Transition Metal Oxygen Research Articles

Role of Annealing Atmosphere on Structure, Dielectric and Magnetic Properties of La2CoMnO6 and La2MgMnO6

AbstractPolycrystalline samples of La2MMnO6 (M = Co and Mg) were prepared by a combined gel‐combustion and high temperature reaction method. The samples were annealed in different oxygen partial pressure (pO2) and characterized by powder XRD, SQUID magnetometry, ac impedance spectroscopy, and electron paramagnetic resonance techniques. Monoclinic (P21/n) and rhombohedral (R) lattices were observed for La2CoMnO6 and La2MgMnO6, respectively. On annealing in inert atmosphere, La2MgMnO6 partially converted to monoclinic La2CoMnO6 type structure, whereas no structural change was observed in La2CoMnO6. Dielectric studies of La2CoMnO6 indicated relaxor like behavior with polaronic conduction, which systematically decreased with the increase in pO2 of the annealing atmosphere. Magnetic studies indicated multiple ferromagnetic phase transitions in La2CoMnO6 and a spin‐glass like phase transition in La2MgMnO6. The fraction of ferromagnetic phases of La2CoMnO6 was significantly dependent on the annealing environments. The variations of magnetic and dielectric properties of samples were related to the fluctuation of oxidation state of transition metal ions and oxygen vacancies in the samples.

Templated fabrication of hierarchically porous Fe–Ti bimetallic solid superacid for efficient photochemical oxidation of azo dyes under visible light

• Hierarchically porous S–Fe 2 − x Ti x O 3 was prepared by critic acid templated method. • Cooperative effect of titanium and sulfate created lots of Lewis acid sites. • Fe–Ti bimetallic solid superacid widened the pH range to 10.0 in Fenton reaction. • Light with wavelength 450–595 nm had significant effect on activity of S–Fe 2 − x Ti x O 3. • Favorable synergetic effect of acid sites and photo improved catalytic activity. A new hierarchically porous solid superacid SO 4 2 - /Fe 2 − x Ti x O 3 was prepared by way of facile citric acid templates and sulfuric acid impregnating strategy. The prepared catalyst exhibited high activity and stability for activation of H 2 O 2 as higher than 90% of organic dyes were decomposed in 30 min at neutral pH under visible light irradiation. UV–vis spectrum and ion chromatography were used to investigate the degradation pathway of dyes. By characterizations, it was found that incorporation of titanium and sulfur had a significant effect on properties and structure of the solid. Large numbers of strong Lewis acid sites were produced on the surface due to cooperative effect of Ti and S, which made significant contributions toward high activity in strong alkaline condition (pH 10.0). Meanwhile, the transition metal doping and oxygen vacancies generation can narrow the band gap of the catalyst and efficiently enhance visible light photocatalytic performance. It was also observed that high catalytic activity can hardly be achieved if acid functionalization or light irradiation was applied individually. This was clearly evidenced that the synergetic effect of acid sites and photo-induced electrons transfer played a crucial role in Fenton reaction, which may be attributed to that the appearance of acid sites (Lewis and Brønsted acidic center) was favorable for the separation of photo-induced electrons and holes and suppressed charge recombination.

Estimating Hybridization of Transition Metal and Oxygen States in Perovskites from O K-edge X-ray Absorption Spectroscopy

The interaction between the transition metal 3d and the oxygen 2p states via hybridization underpins many of the phenomena in transition metal oxide materials. We report the empirical trend of this interaction using the pre-edge feature of the O K-edge X-ray absorption spectrum. Our assessment method is built on the dipole approximation and the configuration interaction between the transition metal 3d and the oxygen 2p states. We found that hybridization increases with the number of 3d electrons, consistent with the expected electronegativity trend. We support this analysis with density functional calculations, which reveal a systematic increase in the transition metal 3d and the oxygen 2p state mixing with increasing 3d-electron number. Oxidation of the transition metal was also found to increase hybridization, which we believe reflects the reduced transition metal 3d and oxygen 2p energy difference, causing increased covalency. We compare the analysis from the surface-sensitive electron-yield and the bu...

Thermodynamic modeling and first-principles calculations of the Mo–O system

Oxidation resistance is the bottleneck to the development of high-performance Mo-based alloys for ultra-high temperature applications. In the present study, thermodynamic assessment of the Mo–O system was carried out using the CALPHAD method on the basis of experimental data in the literature. The derived set of self-consistent parameters gives good representation of the phase-equilibrium and thermodynamic properties of phases in the Mo–O system. Using the developed thermodynamic description, the chemical potential at T=1000K was predicted, and a stability diagram was constructed. The lattice stability of oxide compounds in the Mo–O system was further studied using the first-principles density functional theory method. The impact on the phase stability of various exchange-correlation functional and van der Waals correction was also examined. The agreement in lattice parameters and enthalpy of formation between calculations and experimental data is acceptable except for using the local density functional. The present study also suggests that the value of the so-called correction term used to compensate for the error from calculating the energy of O2 molecule and self-interacting error in oxides is not universal and may vary significantly depending on the individual transition metal–oxygen system. In addition, the present study also reveals that the phase stability among three reported MoO3 polymorphs at low temperatures requires further experimental and theoretical investigation.

Electrochemical potentials of layered oxide and olivine phosphate with aluminum substitution: A first principles study

First-principles prediction of enhancement in the electrochemical potential of LiCoO2 with aluminum substitution has been realized through earlier experiments. For safer and less expensive Li-ion batteries, it is desirable to have a similar enhancement for alternative cathode materials, LiFePO4 and LiCoPO4. Here, we present first-principles density functional theory based analysis of the effects of aluminum substitution on electrochemical potential of LiCoO2, LiFePO4 and LiCoPO4. While Al substitution for transition metal results in increase in electrochemical potential of LiCoO2, it leads to reduction in LiFePO4 and LiCoPO4. Through comparative topological analysis of charge density of these materials, we identify a ratio of Bader charges that correlates with electrochemical potential and determine the chemical origin of these contrasting effects: while electronic charge from lithium is transferred largely to oxygen in LiCoO2, it gets shared by the oxygen and Co/Fe in olivine phosphates due to strong covalency between O and Co/Fe. Our work shows that covalency of transition metal–oxygen bond plays a key role in determining battery potential.

An unprecedented (3,4,24)-connected heteropolyoxozincate organic framework as heterogeneous crystalline Lewis acid catalyst for biodiesel production

A novel 3D hexadecanuclear heteropolyoxozincate organic framework, IFMC-200, has been successfully synthesized based on a late transition metal-oxygen cluster. IFMC-200 not only represents the first example of (3,4,24)-connected framework but also contains the first 24-connected single metal cluster in a crystal structure. It exhibits superior thermal stability, good water-stability, and even insensitivity to the existence of acid and base within a certain range of pH values. Furthermore, it performs as a heterogeneous crystalline Lewis acid catalyst with good activity for the conversion of long-chain fatty acids rather than short-chain ones, and high recycling efficiency for esterification reaction of fatty acids with alcohols to produce biodiesel.

Systematic First-Principles Investigation of Mixed Transition Metal Olivine Phosphates LiM1-yM′yPO4 (M/M′ = Mn, Fe, and Co) as Cathode Materials

Li based batteries are widely used for powering mobile electronic equipment and are considered as highly promising power sources for electrical propulsion. Recent developments in the area of rechargeable lithium ion batteries for electronic devices and transportation have stimulated extensive studies of cathode materials which are the limiting factor in terms of voltage and energy density. In the current work we present a systematic computational study of the geometry, electronic structure, and electrochemical potential for olivines LixM1-yM′yPO4 (M/M′ = Mn, Fe, Co; x = 0.00, 1.00, x = y; y = 0.00, 0.25, 0.50, 0.75, and 1.00). We find that changes in cell volume as a function of transition metal composition may largely be ascribed to changes in the atomic volumes of the oxygen atoms, which modulate the electron charge distribution. Moreover, there is considerable charge transfer from lithium to the transition metal ions and oxygen atoms upon lithiation for all systems studied. The calculated cell potentia...

Site-specific mapping of transition metal oxygen coordination in complex oxides

We demonstrate site-specific mapping of the oxygen coordination number for transition metals in complex oxides using atomically resolved electron energy-loss spectroscopy in an aberration-corrected scanning transmission electron microscope. Pb2Sr2Bi2Fe6O16 contains iron with a constant Fe3+ valency in both octahedral and tetragonal pyramidal coordination and is selected to demonstrate the principle of site-specific coordination mapping. Analysis of the site-specific Fe-L2,3 data reveals distinct variations in the fine structure that are attributed to Fe in a six-fold (octahedron) or five-fold (distorted tetragonal pyramid) oxygen coordination. Using these variations, atomic resolution coordination maps are generated that are in excellent agreement with simulations.

Dynamic disorder and theα-βphase transition in quartz-type FePO4at high temperature investigated by total neutron scattering, Raman spectroscopy, and density functional theory

Quartz-type iron phosphate (FePO${}_{4}$) was studied by total neutron scattering and Raman spectroscopy up to 1150 K in order to investigate disorder and the mechanism of the \ensuremath{\alpha}-\ensuremath{\beta} transition. The increasingly large underestimation of P-O and Fe-O distances in Rietveld refinements of the average long-range structure as compared to the bond lengths obtained from the pair distribution function as a function of temperature is a clear indication of the presence of significant dynamic disorder, particularly from 850 K up to and above the \ensuremath{\alpha}-\ensuremath{\beta} transition near 980 K. A significant broadening of the Fe-O distance distribution is also observed. Reverse Monte Carlo modeling confirms the presence of such disorder with broadened angular distributions in this temperature range, in particular for the Fe-O-P and Fe-P-Fe distributions. The Raman spectrum, calculated using density functional theory, is in very good agreement with experiment. These calculations indicate that there is an inversion of two low-energy A${}_{1}$ vibrational modes with respect to AlPO${}_{4}$. The principle mode, which exhibits strong damping in the Raman spectrum above 850 K, is thus not a tetrahedral libration mode, but a mode that principally involves large amplitude translations of the Fe atoms along with a degree of oxygen displacement. The transition mechanism from a dynamic point of view is thus different from the transitions in SiO${}_{2}$ and AlPO${}_{4}$. The strong damping of this mode is also further evidence of a high degree of dynamic disorder, which is different from the disorder observed in SiO${}_{2}$ and AlPO${}_{4}$. This mode does not exhibit any significant softening with temperature near the phase transition, which is further evidence that the \ensuremath{\alpha}-\ensuremath{\beta} transition is not of the simple displacive type. The difference in behavior between FePO${}_{4}$ and the other quartz homeotypes arises from the weaker bonding between the 3${d}^{5}$ transition metal cation and oxygen.

Oxygen permeation and oxidation states of transition metals in (Fe, Nb)-doped BaCoO 3 − δ perovskites

The influence of cation substitution on the transition metal oxidation states, oxygen nonstoichiometry and oxygen transport properties of Ba(Co, Fe, Nb)O 3 − δ perovskites was investigated with room temperature XRD, X-ray absorption near edge structure and high temperature oxygen permeation measurements. Results show that substitution of Fe for Co is accompanied by a decrease in the lattice constant, whereas Nb substitution leads to the opposite trend. With increasing Fe and/or Nb content, the oxygen permeation flux decreases, whereas the apparent activation energy of the oxygen permeation increases. The oxidation state of Co is smaller than +3 and susceptible to the variation of cation composition. In contrast, the oxidation states of Fe and Nb are larger than +3 and close to +5, respectively, and almost independent of the composition. The influence of Fe and Nb contents on the oxygen transport properties is discussed in terms of the oxygen stoichiometry and metal–oxygen bonding.

Electrical conductivity and defect chemistry of $$ {\hbox{B}}{{\hbox{a}}_x}{\hbox{S}}{{\hbox{r}}_{{1} - x}}{\hbox{C}}{{\hbox{o}}_y}{\hbox{F}}{{\hbox{e}}_{{1} - y}}{{\hbox{O}}_{{3} - \delta }} $$ perovskites

Bulk $$ {\hbox{B}}{{\hbox{a}}_x}{\hbox{S}}{{\hbox{r}}_{{1} - x}}{\hbox{C}}{{\hbox{o}}_y}{\hbox{F}}{{\hbox{e}}_{{1} - y}}{{\hbox{O}}_{{3} - \delta }} $$ compositions (BSCF) were synthesized by the solid-state reaction method. The electrical conductivity of ceramic bars was measured using a dc four-probe method as a function of temperature in air up to 970 °C. All compositions showed thermally activated p-type semi-conductivity up to ~450 °C and then a transition to metal-like conductivity. The small-polaron hopping p-type semi-conductivity depends on the oxygen nonstoichiometry, which increases with increasing temperature. Metal-like conductivity is attributed to the overlap of the transition metal d-electron orbitals with the oxygen p-orbitals. Strontium-rich compositions show higher conductivity. The Co/Fe ratio does not influence much the p-type semi-conduction. Iron-rich compositions revealed more metal-like conduction behavior. The degree of overlap between transition metal d-orbitals and oxygen p-orbitals depends on the Ba/Sr as well as on the Co/Fe ratios.

Defect structure and electrical properties of LaSr 3Fe 3O 10–δ

As a mixed conductor, LaSr 3Fe 3O 10–δ with triple layer perovskite intergrowth structure can be used as an oxygen separation membrane material and cathode material in solid oxide fuell cells. LaSr 3Fe 3O 10–δ was synthesized via citrate acid route. Iodine titration method was used to determine the average valence of transition metal Fe and oxygen nonstoichiometry δ. Conductivities of LaSr 3Fe 3O 10– δ were measured in the oxygen partial pressure range from 10 −2 × 10 5 to 1 × 10 5 Pa, by Ac four probe method. Seebeck coefficient measurement together with conductivity results indicated that LaSr 3Fe 3O 10– δ is a p-type conductor. A linear relationship between the conductivity and logarithm of oxygen partial pressure was observed in the measurement. A defect model was introduced to explain the observed phenomena in the iodine titration, conductivity and Seebeck coefficient measurement. Electrical conductivity relaxation method was used to determine the oxygen chemical diffusion coefficient D chem. The apparent activation energy of oxygen chemical diffusion was calculated to be 61.4 kJ/mol.

Effect of promoters in Co–Mn–Al mixed oxide catalyst on N 2O decomposition

Catalytic decomposition of N 2O over calcined Co–Mn–Al layered double hydroxide (LDH) modified with slight amount of alkali (Li, Na, K), rare earth (La, Ce), or noble metals (Pd, Pt) was studied. Two methods of promoter addition were applied: impregnation of LDH-related Co–Mn–Al mixed oxide by a promoter-containing solution and incorporation of the promoter during coprecipitation of the LDH precursor. The method of preparation affected the reducibility of the obtained mixed oxide catalysts but its effect on the surface area (with the exception of a cerium-contained sample) and N 2O conversion was less evident. The modification of the Co–Mn–Al mixed oxide with K or Na increased N 2O conversion, while no effect or a decrease in the N 2O conversion was observed over the other examined catalysts. Alkali metals act as electron promoters; therefore, the differences in catalytic activity may be related to changes of oxygen-transition metal bond strength. The Co–Mn–Al mixed oxides with 1.8 wt% K exhibited a high catalytic activity even under the simultaneous presence of O 2, H 2O and NO x. The laboratory stability test revealed maintenance of the beneficial effect for 360 h.

Influence of low trivalent iron doping on the electrical characteristics of PZT

Piezoelectric materials based on lead zirconium titanium oxide with a composition near the morphotropic phase boundary, (Pb0.94Sr0.06)(Zr0.53Ti0.47)O3[PSZT] have been synthesized by sol–gel method. The influence of B-site aliovalent dopant Fe3+ on the structure, ferroelectric, dielectric, piezoelectric characteristics and microstructure have been investigated. The influence of the transition metal–oxygen vacancy defect-dipoles on the electrical characteristics have also been investigated.

Ionic Liquid Polyoxometalates as Light Emitting Materials

The low melting point, negligible vapor pressure, good solubility, and thermal and chemical stability make ionic liquids useful materials for a wide variety of applications. Polyoxometalates are early transition metal oxygen clusters that can be synthesized in many different sizes and with a variety of heterometals. The most attractive feature of POMs is that their physical properties, in particular electrical, magnetic, and optical properties, can be easily modified following known procedures. POM ionic liquids can be obtained by selecting the appropriate cation. Different alkyl ammonium and alkyl phosphonium salts are being used to produce new POM ionic liquids together with organic or inorganic luminescent centers to design light emitting materials. Ammonium and phosphonium cations with activated, polymerizable groups are being used to further polymerize the ionic liquid into transparent, solid materials with high metal densities.

Observation of self-regulating response in LixMyMn2−yO4 (M=Mn, Ni): A study using density functional theory

Density functional theory is used to understand the response of the transition metal–oxygen octahedra in LixMn2O4 and LixNi0.5Mn1.5O4 to lithium intercalation and de-intercalation. Electronic structure computations on these compounds for x=0, 0.5 and 1 indicate that the 3d DOS of Mn is almost unaffected to variations in x. On the other hand, the oxygen 2p-DOS and to a lesser extent Ni 3d DOS are found to be sensitive to perturbation. The observations are explained on the grounds of self-regulating response, characteristic of systems having localized d states that communicate with a covalent manifold.

Search for ferromagnetism in SnO2 doped with transition metals (V, Mn, Fe, and Co)

Based on first-principles calculations, we have investigated electronic structures and magnetic propertiesof SnO2 doped with V, Mn, Fe, and Co. Our results show that ferromagnetism is the ground stateand that the Curie temperatures are expected to have high values for Fe- and Co-dopedSnO2, which is in good agreement with the experimental observations (Ogale et al 2003 Phys. Rev.Lett. 91 077205; Coey et al 2004 Appl. Phys. Lett. 84 1332). However, in V- and Mn-dopedSnO2, paramagnetism is more stable than ferromagnetism. In addition, we alsoprobe the effect of an oxygen vacancy. The results exhibit that an oxygenvacancy strongly influences the magnetic properties of these doped systems, andan oxygen vacancy strongly attracts Fe and Co ions. As a result, transitionmetal–oxygen vacancy–transition metal groups will be common in Fe- and Co-dopedSnO2, but this tendency does not exist in the cases of V and Mn doping. In V-dopedSnO2, ferromagnetism becomes more stable than antiferromagnetism afterinducing additional n-type carriers and the Curie temperature increases withincreasing density of n-type carriers. Hence, raising the density of n-typecarriers is a practical way to realize high Curie temperatures in V-dopedSnO2.

Modifications of the structural, electrical and magnetic properties of LaFe0.5Ni0.5O3 thin films by 190 MeV Ag ion irradiation

We have investigated the effect of 190MeV Ag ion irradiation on the structural, electrical and magnetic properties of the pulsed laser deposited thin films of LaFe0.5Ni0.5O3. It is observed that on irradiation the lattice of the composition relaxes with enhanced c-axis orientation. Consequently, the resistivity of the composition, which shows the semiconducting trend, decreases possibly due to the enhanced hybridization between the transition metal 3d and oxygen 2p orbitals. The as grown sample shows the activated variable range-hopping behavior throughout the studied temperature range, though the behavior of irradiated samples deviates from this behavior at lower temperatures. The pristine as well as the irradiated films exhibit room temperature ferromagnetic behavior and the magnetization increases with the irradiation fluence and attains almost double as compared to the pristine value at high fluence. The present studies suggest that the swift heavy ion irradiation can be a good tool to manipulate the structural, electrical and magnetic properties of this ferromagnetic semiconducting composition demonstrating its suitability for a variety of spintronic based applications.

Transition Metal Oxides Using Quantum Monte Carlo

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

A First-Principles Investigation of the Role Played by Oxygen Deficiency in the Electrochemical Properties of LiCu0.5Mn1.5O4 − δ Spinels

The effect of oxygen deficiency in Cu-based spinels was examined using first-principles calculation. Two main results were obtained upon oxygen removal: a progressive unit cell expansion owing to the presence of both more reduced transition metal cation and oxygen vacancies, a progressive decrease in the average lithium deinsertion voltage as the oxygen deficiency increases. Computational results indicate that the oxygen deficiency gives rise to a partial reduction of to , whereas upon lithium deinsertion, this reaction is reversed together with the oxidation of to diamagnetic . The electrochemical tests in lithium cells of two spinels with δ values 0.04 and 0.1 were consistent with these calculations. Computational data indicate that the oxidation of to would be favored in highly oxygen deficient spinels; this contradicts the experimental results that show a decrease of the specific capacity in the high-voltage range with the oxygen deficiency. However, in the prepared spinels, there is an important fraction of Cu ions occupying tetrahedral sites. Therefore, computational investigation points to the electrochemical inactivity of tetrahedral Cu rather than to the oxygen content as the origin of the poor charge capacity observed in the high-voltage range.