5 V Spinel Cathode LiMn1.5Ni0.5O4

Partial substitution of Fe for Ni alone or both for Ni and Mn in the 5 V spinel cathode LiMn1.5Ni0.5O4 has been investigated by characterizing the samples by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), charge−discharge measurements in lithium cells, electrochemical impedance spectroscopy (EIS), chronoamperometry (CA), and X-ray photoelectron spectroscopy (XPS). The Fe-substituted samples exhibit remarkably superior cycling performances and higher rate capabilities than the pristine LiMn1.5Ni0.5O4. The greatly enhanced electrochemical performances by the Fe substitution are attributed to the (i) stabilization of the structure with cation-disorder in the 16d octahedral sites of the spinel lattice, (ii) suppression of the formation of a thick solid-electrolyte interfacial (SEI) layer due to the Fe-enrichment and Ni-deficiency on the surface, (iii) production of Mn3+ and the consequent enhancement in electronic conductivity, and (iv) much reduced polarization loss arising from bo...

Electrochemical properties of LiNi 1− yFe yO 2 synthesized by the combustion method in O 2

LiNi 1− y Fe y O 2 (0.000 ≤ y ≤ 0.025) with small y prepared by a combustion method have only the phase with the α-NaFeO 2 structure of the rhombohedral system (space group; R 3 ¯ m ). Fe is quite continuously detected over the analysis line of EPMA on the surface of LiNi 0.995Fe 0.005O 2, indicating that Fe is distributed quite uniformly. LiNi 0.995Fe 0.005O 2 shows better electrochemical properties than LiNiO 2 with the highest first charge capacity (285 mAh/g) and the highest first discharge capacity (165 mAh/g) at 0.1 C-rate among LiNi 1− y Fe y O 2 (0.000 ≤ y ≤ 0.025). FTIR desorbance curves indicate that Li 2CO 3 remains in LiNiO 2. However, the peaks for Li 2CO 3 are very weak in the Fe-substituted samples. Electron diffraction pattern of LiNi 0.995Fe 0.005O 2 exhibits four diffraction points coming from a local Fm3 m structure made by cation mixing. A larger number of Li + occupying the Li sites, due to the preferential occupation of Fe 3+ in the Li sites, decrease the mixing of Ni 2+ into the Li sites and improve the electrochemical properties of LiNiO 2.

Effect of metal substitution for cobalt on the oxygen adsorption properties of YBaCo4O7

YBaCo4O7 compound is capable to intake and release a large amount of oxygen in the temperature range of 200–400°C. In the present study, the effect of Zn, Ga and Fe substitution for Co on the oxygen adsorption/desorption properties of YBaCo4O7 were investigated by thermogravimetry (TG) method. Due to fixed oxidation state of Zn2+ ions, the substitution of Zn2+ for Co2+ suppresses the oxygen adsorption of YBaCo4−xZnxO7. The substitution of Ga3+ for Co3+ also decreases the oxygen absorption capacity of YBaCo4−xGaxO7. This can be explained by the strong affinity of Ga3+ ions towards the GaO4 tetrahedron. Compared with Zn- and Ga-substituted samples, the drop of oxygen adsorption capacity is smallest for Fe-substituted samples because of the similar changeability of oxidation states of Co and Fe ions.

Characterization of Zn- and Fe-substituted LiMnO 2 as cathode materials in Li-ion cells

Layered LiMn 1− x M x O 2 (M = Zn or Fe) (0 ≤ x ≤ 0.3) samples are synthesized from the corresponding sodium analogues by an ion-exchange method using LiBr in n-hexanol at 160 °C. The samples are subjected to physicochemical and electrochemical characterization. X-ray diffraction data indicate the formation of layered structures for the LiMn 1− x Zn x O 2 samples up to x = 0.3 and for LiMn 1− x Fe x O 2 samples up to x = 0.2. Among these, LiMn 0.95Zn 0.05O 2 and LiMn 0.95Fe 0.05O 2 provide the highest capacity values of 180 and 193 mAh g −1, respectively. Both Zn- and Fe-substituted samples display good capacity retention up to 30 charge–discharge cycles. Electrochemical impedance spectroscopy and galvanostatic intermittent titration data corroborate the results obtained from cyclic volatmmetry and charge–discharge cycling.

Iron Substitution in Lithium-Overstoichiometric “Li1.1CoO2”: Combined 57Fe Mössbauer and 7Li NMR Spectroscopies Studies

Iron substitution was attempted by direct solid-state synthesis in stoichiometric LiCoO 2 and lithium-overstoichiometric 1 1 CoO 2 . Iron substitution was not obtained in stoichiometric LiCo 0 9 8 Fe 0 0 2 O 2 samples, consistent with the fact that the size of Fe 3 + ions is significantly larger than that of Co 3 + ions in the octahedral site. In contrast, up to 8 atom % iron could be substituted in the lithium-overstoichiometric 1 1 CoO 2 samples with an actual composition of Li 1 0 4 Co 0 9 6 O 1 9 6 , which could be rationalized by considering the structural defect model proposed previously by some of us. In the defect model, lithium-overstoichiometric samples consist of excess Li + replacing Co 3 + charge-compensated by an oxygen vacancy in the cobalt layers, which creates two adjacent square-based pyramids containing intermediate-spin (IS) Co 3 + ([Li] i n t e r s l a b [Co 3 + ( L S ) 1 - 3 t Co 3 + ( I S ) 2 t Li t ] s l a b [O 2 - t ], with t = 0.04 and 0.08 IS Co 3 + and LS = low spin). 7 Li MAS NMR showed that the signals associated with intermediate-spin Co 3 + decreased but were not completely suppressed upon iron substitution, even for the 8 atom % Fe-substituted sample, with no new signal appearing in iron-substituted lithium-overstoichiometric samples. Moreover, the values of Mossbauer parameters, isomer shift 0.249 mm.s - 1 and quadrupolar splitting 0.4 mm.s - 1 , revealed that high-spin Fe 3 + was present in the square-pyramidal sites in the 2 atom % iron-substituted lithium-overstoichiometric sample. These results lent further support for the nature of the defects proposed previously for lithium-overstoichiometric 1 1 CoO 2 .

Effects of partial substitution of transition metals for cobalt on the high-temperature thermoelectric properties of Ca3Co4O9+δ

Polycrystalline samples of Ca3Co4−xMxO9+δ (M=Ni, Fe, Mn, Cu; x=0–0.6) have been prepared using sol-gel procedure followed by spark plasma sintering. Their thermoelectric properties have been carefully examined from room temperature to 1000 K. Substitution of Ni and Fe for Co caused the significant increase of electrical resistivity, but the thermopower changed only slightly. The magnitude of the increases in electrical resistivity and thermopower for the Mn-substituted samples was the most pronounced as compared with Ni- and Fe-substituted samples in the same substitution content, which could be explained by the strong localization of charge caused by Mn substitution. Temperature dependence of electrical conductivity showed that the hole hopping conduction mechanism was dominant for all samples. The activation energy increased with increasing substitution content. On the other hand, the Cu substitution resulted in decreases of both electrical resistivity and thermopower, and the activation energy remained unchanged for Cu-substituted sample. These results suggest that Ni, Fe, and Mn substitute the Co in the CoO2 layers, which is the conducting path in the Ca3Co4O9+δ structure, while Cu substitutes the Co in the Ca2CoO3 layers.

Synthesis, Cation Distribution, and Electrochemical Properties of Fe-Substituted Li2MnO3 as a Novel 4 V Positive Electrode Material

solid solution was synthesized using solid-state reaction and hydrothermal-postannealing methods and characterized as a positive electrode material for rechargeable lithium batteries. Although the maximum Fe content was limited up to 30% by solid-state reaction, the content can extend up to 75% by the hydrothermal-postannealing method. Neutron and X-ray Rietveld analysis reveal that the basic structure of the sample is a layered rock-salt structure isostructural with in which Fe ions exist on both Li (3a) and Co (3b) sites. Elemental analysis and Mössbauer spectra show Fe ions exist as 3+/4+ mixed-valence state after the samples were postannealed above 650°C. The initial charge capacity of Li/sample cells was above 100 mAh/g when the upper voltage limit was 4.3 V. The plateau around 4 V was observed for all Li/sample cells on first discharge. The maximum of initial discharge capacity was about 100 mAh/g down to 2.5 V for the Li/(50% Fe-substituted sample) cell, when the positive electrode was obtained by postannealing at 650°C in air. The capacity fading of the 4 V plateau could be suppressed by adjusting the Fe content to less than 50%, postannealing temperature between 600 and 700°C, and by 10% Ni substitution. © 2002 The Electrochemical Society. All rights reserved.

Structure and Electrochemical Properties of LiFexMn2 − x O 4 ( 0 ⩽ x ⩽ 0.5 ) Spinel as 5 V Electrode Material for Lithium Batteries

An electroactive spinel solid solution, was prepared and investigated. X-ray Rietveld refinements showed a small amount of transition metal ions in tetrahedral sites in Fe-substituted samples. The Mössbauer spectrum of at 300 K is composed of two doublets and attributed to trivalent Fe. In the voltage range 3.0 and 5.3 V vs. two reversible plateaus appeared at 4.9 and 3.9 V during discharge. Combined data of X-ray diffraction, Mn K-edge X-ray absorption near-edge structure (XANES), and Mössbauer spectroscopy obtained in situ during charge and discharge demonstrate that the plateau around 4 V on charging is associated with the oxidation of to and the plateau around 5 V with the partial oxidation of to © 2001 The Electrochemical Society. All rights reserved.

Electronic structures and magnetism of LaNi5–xFex compounds

A systematic study of the magnetic moments and hyperfine interactions of the LaNi5–xFex compounds was performed using the spin-polarized tight-binding-linear-muffin-tin-orbital methods (TBLMTO). The calculated results were compared with neutron diffraction and Mössbauer experimental data and found to be in good agreement. Fe atoms exhibit more localized magnetic moments in LaNi5–xFex when the Fe content x is lower than 1.0. The calculated magnetic moments of the Fe atoms approach values as large as 2.5μB due to the dilution effect. The strong s–d hybridization between the Ni and La atoms and the neighboring Fe in the Fe-substituted samples results in a large transferred valence contribution to the total hyperfine field. It is found that the Fe and Ni magnetic moments decrease abruptly at a unit cell volume which is 5% smaller than the observed volume in LaNi4Fe due to the instability of the Fe magnetic moment. LaNi5 becomes ferromagnetic when the unit cell volume is larger than its original volume.

Magnetic and electronic properties of transition-metal-substituted perovskite manganites—La0.7Ca0.3Mn0.95X0.05O3 (X=Fe,Co,Ni)

The magnetic and electronic properties of Fe-, Co-, Ni-substituted La0.7Ca0.3MnO3 were studied. Ferromagnetic–paramagnetic and metal–insulator transitions were significantly affected by Mn-site substitution although no observable differences were found in their crystal structures from x-ray diffraction analysis. Valence-band photoemission studies also showed that there were no appreciable changes in the electronic structures of each sample. Peak temperatures (Tp) at maximum resistivity decreased from 265 K of virgin LaCaMnO to 217, 203, and 171K for Ni-,Co-, and Fe-substituted LaCaMnO samples, respectively. As the Tp decreased, the electrical resistivity increased and the saturation magnetization decreased. The overall experimental results were well explained by considering superexchange interactions induced by substitution with magnetic ions having different magnetic moments. From our study, it is suggested that the magnetic properties of substituting elements should also be considered to deal with the physical properties of these LaCaMnXO systems.

Fe- and Cr-substituted mullites: Mössbauer spectroscopy and rietveld structure refinement

Fe-substituted mullite samples Al 4.5-yFe ySi 1.5O 9.75 (y = 0.25 and 0.50) have been prepared using a sol-gel route involving Si and Al alkoxides and iron nitrate as precursors. Mössbauer spectroscopy reveals the presence of three different Fe(III) sites but does not allow to conclude concerning the ion environments, tetrahedral or octahedral. Rietveld refinements reveal that Fe ions are distributed between octahedral and tetrahedral positions, the proportion of octahedral iron increasing from y = 0.25 to y = 0.50. On the other hand, for Cr-substituted mullite, Cr ions arc mainly located on octahedral positions. This difference between Fe and Cr has been attributed to the fact that Cr shows a clear trend to occupy octahedral sites compared to Fc and that it is incorporated inside the mullite structure only at high temperature.

Structural and superconducting properties of MS (M = Fe, Ni or Zn)-substituted

The effect of CuO substitution by MS ( M = Ni, Fe or Zn) in (Y-123) samples is studied by DC resistivity, AC susceptibility and x-ray emission spectroscopy and diffractometry. All samples show sharp x-ray diffraction patterns with no impurity phase lines for M = Ni, Fe or Zn-doped samples. The cell of Ni-, Zn- or Fe-substituted Y-123 samples remains orthorhombic for all concentrations studied. X-ray emission spectroscopy results show that sulphur atoms enter the lattice of Y-123 compounds and form oxyanion groups. The diamagnetic transitions are sharp for all the small concentrations and become broad for MS doping above 10%. The transition temperature decreases with increasing MS concentration when compared with the undoped sample.

Electronic properties of UNiAl in high magnetic fields.

The ternary compound UNiAl, crystallizing in the ZrNiAl-type structure, is an itinerant antiferromagnet with magnetic moments of less than 0.8${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$ per U atom and with a high \ensuremath{\gamma} value of 164 mJ/mol ${\mathrm{K}}^{2}$. The anisotropic response of the electronic properties to magnetic field is closely connected with the huge uniaxial magnetic anisotropy, observed not only in the magnetically ordered state, but also in the paramagnetic region. The origin of this anisotropy can be found in a strongly anisotropic hybridization. The magnetic ordering temperature, ${\mathit{T}}_{\mathit{N}}$=19.3 K, is reduced upon applying the field along the c axis of the hexagonal structure, which is the easy-magnetization direction. Antiferromagnetic ordering is suppressed by magnetic fields higher than the critical field of the metamagnetic transition, which is 11.35 T at 1.4 K. The electronic contribution to the specific heat is gradually enhanced by the magnetic field up to the transition, where a \ensuremath{\gamma} value of about 260 mJ/mol ${\mathrm{K}}^{2}$ is recorded. In higher fields, the \ensuremath{\gamma} coefficient is gradually suppressed even below the zero-field value and reaches 143 mJ/mol ${\mathrm{K}}^{2}$ at 20 T. The magnetic field applied in the basal plane has a negligible effect on both ${\mathit{T}}_{\mathit{N}}$ and \ensuremath{\gamma}. The pronounced anisotropy is also found in the low-temperature electrical resistivity, which is considerably reduced in fields above the metamagnetic transition. Strong indications of anisotropic magnetic fluctuations can be traced in a number of experimental findings on pure UNiAl and also on Y-, Co-, or Fe-substituted samples, in which a fast decay of magnetic ordering is observed.

Evidence for superconducting fluctuations at 180 K: influence of Zn, Fe and Co doping on the phonon renormalization of YBa2Cu3O7- delta

Detailed studies on the mid-infrared-active phonon spectra of YBa2(Cu1-xMx)3O7- delta ; M=Co, Fe and Zn for 0<or=x<or=10%, have been carried out from 30 K to room temperature. The phonon mode near 580 cm-1 softens at the superconducting transition temperature (Tc), as reported earlier. The new observation is a marked difference in the temperature dependence of the phonon in Co- and Fe-doped compounds as opposed to Zn-doped and undoped material. The authors observe that the phonon begins to soften at temperatures well above Tc-in some cases approximately 2Tc( approximately=180 K)-in the Co- and Fe-doped YBa2Cu3O7- delta , whilst no such behaviour is detected in Zn-doped and undoped samples. They argue that this precursor phonon softening above Tc in Co- and Fe-substituted samples is due to superconducting fluctuations. Their proposal is that the transition-metal-ion substitution affects the dimensionality of the system and the ramifications of this are crucial for explaining the observed differences between Zn/undoped YBa2Cu3O7- delta and Co/Fe-doped compounds. Their observations cast doubt on the idea that 2D superconductors like YBa2Cu3O7- delta show a Kosterlitz-Thouless behaviour. Instead, it appears that Tc to 0 K and fluctuations are reduced for the fully decoupled layers.

Distorted chain sites for Co- and Fe-substituted YBa2Cu3O7- delta.

We present x-ray-absorption fine-structure (XAFS) measurements for a series of Co- and Fe-substituted samples of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (Y-Ba-Cu-O). Our analysis of the first- and second-neighbor environments indicates that the Co atoms primarily replace the Cu in the chain sites, the Cu(1) atoms, in Y-Ba-Cu-O, but many of these Co(1) sites and their neighboring oxygen sites are highly distorted. The first-neighbor Co-O peak consists of \ensuremath{\sim}3.5 oxygen at 1.8 \AA{} and \ensuremath{\sim}1.3 oxygen at 2.4 \AA{}, while the second-neighbor multipeak in the XAFS data is unexpectedly low in amplitude. Structure in this peak is inconsistent with a simple Gaussian broadening and indicates that several Co(1)-Ba distances exist. We propose an aggregation of the Co atoms into distorted, zigzag chains along the $〈110〉$ directions, with some of the Co displaced off center by \ensuremath{\sim}0.4 \AA{} along a perpendicular $〈110〉$ direction. This model is consistent with the second-neighbor XAFS data, provides an explanation for the tetragonal structure via twinning on a microscopic scale, and accommodates excess oxygen within the Co chains. The Fe data suggest that similar chains also exist in the Fe-substituted samples. There are, however, some differences between the local environments of the Fe and Co. The primary difference is that a small but significant number of Fe atoms occupy the Cu(2) plane sites while no appreciable number of Co atoms are found on the Cu(2) sites in the more dilute samples. Finally, near-edge measurements on the Co and Fe $K$-absorption edges indicate that the valence is primarily +3, but a mixture of valences exists. For Co, the edge position corresponds to a mixture of +2 and +3 valences, while Fe exists in a mixture of +2, +3, and +4 states.

Magnetic susceptibility, electrical resistivity and thermoelectric power of pure and Fe-substituted YBa 2Cu 3O 7−y at high temperatures

High-temperature magnetic susceptibility (X), electrical resistivity (9) and thermoelectric power (S) measurements for pure and Fe-substituted YBa 2Cu 3O 7−y show an orthorhombic-to-tetragonal transition at 600∼650 °C under 0.2 atm O 2. The other distinct anomalies also are observed at about 700 and 800 °C in all samples. The values of S of undoped samples are negative below about 500°C, and positive at the higher temperatures; the S–T curve has a minimum near 450 °C. S of Fe-substituted samples shows the similar temperature dependence, while it is shifted to the positive higher values.