Cation Distribution In Spinels Research Articles

Origin of metallic behavior in NiCo2O4 ferrimagnet.

Predicting and understanding the cation distribution in spinels has been one of the most interesting problems in materials science. The present work investigates the effect of cation redistribution on the structural, electrical, optical and magnetic properties of mixed-valent inverse spinel NiCo2O4(NCO) thin films. It is observed that the films grown at low temperatures (T < 400 °C) exhibit metallic behavior while that grown at higher temperatures (T > 400 °C) are insulators with lower ferrimagnetic-paramagnetic phase transition temperature. So far, n-type Fe3O4 has been used as a conducting layer for the spinel thin films based devices and the search for a p-type counterpart still remains elusive. The inherent coexistence and coupling of ferrimagnetic order and the metallic nature in p-type NCO makes it a promising candidate for spintronic devices. Detailed X-ray Absorption and X–ray Magnetic Circular Dichroism studies revealed a strong correlation between the mixed-valent cation distribution and the resulting ferrimagnetic-metallic/insulating behavior. Our study clearly demonstrates that it is the concentration of Ni3+ions and the Ni3+–O2−Ni2+ double exchange interaction that is crucial in dictating the metallic behavior in NCO ferrimagnet. The metal-insulator and the associated magnetic order-disorder transitions can be tuned by the degree of cation site disorder via growth conditions.

Evolution of the Ni2+ Environment During the Formation of a MgO–Al2O3–SiO2 Glass‐Ceramic: A Combined XRD and Diffuse Reflectance Spectroscopy Approach

The effect of the addition of NiO on the crystallization mechanisms has been studied for SiO2–Al2O3–MgO glass‐ceramics. A detailed Rietveld analysis has been made to determine the chemical composition and the cation distribution in the spinel phase and the evolution of Ni2+ coordination site using diffuse reflectance spectroscopy. Powder X‐ray diffraction (XRD) results show the formation of β‐quartz and spinel and the nickel fraction in the spinel phase decreasing as the plateau temperature increases. The determination of cation distribution in spinel using XRD reveals a preference of Ni2+ and Al3+ for octahedral sites and an increasing proportion of Ni2+ in tetrahedral sites for heat treatments at high temperature (1100°C–1200°C). Diffuse reflectance spectroscopy shows that Ni2+ is mainly fivefold coordinated in the starting glass, playing an important role in the crystallization of the spinel phase and it confirms the presence of tetrahedral and octahedral Ni2+ in the glass‐ceramics. The combination of these two techniques also indicates a chemical heterogeneity of spinel crystals with Ni2+‐rich and Mg2+‐rich zones.

Analysis of the magnetic corrosion product deposits on a boiling water reactor cladding

The buildup of corrosion product deposits (CRUD) on the fuel cladding of the boiling water reactor (BWR) before and after zinc injection has been investigated by applying local experimental analytical techniques. Under the BWR water chemistry conditions, Zn addition together with the presence of Ni and Mn induce the formation of (Zn,Ni,Mn)[Fe2O4] spinel solid solutions. X-ray absorption spectroscopy (XAS) revealed inversion ratios of cation distribution in spinels deposited from the solid solution. Based on this information, a two-site ferrite spinel solid solution model is proposed. Electron probe microanalysis (EPMA) and extended X-ray absorption fine structure (EXAFS) findings suggest the zinc-rich ferrite spinels formation on BWR fuel cladding mainly at lower pin.

Structural refinement and crystal chemistry of Mn-doped spinel: A case for tetrahedrally coordinated Mn3+ in an oxygen-based structure

Spinel single crystals of four compositions along the MgAl2O4-MgMn2O4 join, with Mn3+ up to 0.25 apfu, were synthesized by use of a flux-growth method. The crystals were analyzed by electron microprobe, X-ray single-crystal diffraction, and optical absorption spectroscopy. Results revealed that Mg contents vary from 0.90 to 0.99 apfu, Mn2+ ≤ 0.11 apfu, Mn3+ varies from zero to 0.25 apfu, and Al-contents from 1.75 to 1.99 apfu. The unit-cell parameter increases linearly from 8.0883(3) to 8.1413(4) Å with increasing Mn3+ content. The crystals show moderately disordered cation distributions, with i = 0.23(1), and different distribution trends have been observed: the Mg content is constant at the T site and is replaced by Mn2+ at the M-site; Al decreases while Mn3+ increases in T- and M-sites. Mn3+ shows a preference for the T site, and a specific bond distance was refined, IVMn3+-O = 1.88(1) Å. Unpolarized room-temperature single-crystal spectra reveal two relatively broad absorption features at ca. 23 000 and 10 800 cm-1, which are assigned to spin-allowed d-d transitions in Mn3+ located at octahedral and tetrahedral sites, respectively. The bond valence approach shows that the bonds are strained in the tetrahedron indicating underbonding in T, whereas the bonds are unstrained in the octahedron. To reduce the M-M repulsion, the steric effect is driven by the movement of the oxygen atoms, which improves the shielding effect around the M-site, thus increasing the distortion of the structure relative to the CCP. As a result the tetrahedron undergoes an isotropic expansion, which constrains the structure to incorporate larger cations such as Mn3+ rather than smaller cations like Al at the T-site. This behavior, which is in disagreement with predictions based on crystal field energy considerations, illustrates the greater importance of steric factors on the cation distribution in spinels.

Cation distribution in natural Zn-aluminate spinels

AbstractThe intracrystalline cation distributions in fourteen natural Zn-aluminate spinels were determined by means of X-ray single-crystal structural refinement, supported for some samples by Mössbauer spectroscopy.Zinc substitutes for Mg and subordinately Fe2+ and its relevant changes in content, from 0.10 to 0.96 atoms per formula unit (apfu), are not related to variations of cell parameter. The latter is determined mainly by the substitution Fe3+ ⇌ Al. In agreement with data from synthetic samples, a small but definite amount of Zn (up to 0.06 apfu) is located in the octahedral M site. Fe2+, when present, shows a preference for tetrahedral coordination.An improved value of the tetrahedral Zn(T)-O distance (1.960 Å) was obtained, integrating the set of interatomic distances used for the determination of cation distribution in spinels.

Contrasting Cation Distributions in Spinels of Similar Composition

Abstract Chromite mined from the Great Dyke in Zimbabwe is used for the production of about 180 000 tonnes of ferroalloy per annum, equivalent to about 6% of the world’s supply. The chemical and physical characteristics of the ores, which vary even within the same seam, affect the carbon content of the ferroalloys and hence their commercial value. The purpose of this study is to examine some of the differences between two extremes of chromitites, sheared and unsheared, used in the ferroalloy industry in Zimbabwe to provide a better understanding of the factors that influence the composition of the alloys as a basis for process mineralogy. The ores consist of chromium spinels (MgAlCrFe)3O4 with a lattice of alternating non-equivalent planes of tetrahedrally coordinated A sites and octahedrally coordinated B sites. The distribution of five major cations (Mg2+, Al3+, Fe2+, Fe3+ and Cr3+) was examined by the technique of electron channelling.

Recovery of neutron-induced defects in near-stoichiometric spinel ceramics irradiated at around 500°C

Magnesium aluminate spinel ceramics with different stoichiometry were irradiated with fast neutrons up to 2.3 × 10 24 n/m 2 ( E > 0.1 MeV) at around 500°C. Near stoichiometric compositions, average length change and corresponding lattice parameter change by the irradiation were very small, but some specimens contracted relatively large amount. These specimens were isochronally annealed up to 1000°C. During the annealing, macroscopic length of the specimens whose length reduced by the irradiation showed stepwise increase at around 650°C, and returned to the pre-irradiation value. On the other hand, macroscopic length of the specimens whose length increased very slightly due to the irradiation almost unchanged. The changes in macroscopic length matched well with the changes in lattice parameter of all specimens due to the irradiation and post-irradiation annealing. The reason of the stepwise increase in length and lattice parameter during annealing may be related to the order-disorder transition of cation distribution in spinel.

Crystal structures and cation distributions in simple spinels from powder XRD structural refinements: MgCr2O4, ZnCr2O4, Fe3O4 and the temperature dependence of the cation distribution in ZnAl2O4

The crystal structure and cation distributions in the spinels MgCr2O4, ZnCr2O4, Fe3O4 and a suite of ZnAl2O4 samples annealed at 900 to 1400° C and then rapidly quenched, have been determined by powder X-ray diffraction, using several different X-ray procedures and both conventional structure-factor refinement and whole-pattern (or Rietveld) refinement methods. The chromite spinels are expected from crystal chemical considerations to have an almost completely normal cation distribution (inversion parameter, x, equal to zero). In agreement with this expectation, three samples of MgCr2O4 annealed at 900, 1100 and 1300° C, and ZnCr2O4 were all found to have x=0 within two estimated standard deviations (esd), suggesting that the accuracy with which cation distributions in spinels may be determined by powder XRD is close to the estimated precision. Slightly better results are obtained assuming neutral-atom scattering curves rather than half-ionized or fully ionized, but the differences are small (within the esd). The results from the Rietveld refinements are similarly in good agreement with those using the conventional structure factor refinement approach (agreement within the combined esd's), although in detail the Rietveld procedure sometimes produces small systematic differences in refined parameters. The suite of ZnAl2O4 spinels show a smooth increase in x from 0.01 at 900° C to 0.05 at 1300° C, and this behaviour is well described by the simple thermodynamic model for disordering in spinels with αZn-Al=89 kJ/mol, assuming β=−20 kJ/mol. The oxygen positional parameters for Fe3O4 are similar to those from published single crystal studies, indicating that the powder method also yields accurate interatomic distances in spinels.

EXAFS: Determination of cation distribution in spinels

In oxidic spinels containing transition metal ions, the tetrahedral and octahedral bond distances differ by less than 0·25 Å. Therefore, in the case of a cation occupying both the sites, the normalized EXAFS for first coordination shell gives the average bond distance and average coordination number respectively over all the bonds and the positions of the cation under investigation. EXAFS analysis of fourteen mixed lithium ferrite spinels was carried out to show how the average bond distance can fruitfully be used to obtain the exact cation distribution, in conformity with X-ray diffraction data.

Isostructural phase transitions in spinels described by two internal parameters

Equations describing cation distribution in spinels are studied by numerical methods. All types of the dependence of the conversion degree on temperature are determined. It is shown that complex sequences of isostructural phase transitions are realized in spinels with two conversion degrees. Several critical points of “liquid-vapour” type may exist on the phase diagram.

Spectroscopic in situ investigations of solids at high temperatures: A Mössbauer and optical spectroscopy study of diffusion and of the kinetics of solid state reactions

We report on Mössbauer and on optical in situ investigations of diffusion dynamics and of solid state reaction kinetics. The first example refers to the cation diffusion in magnetite, Fe 3−δO 4. From the isothermal oxygen activity dependence of the Mössbauer spectra detailed information is deduced on cation diffusion taking place on the two regular cation sublattices of the spinel structure. The second example concerns the homogeneous disorder reaction establishing cation distributions in spinels. Our results obtained from optically detected temperature-jump experiments indicate surprisingly rapid kinetics for this reequilibration process in CoAl 2O 4 and NiAl 2O 4 at high temperatures. Finally, results are reported of a Mössbauer high-temperature investigation of the internal oxidation reaction in the system Mg-Fe-O induced by rapid changes in oxygen activity.

Factors determining the cation distribution in spinels: The case of spinels containing several high-valency cations

Article de synthese concernant la distribution des cations de haute valence dans les structures de type spinelle. Application de la regle electrostatique de Pauling a quelques structures

Application of a Monte Carlo method to the determination of cation distribution in spinels by powder diffractometry

Use of a Monte Carlo method is proposed for assessing the reliability of the experimental determination of cation distribution in spinels as deduced from X-ray powder diffraction data. The method is demonstrated on the ternary spinels Cu 0.6Zn 0.4Al 2O 4 and Cu 0.2Mg 0.8Al 2O 4, which present the complication of having two cations with very close X-ray scattering power. It is shown that the proposed method tests out the precision of the cation distribution results with greater detail than what can be obtained by simple consideration of a residual function.

Isostructural phase transitions in spinels

Equations are studied, which describe cation distribution in spinels. All types of the dependence of the inversion parameter on temperature are determined. The possibility of isostructural phase transitions in spinels is shown and the phase diagram is built.

X-Ray diffraction determination of the cation distribution and oxygen positional parameter in polycrystalline spinels

A critical assessment is made of the calculation procedures currently used for determining the cation distribution in spinels from X-ray powder diffraction data. Both binary and ternary spinels are considered. It is shown that X-ray powder diffraction, in conjunction with appropriate computational methods, has enough sensitivity to determine the cation distribution and oxygen positional parameter. A difference of one electron between two different cations can be enough to render them distinguishable.

The factors influencing cation site-preferences in spinels a new mendelyevian approach

Pseudopotential orbital radii rs, rp are used to construct an index, rσ=rs+rp, which characterizes the average potential experienced by atomic valence electrons. A plot of rAσverses rBσfor 172 chalogenide spinels (AB2X4, X=O, S, Se, Te) leads to two well defined areas, which separate normal and inverse spinels, with only four errors (a predictive success rate of 98%). The gross sorting is achieved without recourse either to the number of d-electrons or an orbital radius rd, from which it is inferred that it is the energies and extents of the cation s and p-orbitals which primarily determine coordination number in these systems. This approach to the problem of cation distribution in spinels is contrasted with the less generally applicable, traditional, crystal field ideas. The relevance of both rσ and crystal field stabilization energies to the thermodynamics of spinel reactions is also discussed.

Cation distributions in spinels: Effective electrostatic energy versus crystal field stabilization energy

Cation distributions in spinels: Effective electrostatic energy versus crystal field stabilization energy

Three different methods of determining the cation distribution in spinels: A comparison

MgIn 2S 4 crystallizes in the Fd3 m ( O 7 h ) space group with a = 10.7108(8), Å. The cation distribution ( x) and the deformation parameter ( u) have been determined by means of three methods: Bertaut, Furuhashi, and Baltzer. A critical discussion about the coupled use of the three methods is presented.

Chemical shifts and effective charges in ternary and complex systems

Effective charges, q, in ternary and complex systems are estimated. The semiempirical method of Suchet (1965) is extended to ternary compounds such as manganites and ferrites while the semi-experimental method of Gianturco and Coulson (1968) is applied to complex Co-Zn ferrites and cobalt complexes. An attempt is made to obtain correlation between the estimated values of q and the measured chemical shifts, Delta E. It is found that although the effective charge governs the chemical shift, its magnitude is profoundly affected by the cation distribution in spinels. A comparison of the values of q obtained from the Suchet's method with those estimated from Gianturco and Coulson's method for the same family of compounds shows good agreement.

Determination of cation distribution in spinels by X-ray diffraction method

A method for the determination of the cation distribution in spinels by X-ray powder diffraction technique was modified so as to take into consideration the effective temperature factor for integrated diffraction intensities of the sample. The optimum values of the oxygen parameter u and degree of inversion x for the spinel sample were determined as the ones gave the best linearity of the relation between log ( I obs I calc ) and sin 2 θ λ 2 . Eight diffraction lines were used, five of which were attributed to the cations in tetrahedral site and other three to all the cations in the spinel. The modified method was applied to three samples GeCo 2O 4, CoAl 2O 4 and NiAl 2O 4.