Mid- and near-infrared spectroscopic investigation of homogeneous cation distribution in MgxZnyAl(x+y)/2-layered double hydroxide (LDH)

In this report, a detailed FTIR fitting analysis was used to recognize Mg, Zn and Al homogeneous distribution in MgxZnyAl(x+y)/2-Layered double hydroxide (LDH) hydroxyl layer. In detail, OH-Mg2Al:OH-Mg3 ratios decreased from 95.2:4.8 (MIR) and 94.2:5.8 (NIR) to 58.9:41.1 (MIR) and 61.8:38.2 (NIR), when Mg:Al increased from 2.2:1.0 to 4.1:1.0 in MgAl-LDHs. These fitting results were similar with theoretical calculations of 94.3:5.7 and 59.0:41.0. In a further analysis of MgxZnyAl(x+y)/2-LDHs, OH bonded Zn2Mg, Zn2Al, MgZnAl, Mg2Al and Mg2Zn peaks were identified at 3420, 3430, 3445–3450, 3454 and 3545 cm-1, respectively. With the decrease of Mg:Zn from 3:1 to 1:3, metal-hydroxyl bands changed from OH-Mg2Al and MgZnAl (with a ratio of 49.4:50.6) to OH-MgZnAl and Zn2Al (with a ratio of 55.0:45.0). They were also similar with theoretical calculations of 47.6:52.4 and 54.6:45.4. As a result, these results show that there is an ordered cation distribution in MgxZnyAl(x+y)/2-LDH, and FTIR is feasible in recognizing this structure.

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 + NixMg1−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.

An improved X-ray structural analysis for determining cation distribution in ZnFe2O4 and ZnFe2O4- Fe3O4 solid solutions

The distribution of zinc cation between crystallographically nonequivalent positions in ZnFe204 has been determined by anomalous X-ray scattering near the Zn K absorption edge. Measured intensity ratio with two energies close to the edge can be quantitatively explained only by assigning all zinc cations to the tetrahedral position in the approximately cubic close packed array of oxygen ions. A similar conclusion has also been reached for ZnxFe3-x04 solid solutions with x = 0.73, 0.54 and 0.35 employing the improved X-ray method. This is consistent with the EXAFS results which indicate an almost unchanged environmental structure around zinc cation in these solid solutions.

Cation distribution and mixing thermodynamics in Fe/Ni thiospinels

The structural analogy between Ni-doped greigite minerals (Fe3S4) and the (Fe,Ni)S clusters present in biological enzymes has led to suggestions that these minerals could have acted as catalysts for the origin of life. However, little is known about the distribution and stability of Ni dopants in the greigite structure. We present here a theoretical investigation of mixed thiospinels (Fe1

Cation distribution and magnetic characterization of the multiferroic cobalt manganese Co2MnO4 spinel doped with bismuth

The structural and magnetic properties of the cubic spinel oxide Co 2MnO4 (Fd3m space group) doped with different concentrations of bismuth, were investigated by X-ray diffraction and SQUID magnetometry. The Bi3+ ions entering into the CoIII octahedral sites do not alter the effective moment, μeff ∼8.2 μB, whereas both the magnetization M50 kOe at the highest field (50 kOe) and the field-cooled MFC magnetizations increased when increasing the Bi content. The ferrimagnetic character of the parent compound, Co2MnO4, is maintained for all materials although the antiferromagnetic interactions Co2+-Co2+ are affected, resulting in higher values of the Curie-Weiss temperature. Due to the large ionic radius of Bi, octahedra distortions occur as well as valence fluctuations of the Mn ions, giving rise to Jahn-Teller effects and enhancing the exchange interactions. The off-center Bi3+ ion is responsible of non-centrosymmetric charge ordering and should lead to multiferroïsme conditions for the BixCo2-xMnO4 material. © 2012 Elsevier B.V.

Cation Distribution and Local Order in Mixed Sodium Metaphosphate Glasses

Two structural properties in mixed alkali metal phosphate glasses that seem to be crucial to the development of the mixed ion effect in dc conductivity were systematically analyzed in Na mixed metaphosphates: the local order around the mobile species, and their distribution and mixing in the glass network. The set of glasses considered here, Na1-xMxPO3 with M = Li, Ag, K, Rb, and Cs and 0 <= x <= 1, encompass a broad degree of size mismatch between the mixed cation species. A comprehensive solid-state nuclear magnetic resonance study was carried out using P-31 MAS, Na-23 triple quantum MAS, Rb-87 QCPMG, P-31-Na-23 REDOR, Na-23-Li-7 and Li-7-Li-6 SEDOR, and Na-23 spin echo decay. It was observed that the arrangement of P atoms around Na in the mixed glasses was indistinguishable from that observed in the NaPO3 glass. However, systematic distortions in the local structure of the 0 environments around Na were observed, related to the presence of the second cation. The average Na-O distances show an expansion/compression When Na+ ions are replaced by cations with respectively smaller/bigger radii. The behavior of the nuclear electric quadrupole coupling. constants indicates that this expansion reduces the local symmetry, while the compression produces the opposite effect These effects become marginally small when the site mismatch between the cations is small, as in Na-Ag mixed glasses. The present study confirms the intimate mixing of cation species at the atomic scale, but clear deviations from random mixing were detected in systems with larger alkali metal ions (Cs-Na, K-Na, Rb-Na). In contrast, no deviations from the statistical ion mixture were found in the systems Ag-Na and Li-Na, where mixed cations are either of radii comparable to (Ag+) or smaller than (Li+) Na+. The set of results supports two fundamental structural features of the models proposed to explain the mixed ion effect: the. structural specificity of the sites occupied by each cation species and their mixing at the atomic scale.

Average structure of an An (sub 48) plagioclase from the Hogarth Ranges

The average structure (CI) of a volcanic plagioclase megacryst with composition Ano, from the Hogarth Ranges, Australia, has been determined using three-dimensional, singlecrystal neutron and X-ray diffraction data. Least squaresr efinements, incorporating anisotropic thermal motion of all atoms and an extinction correction, resulted in weighted R factors (based on intensities) of 0.076 and 0.056, respectively, for the neutron and X-ray data. Very weak e reflections could be detected in long-exposure X-ray and electron diffraction photographs of this crystal, but the refined average structure is believed to be unaffected by the presence of such a weak superstructure. The ratio of the scattering power of Na to that of Ca is different for X ray and neutron radiation, and this radiation-dependence of scattering power has been used to determine the distribution of Na and Ca over a split-atom M site (two sites designated M' and M") in this Ano, plagioclase. Relative peak-height ratios M'/M", revealed in difference Fourier sections calculated from neutron and X-ray data, formed the basis for the cation-distribution analysis. As neutron and X-ray data sets were directly compared in this analysis, it was important that systematic bias between refined neutron and X-ray positional parameters could be demonstrated to be absent. In summary, with an M-site model constrained only by the electron-microprobedetermined bulk composition of the crystal, the following values were obtained for the M-site occupanciesN: ar, : 0.29(7),N ar. : 0.23(7),C ar, : 0.15(4),a nd Car" : 0.33(4). These results indicate that restrictive assumptions about M sites, on which previous plagioclase refinements have been based, are not applicable to this Ano, and possibly not to the entire compositional range. T-site ordering determined by (T-O) bond-length variation-t,o : 0.51(l), trm = t2o = t2m = 0.32(l)-is weak, as might be expectedf rom the volcanic origin of this megacryst.

Gibbs Energy of Formation of Cobalt Divanadium Tetroxide

The Gibbs energy of formation of V2O3-saturated spinel CoV2O4 has been measured in the temperature range 900–1700 K using a solid state galvanic cell, which can be represented as Pt, Co + CoV2O4 + V2O3/(CaO) ZrO2/Co + CoO, Pt. The standard free energy of formation of cobalt vanadite from component oxides can be represented as CoO (rs) + V2O3 (cor) → CoV2O4 (sp), ΔG° = −30,125 − 5.06T (± 150) J mole−1. Cation mixing on crystallographically nonequivalent sites of the spinel is responsible for the decrease in free energy with increasing temperature. A correlation between “second law” entropies of formation of cubic 2–3 spinels from component oxides with rock salt and corundum structures and cation distribution is presented. Based on the information obtained in this study and trends in the stability of aluminate and chromite spinels, it can be deduced that copper vanadite is unstable.

Phase relations between Al2O3---Cr2O3 and FeAl2O4---FeCr2O4 solid solutions at 1823 K

Tie-lines between the corundum and spinel solid solutions have been determined experimentally at 1823 K. Next, activities of FeCr2O4 and FeAl2O4 in the spinel solid solution were determined by combining the tie-line data with literature values for the activities of Cr2O3 and Al2O3 in the corundum phase. Activities and the Gibbs energy of mixing for the spinel solid solution were also obtained from a model based on cation distribution between nonequivalent crystallographic sites in the oxide lattice. The difference between the Gibbs energy of mixing obtained experimentally and from the model has been attributed to a strain enthalpy term which is relatively unchanged in magnitude from the reported at 1373 K. The integral enthalpy of mixing obtained from experimental data at 1373 and 1823 K using the second law is compared with the model result.

Spinel-Corundum Phase Equilibria in the Systems Mn-Cr-Al-O and Co-Cr-Al-O at 1373 K

The tie lines delineating ion-exchange equilibria between MCr2O4-MAl2O4 spinel solid solution, where M is either Mn or Co, and Cr2O3-Al2O3 solid solution with the corundum structure were determined at 1373 K by electron microprobe and E0AX point count analysis of the oxide phases equilibrated with metallic Co and Au-5% Mn. The component activities in the spinel solid solutions are derived from the tie lines and the thernodynamic data for Cr2O3-Al2O3 soiid solutions available hi the literature. The Gibbs free energies of mixing calculated from the experimental data are discussed in relation to the values derived from the cation distribution a.odel based on the site preference energies and assuming random mixing on both tetrahedral and octahedral sites. Positive deviations from ideality observed in this study suggest a miscibility gap for both series of spinel solid solutions at low temperatures in the absence of oxidation.

New A2+Mo4+O3 oxides with defect spinel structure

New A2+Mo4+O3 oxides for A = Mn, Co and Zn crystallizing in a defect spinel structure have been prepared by hydrogen-reduction of the corresponding AMoO4 oxides. X-ray powder diffraction intensity analysis of the zinc compound indicates that the cation distribution is (Zn)t[Zn1/3Mo4/3□1/3]oO4. The defect spinels are metastable decomposing to a mixture of A2Mo3O8 and AO at high temperatures. Electrical and magnetic properties of the spinel phases are reported.

Study of (Ba3MMWO9)-M-II-W-IV (M-II = Ca, Zn; M-IV = Ti, Zr) perovskite oxides: Competition between 3C and 6H perovskite structures

We describe an investigation of (Ba3MMWO9)-M-II-W-IV oxides for M-II = Ca, Zn, and other divalent metals and M-IV = Ti, Zr. In general, a 1:2-ordered 6H (hexagonal, P6(3)/mmc) perovskite structure is stabilized at high temperatures (1300 degrees C) for all of the (Ba3MTiWO9)-Ti-II oxides investigated. An intermediate phase possessing a partially ordered 1:1 double perovskite (3C) structure with the cation distribution, Ba-2(Zn2/3Ti1/3)(W2/3Ti1/3)O-6, is obtained at 1200 degrees C for Ba3ZnTiWO9. Sr substitution for Ba in the latter stabilizes the cubic 3C structure instead of the 6H structure. A metastable Ba3CaZrWO9 that adopts the 3C (cubic, Fm (3) over barm) structure has also been synthesized by a low-temperature metathesis route. Besides yielding several new perovskite oxides that may be useful as dielectric ceramics, the present investigation provides new insights into the complex interplay of crystal chemistry (tolerance factor) and chemical bonding (anion polarization and d(0)-induced distortion of metal-oxygen octahedra) in the stabilization of 6H versus 3C perovskite structures for the (Ba3MMWO9)-M-II-W-IV series.

Metal-spinel-corundum three phase equilibria in the system Ni-Cr-Al-O at 1373 K

The tie-lines representing the inter-crystalline ion exchange equilibria between the NiCr2O4-NiAl2O4 spinet solid solution and Cr2O3-Al2O3 corundum solid solution are determined by electron microprobe andEDAX pointcountanalysis of the oxide phases equilibrated with metallic Ni at 1373 K. The component activities in the spinet solid solution are derived from the tie-lines and thermodynamic data for Cr2O3-Al2O3 solid solution available in the literature. The Gibbs energy of mixing of the spinet solid solution calculated from the experimental data is discussed in relation to the values derived from the cation distribution models which assume random mixing of cations on both tetrahedral and octahedral sites. Positive deviation from the models is observed indicating significant positive enthalpy contribution arising form the size mismatch between Al+3 and Ni+2 ions on the tetrahedral site and Al+3, Ni+2 and Cr+3 on the octahedral site. Variation of the oxygen potential for threephase equilibrium involving metallic nickel, spinet solid solution and corundum solid solution is computed as a function of composition of the solid solutions at 1373 K. The oxygen potential exhibits a minimum at aluminum cationic fraction eta(Al)/(eta(Al) + eta(Cr)) = 0.524 in the oxide solid solutions.

Study of (Ba3MMWO9)-M-II-W-IV (M-II = Ca, Zn; M-IV = Ti, Zr) perovskite oxides: Competition between 3C and 6H perovskite structures

We describe an investigation of (Ba3MMWO9)-M-II-W-IV oxides for M-II = Ca, Zn, and other divalent metals and M-IV = Ti, Zr. In general, a 1:2-ordered 6H (hexagonal, P6(3)/mmc) perovskite structure is stabilized at high temperatures (1300 degrees C) for all of the (Ba3MTiWO9)-Ti-II oxides investigated. An intermediate phase possessing a partially ordered 1:1 double perovskite (3C) structure with the cation distribution, Ba-2(Zn2/3Ti1/3)(W2/3Ti1/3)O-6, is obtained at 1200 degrees C for Ba3ZnTiWO9. Sr substitution for Ba in the latter stabilizes the cubic 3C structure instead of the 6H structure. A metastable Ba3CaZrWO9 that adopts the 3C (cubic, Fm (3) over barm) structure has also been synthesized by a low-temperature metathesis route. Besides yielding several new perovskite oxides that may be useful as dielectric ceramics, the present investigation provides new insights into the complex interplay of crystal chemistry (tolerance factor) and chemical bonding (anion polarization and d(0)-induced distortion of metal-oxygen octahedra) in the stabilization of 6H versus 3C perovskite structures for the (Ba3MMWO9)-M-II-W-IV series.

Experimental and Computational Characterization of Alloy--Spinel--Corundum Equilibrium in the System Fe--Co--Al--O at 1873K

The three-phase equilibrium between alloy, spinel solid solution and alpha -Al sub 2 O sub 3 in the Fe--Co--Al--O system at 1873k was fully characterized as a function of alloy composition using both experimental and computational methods. The equilibrium oxygen content of the liquid alloy was measured by suction sampling and inert gas fusion analysis. The O potential corresponding to the three-phase equilibrium was determined by emf measurements on a solid state galvanic cell incorporating (Y sub 2 O sub 3 )ThO sub 2 as the solid electrolyte and Cr + Cr sub 2 O sub 3 as the reference electrode. The equilibrium composition of the spinel phase formed at the interface between the alloy and alumina crucible was measured by electron probe microanalysis (EPMA). The experimental results were compared with the values computed using a thermodynamic model. The model used values for standard Gibbs energies of formation of pure end-member spinels and Gibbs energies of solution of gaseous O in liquid Fe and cobalt available in the literature. The activity--composition relationship in the spinel solid solution was computed using a cation distribution model. The variation of the activity coefficient of O with alloy composition in the Fe--Co--O system was estimated using both the quasichemical model of Jacob and Alcock and Wagner's model along with the correlations of Chiang and Chang and Kuo and Chang. The computed results of spinel composition and O potential are in excellent agreement with the experimental data. Graphs. 29 ref.--AA