Structural study of copper-nickel aluminate (CuxNi1−xAl2O4) spinels

CuAl₂O₄, NiAl₂O₄, and three ternary spinels Cu_xNi_1?xAl₂O₃ have been prepared, in polycrystalline form, by solid-state reaction of mixtures of CuO, NiO, and Al₂O₃ at 1223 K. X-Ray powder diffractometry, coupled with adequate computational methods, allowed determination of the unit-cell length, oxygen positional parameter, and cation distribution for each compound. Interdependence of these structural parameters is closely analyzed on the ternary oxide spinels. The one-electron difference between the Cu²⁺ and Ni²⁺ ions was found to be enough to render them distinguishable by X-ray powder diffraction.     

The distribution of nickel ions among octahedral and tetrahedral sites in NiAl2O4MgAl2O4 solid solutions

The cation distribution in NiAl₂O₄ and in the solid solutions Ni_xMg_1?xAl₂O₄ with x ranging from 0.01 to 0.55 has been studied by X-ray analysis, magnetic susceptibility and reflectance spectroscopy.The relative X-ray intensities of various reflections for each compound were measured and compared with intensities calculated for various models of cation distribution. Lattice parameters, magnetic moments of Ni²⁺, and the position of nickel absorption bands have also been measured. The results show that all specimens have a predominantly octahedral distribution for their nickel ions, and that there is a small change towards a random distribution when the quenching temperature is increased from 1273 to 1673°K.Both X-ray and magnetic results show that the fraction, α, of Ni²⁺ ions on tetrahedral sites varies with nickel content. For samples quenched from 1273°K, NiAl₂O₄ is 78% inverse, but between x = 1.0 and x = 0.25 the value of α decreases from 0.22 towards 0.1. Thus, the fraction of nickel on octahedral sites in the solid solutions is higher than in NiAl₂O₄. The effect is linked with an increase in Dq for the Ni²⁺ ion. Below x = 0.25, there is tentative evidence that α may increase. In nickel-dilute solutions the matrix as a whole is nearly normal, and it is suggested that Madelung energy may then be more effective in counteracting the octahedral stabilization of nickel. The same trend is observed in the specimens quenched from 1673°K.     

Multiple scattering method applied to the electronic structure and spectra of the impurity ions Cu2+, Ni2+, Co2+, Fe2+ and their tetrahedral environments in ZnO,ZnS and CdS

The multiple scattering method with local exchange and muffin-tin potentials is applied to tetrahedral clusters with Cu²⁺, Ni²⁺, Co²⁺ and Fe²⁺ as central ions and oxygen and sulphur as ligands. The calculated orbital energy differences ?(t₂^*) - ?(e^*) are compared to Δ_t values deduced from optical spectra. The agreement is fairly good.     

Stabilization of unusual electronic configurations of transition elements in elongated six-coordinated oxygen sites of a K2NiF4 structure

The A′_xA_2?xLi_0.5M_0.5O₄ matrix deriving from the K₂NiF₄ structure has an elongated MO₆ octahedron; it has been used to stabilize anisotropic electronic configurations such as high-spin Fe⁴⁺, low-spin Co⁴⁺, medium-spin Co³⁺, low-spin Ni³⁺, or low-spin Cu³⁺. The choice of ions to be stabilized was based on a simple model for predicting the electronic configuration of a dⁿ ion in a tetragonally distorted octahedral environment.     

Activities and their relation to cation distribution in NiAl2O4MgAl2O4 spinel solid solutions

The activity of NiAl₂O₄ in NiAl₂O₄MgAl₂O₄ solid solutions has been measured by using a solid oxide galvanic cell of the type, Pt, Ni + NiAl₂O₄ + Al₂O₃(α)/CaOZrO₂/Ni + Ni_xMg_1?xAl₂O₄ + Al₂O₃(α). Pt, in the temperature range 750–1150°C. The activities in the spinel solid solutions show negative deviations from Raoult's law. The cation distribution in the solid solutions has been calculated using site preference energies independent of composition for Ni²⁺, Mg²⁺, and Al³⁺ ions obtained from crystal field theory and measured cation disorder in pure NiAl₂O₄ and MgAl₂O₄, and assumi g ideal mixing of cations on the tetrahedral and octahedral positions. The calculated values correctly predict the decrease in the fraction, α, of Ni²⁺ ions on tetrahedral sites for 1>x>0.25, observed by Porta et al. [J. Solid State Chem.11, 135 (1974)] but do not support their tentative evidence for an increase in α for x < 0.25. The measured excess free energy of mixing can be completely accounted for by using either the calculated or the measured cation distributions. This suggests that the Madelung energy is approximately a linear function of composition in the solid solutions. The composition of NiOMgO solid solutions in equilibrium with NiAl₂O₄MgAl₂O₄ solid solutions has been calculated from the results and information available in literature.     

Fluctuation in occupancy of Cu2+ ions in Zn- and Cd-substituted Cu-ferrites

Mössbauer effect technique has been used for the comparative study of Cu_1?x Zn _x Fe₂O₄ and Cu_1?x Cd _x Fe₂O₄ ( _x = 0.0?1.0) ferrites. Both Zn²⁺ and Cd²⁺ cations are divalent, non-magnetic ions with different ionic radii. With the substitution of these non-magnetic cations the average internal magnetic field decreases and paramagnetic behavior is dominated at x = 0.7 in both series. It is observed that the occupancy of Cu²⁺ ions for tetrahedral site is not constant for all compositions but fluctuate between 8–15%. It is also found that Cu²⁺ ions have more preference for tetrahedral site in Cu-Zn system as compared to the Cu-Cd system. Zn²⁺ and Cd²⁺ both ions occupy tetrahedral site completely and form normal spinels for x = 1.0.     

Availability of Fe2+ ions in Cr- or Al-substituted magnetites with relevance to the process of oxidation in defect phase γ

During oxidation of the aluminum- or chromium-substituted magnetites, (Fe²⁺Fe³⁺_2?xM^3f_x)O^2?₄, with 0.4 < x < 1.8 in defect phase γ of the same spinel structure, the availability for oxidation of Fe²⁺ ions in the tetrahedral sites (A sites) of the spinel structure is much less than that in octahedral sites (B sites) and in both cases depends on the extent of aluminum or chromium substitution. The influence of cation distribution in A and B sites on the oxidation temperature is shown directly by differential thermogravimetric analysis and by electrical conductivity.     

Lithium-substituted cobalt oxide spinels LixM1−xCo2O4 (M = Co2+, Zn2+; 0 ≤ x ≤ 0.4)

Substitution of Li⁺ into Co₃O₄ and ZnCo₂O₄ gives rise to the solid solution series Li_xM_1?xCo₂O₄ (M = Co²⁺ or Zn²⁺) having the spinel structure upto x = 0.4. X-Ray diffraction intensities show that the spinel solid solutions are likely to have the following cation distributions: (Co²⁺)_t[Li⁺_xCo³⁺_2?3xCo⁴⁺_2x]₀O₄ and (Zn²⁺_1?xCo²⁺_x)_t[Li⁺_xCo³⁺_2?3xCo⁴⁺_2x]₀O₄. Electrical resistivity and Seebeck coefficient data indicate that the electron transport in these systems occurs by a small-polaron hopping mechanism.     

Electrochemical production of NH2• radicals by the system Cu2+/VO2+/NH2OH and their reactions with benzene and toluene

Electroinitiated production of NH₂^. radicals was demonstrated to occur in aqueous acid solution when in the system Cu²⁺/VO²⁺/NH₂OH Cu²⁺ is reduced to metallic copper on a mercury cathode. The first stage of this process involves the reduction of vanadyl to V³⁺ by metallic copper. The electrochemical system Cu²⁺/VO²⁺ was demonstrated to follow a depolarization scheme of the type of the catalyzed electrode process.For the reaction Cu+2 VO²⁺k₁→Cu²⁺+2 V³⁺ a k₁ value of 32 M^?1 s^?1 at 50°C was calculated.The second stage of the process is the reduction of NH₂OH by V³⁺ to give aminyl radicals. In presence of benzene and toluene these radicals add to the aromatic ring to give new radicals which are oxidized by Cu²⁺ to the corresponding amines.The cuzrent yield of this process attains values as high as 70%. The yield dependence on H₂SO₄ concentration, electrolytic current and dioxane concentration is discussed.     

The stereochemistry of Tutton's salts X2[M(H2O)6](YO4)2

Neutron structure determinations have been made of Tutton's salts, X₂[M(H₂O)₆] (YO₄)₂, where Y = Se, X = K⁺, M = Cu²⁺; Y = S, X = K⁺, M = Ni²⁺, Cu²⁺, Zn²⁺; X = Rb⁺, Cs⁺, M = Cu²⁺. This work has shown that there are extensive hydrogen networks with almost linear hydrogen bonds from [M(H₂O)₆]²⁺ to (YO₄)^2?. The (H … O) distance increases in the Cu²⁺ series for X = K⁺ to Cs⁺ but there is no difference for the potassium copper salts when Y = Se or S. Three different distorted [M(H₂O)₆]²⁺ octahedra were found in the series (orthorhombic, tetragonal with two long and four short, or four long and two short bonds). The interatomic distances from X⁺ to the neighboring O in a distorted XO₈⁺ dodecahedron increases with increased cation size, implying that the X⁺ polyhedron is maintaining its shape.     

Influence of M2+ ions substitution on the structure of lanthanum hexaaluminates with magnetoplumbite structure

A series of compounds with the initial composition LaM_xAl₁₁O_18+x has been studied with x ranging from 0 to 1, and M = Mn, Co, Cu. They exhibit a more or less distorted and defective magnetoplumbite arrangement. Their effective compositions have been determined by X-ray structure refinement. The smaller the content of M²⁺, the higher the disorder in the crystal lattice, the worse the crystal growth, and more intense the diffuse scattering in (001) planes. The role of M²⁺ is compared to that of M²⁺ stabilizing γ-Al₂O₃ to give MAl₂O₄ spinels.     

A potentiometric and calorimetric study of the protonation and complexation with Cu2+ and Ni2+ of some sulphur-containing α,ω-amino acids

The behaviour in aqueous solution of some aminocarboxylates of the type NH₂(CH₂)_nS(CH₂)_m-1COO^? (abbreviated as n,m-NSO; n and m = 2 or 3) in equilibria with H⁺, Cu²⁺ and Ni²⁺ ions has been investigated potentiometrically and calorimetrically at 25° C in a 0.5 M KNO₃ medium.The protonation of the amino function and especially the carboxylate function is attended by strong desolvation effects, which are characterized by low exothermic enthalpies and strongly positive entropies of protonation.In [Cu(n,m-NSO)]⁺ and [Ni(n,m-NSO)]⁺ the aminocarboxylates act as tridentate ligands, forming complexes with a strong hard—hard character. The biligand species [Ni(n,m-NSO)₂] behave as six-coordinated complexes whereas in [Cu(n,m-NSO)₂] the second ligand is bound only through the N and S donor, forming a five-coordinated species.Finally, the n,m-NSO ligands also form protonated species with the Cu²⁺ ion.     

ESR and optical spectra of Cu2+:NH4Br

ESR and optical studies of Cu²⁺ ions doped in NH₄Br single crystals have been carried out at room temperature. A new centre has been identified for the crystal grown from acidic medium. The room temperature data reveal that Cu²⁺ ions go to interstitial sites having square planar coordination of four Br^? ions.     

Thermodynamic determination of the cation distribution in NixMn1−γ−xFe2+γO4 ferrites

The cation distribution in the spinel ferrite system Ni_xMn_1−γ−xFe_2+γO₄ (x=0, 0.25, 0.5, 0.75 and γ=0.137) has been calculated analytically in complete form as a function of thermodynamic parameters. A generalized theoretical framework based on the O’Neill-Navrotsky model and Newton methods was used to solve a multicomponent system for up to 10 cation species. The relationship between the cation distribution and composition is given. The results are shown to agree with the available experimental results.     

Structural and magnetic properties of nanocrystalline Ni1−xCuxFe2O4 prepared through oxalates precursors

Spinel ferrites of the composition Ni_1−xCu_xFe₂O₄ (x = 0.0-1.0) have been prepared through the thermal decomposition of their respective impregnated oxalates. The oxalate decomposition process was followed using differential thermal analysis-thermogravimetry techniques (DTA-TG). The synthesized nanocrystallites were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR). The formation of single-phase ferrite is confirmed by XRD. Tetragonal deformation is observed for samples with composition x ? 0.7. The increase in the lattice parameter with increasing Cu content can be explained based on the relative ionic radius. The TEM image shows spherically non-agglomerated particles with an average crystallite size that agrees well with that obtained from XRD. FT-IR studies show two absorption bands (ν₁ and ν₂) near to 600 and 400 cm⁻¹ for the tetrahedral and octahedral sites, respectively. The hysteresis measurements were done using a vibrating sample magnetometer (VSM). The cation distribution in these compositions is calculated from the magnetization data. With increasing Cu content, the saturation magnetization (M_s) was observed to decrease while the coercivity (H_c) increases. The possible reasons responsible for the composition dependence of the magnetic properties were discussed. The Curie temperature, measured through the temperature dependence of the dc-molar magnetic susceptibility, was found to decrease with increasing Cu content.     

Ni_xCu_(1-x)Fe_2O_4的H_2/CO_2氧化还原反应性能

完全还原的NixCu1-xFe2O4(x=0.25,0.5,0.75)可用来分解CO2。采用共沉淀法制备了NixCu1-xFe2O4(x=0.25,0.5,0.75)系列铁酸盐的纳米微粒,利用H2-TG数据分析NixCu1-xFe2O4(x=0.25,0.5,0.75)中Cu2+的还原反应动力学数据,得出表观活化能Ea并不是单纯随着Ni2+或Cu2+含量的变化而变化,当x=0.25时反应活化能具有最大值。通过CO2-TG比较不同样品经H2完全还原后分解CO2的活性,得出Ni0.5Cu0.5Fe2O4具有最低的起始反应温度和最大的分解活性,并采用XRDRietveld拟合方法对Ni0.5Cu0.5Fe2O4的完全还原产物和分解CO2产物进行物相分析。     

Cation ordering in the fluorite-like transparent conductors In4+xSn3−2xSbxO12 and In6TeO12

The cation ordering in the fluorite-like transparent conductors In_4+xSn_3−2xSb_xO₁₂ and In₆TeO₁₂, was investigated by Time of Flight Neutron Powder Diffraction and X-ray Powder Diffraction (tellurate). The structural results including atomic positions, cation distributions, metal-oxygen distances and metal-oxygen-metal angles point to a progressive cation ordering on both sites of the Tb₇O₁₂-type structure with a strong preference of the smaller 4d¹⁰ cations (Sn⁴⁺, Sb⁵⁺, Te⁶⁺) for the octahedral sites. The corresponding increase of the overall structure-bonding anisotropy is analyzed in terms of the crystal chemical properties of the OM₄ tetrahedral network of the antistructure. The relationships between the M₇O₁₂ and the M₂O₃ bixbyite-type structures are explored. Within the whole series of compositions In_4+xM_3−xO₁₂ (M=Sn, Sb, Te) there exists an increase of the symmetry gap between the more symmetrical bixbyite structure and the M₇O₁₂ type. This is tentatively correlated with the progressive weakening of thermal stability of these compositions from Sn to Te via Sb.     

Preparation and thermal stability of trioxalatocobaltates of bivalent metals KM2+ [Co(C2O4)3]·xH2O

Trioxalatocobaltates of bivalent metals KM²⁺[Co(C₂O₄)₃]·x H₂O, with M²⁺ = Ba, Sr, Ca and Pb, have been prepared, characterized and their thermal behaviour studied. The compounds decompose to yield potassium carbonate, bivalent metal carbonate or oxide and cobalt oxide as final products. The formation of the final products of decomposition is influenced by the surrounding atmosphere. Bivalent metal cobaltites of the types KM²⁺CoO₃ and M²⁺CoO_3—x are not identified among the final products of decomposition. The study brings out the importance of the decomposition mode of the precursor in producing the desired end products.     

Li and Li MAS NMR Studies on Fast Ionic Conducting Spinel-Type Li2MgCl4, Li2-xCuxMgCl4, Li2-xNaxMgCl4, and Li2ZnCl4

⁶Li and ⁷Li MAS NMR spectra including 1D-EXSY (exchange spectroscopy) and inversion recovery experiments of fast ionic conducting Li₂MgCl₄, Li_2-xCu_xMgCl₄, Li_2-xNa_xMgCl₄, and Li₂ZnCl₄ have been recorded and discussed with respect to the dynamics and local structure of the lithium ions. The chemical shifts, intensities, and half-widths of the Li MAS NMR signals of the inverse spinel-type solid solutions Li_2-xM^I_xMgCl₄ (M^I=Cu, Na) with the copper ions solely at tetrahedral sites and sodium ions at octahedral sites and the normal spinel-type zinc compound, respectively, confirm the assignment of the low-field signal to Li^tet of inverse spinel-type Li₂MgCl₄ and the high-field signal to Lioct as proposed by Nagel et al. (2000). In contrast to spinel-type Li_2-2xMg_1+xCl₄ solid solutions with clustering of the vacancies and Mg²⁺ ions, the Cu⁺ and Na⁺ ions are randomly distributed on the tetrahedral and octahedral sites, respectively. The activation energies due to the various dynamic processes of the lithium ions in inverse spinel-type chlorides obtained by the NMR experiments are E_a=6.6-6.9 and ΔG^*>79 KJ mol⁻¹ (in addition to 23, 29, and 75 kJmol-1 obtained by other techniques), respectively. The largest activation energy of >79 KJ mol⁻¹ corresponds to hopping exchange processes of Li ions between the tetrahedral 8a sites and the octahedral 16d sites. The smallest value of 6.6-6.9 KJ mol⁻¹, which was derived from the temperature dependence of both the spin-lattice relaxation times T₁ and the correlation times τ_C of Li^tet, reveals a dynamic process for the Li^tet ions inside the tetrahedral voids of the structure, probably between fourfold 32e split sites around the tetrahedral 8a site.     

Carbon and proton magnetic resonance studies on the sites of Mn2+ and Ni2+ ion binding with thiamin and thiamin pyrophosphate and on the solution conformation of the coenzyme

Carbon magnetic resonance was employed to determine the Mn²⁺ and Ni²⁺ binding sites on thiamin and thiamin pyrophosphate and to help elucidate the solution conformation of the latter with Mn²⁺. In comparison with previous proton magnetic resonance studies, the results here presented elucidate direct binding to thiamin even without the presence of the pyrophosphate. In thiamin Mn²⁺ is bound to both the pyrimidine ring and to the oxygen of the β-hydroxyethyl chain, albeit more weakly to the former, Ni²⁺, on the other hand is indicated in binding to the pyrimidine ring only. From observation of water relaxation data, potentiometric determination of the pyrimidinium pK_a with metal ions added and carbon magnetic resonance (competition of thiamin and thiamin pyrophosphate for Mn²⁺) we have very strong evidence to indicate that Mn²⁺ is separated from the pyrimidine ring by a solvent molecule. We were able to estimate association constant between the pyrimidine and Mn²⁺ (50 M^?1) and OH and Mn²⁺ (100 M^?1) in thiamin.In thiamin pyrophosphate both Mn²⁺ and Ni²⁺ much prefer the pyrophosphate as binding sites and at least in the former an inner sphere complex is indicated. Also with Mn²⁺ present the dimethylene pyrophosphate is folded back onto the thiazolium ring according to analysis of proton broadening data and relaxation mechanisms.