Chromium Spinel Research Articles

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.

High-temperature expansion of the correlation function and short-range ordering in a spinel lattice

The short-range ordering in a pure and in a diluted spinel lattice in the paramagnetic phase is studied by the high-temperature series expansion of the zero-field static spin correlation function γ i = < S 0 − S i> S(S + 1) . The first four terms of γ i are derived using first, second and third nearest-neighbour (n.n) interactions. The results are presented for various neighbour correlations (up to the third). Their behaviour with the temperature and the site dilution is presented. We have compared our theoretical results with experimental data in some chromium spinel compounds having different kinds of spin ordering, namely helimagnetic and ferromagnetic at low temperature. The paramagnetic short-range order is shown to be compatible with the long-range one both for the pure and diluted systems.

Exchange Integrals and Magnetic Properties of Mixed Chromium Spinel Systems: Zn1−xAxCr2X4 (A = Cd, Hg and X  S, Se)

AbstractA lot of magnetic measurements are accomplished in order to improve the phase diagrams of the systems Zn1‐xCdxCr2S4 and Zn1‐xCdxCr2Se4 particularly in the region of intermediate concentrations with mixed structure. The exchange integrals for the first and second coordination spheres (J and K) are calculated in terms of dilution for the three systems Zn1‐xCdxCr2Se4, Zn1‐xCdxCr2Se4, and Zn1‐xHgxCr2Se4. The evolution of the structure and the magnetic properties with the substitution of diamagnetic ions at tetrahedral sites is the result of the competition between the ferromagnetic and antiferromagnetic interactions. Besides, the magnetic properties are compared with the phase diagrams (T, x) of these systems. Because of its complicated antiferromagnetic structure, the sulfur compound seems to have particular characteristics among the mixed spinel systems.

The extent of zinc oxide solution in zinc chromate spinel

Abstract Atomistic simulation methods have been applied to an ionic model and used to investigate the enthalpies of formation and reaction of defects in the spinel, ZnCr2O4. For the stoichiometric material our results (i) suggest that small amounts of exchange disorder are expected and (ii) show that the formation enthalpies of Schottky and Frenkel defects are sufficiently high that negligible intrinsic disorder of these kinds is expected. The possibility, suggested by experiment, that large amounts of ZnO may dissolve in the ZnCr2O4 lattice has also been investigated. However, the results of the calculations presented here rule out straightforward dissolution. On the other hand, they show that it is likely that the solubility of ZnO is substantially increased when the material is simultaneously oxidized. Unfortunately, the methods used to calculate the corresponding reaction energies in non-stoichiometric ZnCr2O4 are subject to some uncertainty arising from the variable nature of the second electron affinity of the oxygen atom. Nevertheless, the oxidation-driven mechanism is supported by the calculated variations of lattice parameter with composition, which are not subject to the same uncertainty.

Electronic and magnetic properties of the spinel semiconductor CdCr2Se4.

We present a first-principles study of the electronic and magnetic properties of the chromium spinel ${\mathrm{CdCr}}_{2}$${\mathrm{Se}}_{4}$, a ferromagnetic semiconductor, which, for its interesting magnetic and electric properties has been the object of many experimental studies. Using calculations based on the full-potential linearized augmented plane wave relativistic scheme we are able to reproduce the structural equilibrium properties and to explain many of the features measured by resonant photoemission spectroscopy. In particular, we analyze the contribution of the Cr 3d electrons in a density-of-states study including the two spin orientations and a comparison with the binary analog CdSe.

Accessory chromium spinels: Their coexistence and alteration in serpentinites

Accessory chromium spinel occurring in serpentinites is usually altered. The extent of this alteration and its relationship to alteration processes such as serpentinization and metamorphism is not defined, complicating the use of altered spinel as a petrogenetic indicator. Here, the relationship between different alteration processes and spinel alteration is evaluated in terms of accessory chromium spinel occurring in serpentinites of progressive metamorphism in the eastern Central Alps. Two coexisting Cr-spinels were identified, i.e., an altered, Fe-rich Cr-spinel (I) with Cr > Al, and an Fe-poor, Cr-spinel (II) with Al > Cr, which exsolved a Cr-Fe 3+ phase and which resisted alteration. Alteration of Cr-spinel (I) is evident from decreasing Al, Mg, and Cr values, with a trend leading to ferrite-chromite (trend a). An enrichment in Cr (trend b) related to CO 2 in the serpentinizing fluid, indicated by the presence of carbonate, talc or tremolite, is superimposed to (a). Alteration is a decomposition of the spinel structure caused by the dissolution of Mg, Al, and Cr during serpentinization. Dissolution of Cr is favored in a reduced environment whereas reprecipitation occurs under locally oxidizing conditions, indicated by the presence of CO 2. SiO 2 detected in spinel is a relative measure of the degree of alteration. Metamorphism up to amphibolite facies recrystallized and homogenized preexisting spinel phases to various degrees. Whether or not and to what extent a spinel alters depends on its Fe content. Fe 2+, stabilized in the octahedral site (high CFSE), is responsible for an inverse and less stable spinel structure. Fe-rich spinels (Cr-spinel I) are, therefore, readily subjected to alteration, whereas Fe-poor spinels (Cr-spinel II) are resistant to alteration during the serpentinization processes.

Neutron diffraction study of the structure of chalcogen spinels Cu 1+yCr 2X 4 (X Se, Te)

The structure of the copper excess chromium spinels Cu 2Cr 2Se 4 and Cu 1.81Cr 2Te 4 was determined by neutron diffraction at 300 K and at 4 K. The structure refinement reveals a simultaneous occupation of tetrahedral as well as of pseudo-octahedral sites by the excess y copper ions in Cu 1+yCr 2X 4. The relatively high temperature factors are discussed in terms of a specific local disorder model within the spinel space group Fd3m. From these results and the change in the lattice parameter α o with y a model was derived that is able to describe the sequence of occupation of tetrahedral and pseudo-octahedral lattice sites in the course of copper intercalation.

Formation of silver chromium spinels and structure determination of ferromagnetic AgCr 2Te 4

The silver chromium chalcogen spinel AgCr 2Te 4 has been prepared via ion exchange of the corresponding known copper chalcogen spinel CuCr 2Te 4 in aprotic Ag + electrolyte. Electrochemical intercalation of Ag + into AgCr 2Te 4 resulted in the formation of Ag 1+yCr 2Te 4 with 0≦ y≦1 , while ion exchange of Cu 2Cr 2Te 4 with Ag + electrolyte yielded directly the end member Ag 2Cr 2Te 4 of the non-stoichiometric system. The structure of AgCr 2Te 4 was determined from X-ray powder data. The ferromagnetic properties of these new phases are discussed in terms of the concentration of anion p-band holes.

Exchange interactions in the helimagnetic spinel solid solutions [formula omitted] (where 0.0≤ x≤0.5)

Magnetic phase transition from the magnetic simple spiral state (for x = 0.0) to the spin glass state (for x = 0.5) results from the high degree of frustration of exchange interactions in the chromium spinel sublattice. Using experimental values of: the Curie-Weiss paramagnetic temperature, the Néel temperature and the rotation angle of the spiral, the exchange parameters are estimated (using a mean-field theory) up to the third coordination sphere for the whole range of concentration.

Formation of Cu 2+3 clusters: Structure, bonding, and topotactic reactivity of chalcogen spinels Cu 1± yCr 2Se 3Br

The semiconducting chromium spinel CuCr 2Se 3Br is shown to be accessible to reversible electrochemical and chemical reduction as well as to oxidation in Cu electrolytes by topotactic electron/ion transfer reactions leading to nonstoichiometric phases Cu 1+ y Cr 2Se 3Br (0 ≤ y ≤ 0.50) and Cu 1− y CrSeBr (0 ≤ y ≤ 0.23). The oxidation proceeds under partial deintercalation of Cu and formation of valence band holes with a concurrent significant increase in the Curie temperatures T c, which demonstrates the strong correlation between magnetic ordering and band hole concentration. The final product obtained by cathodic reduction is the new metastable spinel Cu 1.5Cr 2Se 3Br. Neutron diffraction and electron spectroscopy studies reveal that copper intercalation leads to the formation of Cu 2+ 3 clusters with electron defect bonding. Changes in the electronic structure upon reduction and oxidation processes are discussed on grounds of band structure schemes.

Key areas for alluvial diamond and sapphire exploration in the New England gem fields, New South Wales, Australia

Key areas for diamond and sapphire exploration may be defined within the New England gem fields of eastern Australia through an understanding of the geologic and geomorphic processes that operated within the Tertiary Central volcanic province.At least two distinct periods of volcanic activity--one at 32 to 38 Ma and the other at 19 to 23 Ma--were controlled by south-southeast-north-northwest-trending fracture sets revealed by Landsat data. The locus of volcanism stepped westward with time from the Glen Innes-Ben Lomond-Guyra area (the East Central province) to the area west of Inverell and Armidale (the West Central province). Drainage and prevolcanic topography indicate a radial pattern of drainage associated with East Central province uplift and volcanism (32-38 Ma). This was modified by the later West Central province volcanism (19-23 Ma) which established its own radial pattern. The division of the Central province defines broad regions suitable for sapphire and diamond exploration.Highly prospective diamond-bearing deep leads associated with the paleo-Gwydir River can be traced under the younger basalts of the West Central province via reconstruction of the prevolcanic topography.In the East Central province the posteruptive fluvial history is seen as vital in controlling the concentration of economic sapphire deposits from lower grade source rocks. The sapphire-bearing placer deposits occur as shoestring-type accumulations occupying channels within broad, flat, basalt-filled valleys. Higher grades generally correspond to areas where channels are deepest and sapphire grades in excess of 500 g per m 3 have been recorded. Heavy minerals associated with sapphire in the placer deposits are zircon, pleonaste, chromium spinel, titanium magnetite, magnetite, ilmenite, and rare chrysoberyl. Additional minerals found in nearby in situ basaltic soils are olivine, clinopyroxene, enstatite, and amphibole, with zircon and sapphire being extremely rare.Investigation of heavy mineral suites from 12 collection sites indicates significant variability in mineral species within and between particular drainage catchments. Visual characteristics of the sapphires also vary within and between catchments. This implies that the sources are local to the placer deposits. The similarity in composition of minerals from soil and placer sites indicates that the sapphire-bearing placers have formed through extensive reworking of the immediately surrounding alkali basaltic and volcaniclastic rocks. Concentration was largely through vertical movement, with fine or light material being winnowed downstream leaving behind a heavy gem-bearing residue, within low-gradient, low-energy drainage systems.

Synthesis and physical properties of chalcogenide chromium spinel films

This work is a report of the present authors' experimental results on the synthesis and investigation of thin films of the chalcogenide chromium spinels CdCr2Se4, CuCr2Se4 and CuCr2Te4. Films were made by the method of vacuum condensation from separate sources. As substrates glass, mica, MgO, CaF2, Y2O3 and NaCl were used. With the last of these vacuum condensation results in the formation of a single-crystal film CdCr2Se4, the physical characteristics of which are, however, worse than those of polycrystalline films.The measured temperature dependences of magnetization, ferromagnetic resonance and electron paramagnetic resonance linewidths, electric resistance and magnetic conductivity are given. We have realized and investigated stripe domain structures in CdCr2Se4. Photoelectric, photomagnetic and magneto-optical properties are studied. The magnetic and electric characteristics of stoichiometric polycrystalline films are close to those of bulk materials.The effects of diffuse substitution with indium, gallium (donors) and silver, copper (acceptors) and of non-stoichiometry on the magnetic and electric properties of CdCr2Se4 semiconductor films are investigated.

Anion valence band holes in the copper chalcogen spinels CuCr 2X 4

X-ray photoelectron spectroscopy and polarized neutron diffraction studies have been used for an investigation of chemical bonding and ion valencies in the ferromagnetic metallic copper chromium spinels CuCr 2X 4. The results obtained demonstrate the presence of trivalent chromium, monovalent copper and one hole in the anion valence band per formula unit i.e. formally Cu +(Cr 3+) 2(S 2−) 3(S ·−). The significance of the ground state holes for the strength of the ferromagnetic coupling (as compared to the semiconducting chromium spinels with low Curie temperatures) is discussed.

The effect of alloy chemistry on creep behaviour in a helium environment with low oxygen partial pressure

The alloy NiCr22W12Fe (Thermon 4972) has mechanical and creep rupture properties similar to those of the alloy NiCr22Co12Mo (Inconel 617) when air tested. In atmospheres with low oxygen partial pressure, such as helium with reactive components, the high-temperature corrosion resistance of Thermon 4972 was improved over Inconel 617 due to the formation of a protective oxide layer consisting of a manganese chromium spinel, which preserved the strength. For Inconel 617 the oxide layers are not protective. Strong internal oxidation and decarburization occurred, leading to a weakening of the material with increasing creep rupture time.

The pulsed fields in studies of spinels

The pulsed field technique is discussed for studies of magnetization, magnetic phase transitions induced by strong fields, high-field susceptibility as well as magnetocrystalline anisotropy for three chosen chromium spinel systems.

Structure, ionic conductivity and phase transformation of double chloride spinels

New double chlorides, Li2MCl4 (M=Cr, Co, Fe) were prepared and were characterized by X-ray structure refinement, differential thermal analysis, high-temperature X-ray diffraction analysis and electrical conductivity measurement. The new phases have a distorted spinel structure; their orthorhombic and monoclinic distortions determined respectively for the cobalt and iron spinels, and for the chromium spinel were caused by a 1:1 ordering of lithium and metal divalent ions on the octahedral B sites. Conductivity measurements on both the distorted spinels Li2MCl4 (M=Cr, Co, Fe) and the cubic spinels Li2MCl4 (M=Mg, Ti, Mn, Fe, Cd) proved that the distortion causes a considerable degradation of lithium ionic conduction. The ionic conduction of the halide spinels is discussed in terms of dimensionality of conduction pathway, position of lithium ion, polarizability, cation vacancies, and bottleneck size.

Microstructure of chromium spinel and pyrite as indicators of thermomechanical history of chromitite from Kenting melange, Taiwan

Transmission electron microscopy was used to characterize the microstructure of chromium spinel and pyrite in the serpentinized chromitite occurrence of the Kenting melange at Hengchun, southern Taiwan. The deformation episodes of the Hengchun chromitite were then inferred from the defect microstructures on the basis of the known creep behaviour of analogue spinel materials and pyrite. Dislocations arranged as dipoles, tangles and subgrain boundaries were found in chromium spinel indicating that the chromium spinel was probably plastically deformed and polygonized later in the upper mantle. The dislocations arranged as dipoles, loops, tangles and subgrain boundaries in pyrite correspond probably to later episodes of the ocean floor metamorphism during which dynamic recrystallization of pyrite occurred, together with brittle deformation of chromium spinel. The breccias of chromium spinel and pyrite are in part due to brittle deformation during emplacement of the ophiolite at a convergent plate boundary.

Phase relationships in the quaternary systems MSCr 2S 3In 2S 3 (M=Mn, Fe, Co, Ni), miscibility gaps in spinel solid solutions

The phase diagrams of the quaternary systems MSCr 2S 3 with M=Mn, Fe, Ni were studied by X-ray powder photographs of quenched samples. In the quasibinary sections MnIn 2S 4MnCr 2Su 4 and FeIn 2S 4FeCr 2S 4, immiscibility domes exist below 800 and 850 °C, respectively. At higher temperatures complete series of spinel type solid solutions are formed, which can be quenched to ambient temperature without decomposition. In the NiIn 2S 4NiCr 2S 4 section, spinel type solid solutions are formed in the range from NiIn 2S 4 to NiCr 1.7In 0.3S 4 (800°C). At lower temperatures decomposition to chromium and indium rich spinel solid solutions occurs. The phase width of Cr 3S 4 type NiCr 2S 4 is very small. The system NiCr 2S 4In 2S 3 is similar to the NiCr 2S 4NiIn 2S 4 system, but probably not quasibinary. The FIR spectra of the NiCr 2−2x In 2xS 4 (and NiCr 2−2xGa 2xS 4) solid solutions reveal that the hypothetical spinel NiCr 2S 4 is probably metallic. In the sections CoSIn 2S 3 and NiSIn 2S 3, spinel type solid solutions are formed in the range from In 2S 3 to MIn 2S 4. The unusually small reaction rates of the solid state equilibration reactions are discussed in terms of the Gibbs 1 enthalpies of forming the spinel mixed crystals and the kinetic retardation of such reactions.

Effects of radiation disorder in chromium spinels

Polycrystalline MgCr2O4 and CuCr2O4 samples are investigated by X-ray and neutron diffraction methods before and after irradiation by fast neutrons (En ≧ 1 MeV) of fluence 2 × 1024 m−2. It is found that under irradiation in cubic magnesium and tetragonal copper chromites with the corresponding space groups O – Fd3m and D – I4/amd different types of cations are redistributed both, over “allowed” and “forbidden” sites of the initial spinel lattice. As a result, the lattice periods and crystal symmetries are changed. The structures of the irradiated chromites may be described in terms of the space group O – Fm3m. Under annealing the irradiated samples restore their initial crystal structures. Structural changes occurring on different stages of annealing are analysed by X-ray. Polykristalline MgCr2O4- and CuCr2O4-Proben werden mittels Rontgen- und Neutronen-Beugungs-methoden vor und nach Bestrahlung mit schnellen Neutronen (En ≧ 1 MeV) einer Dosis von 2 × 1024 m−2 untersucht. Es wird gefunden, das unter Bestrahlung in den kubischen Magnesium- und tetragonalen Kupfer-Chromiten mit den entsprechenden Raumgruppen O – Fd3m und D – I4/amd verschiedene Kationentypen sowohl uber „erlaubte” als auch „verbotene” Platze des ursprunglichen Spinelgitters umverteilt werden. Als Ergebnis andern sich die Gitterperioden und Kristallsymmetrien. Die Strukturen der betrachteten Chromite konnen mit der Raumgruppe O –Fm3m beschrieben werden. Bei Temperung kehren die bestrahlten Proben zu ihrer ursprunglichen Kristallstruktur zuruck. Strukturanderungen, die bei verschiedenen Stufen der Ausheilung auftreten, werden mittels Rontgenstrahlen analysiert.

Spin-glass like behaviour in a concentrated chromium spinel: Zn 0.5Cd 0.5Cr 2S 4

Low field d.c. and a.c. susceptibility measurements are reported for the non magnetically diluted spinel Zn 0.5Cd 0.5Cr 2S 4. A spin-glass like behaviour is observed at low temperature ( T F = 15.4 K at v = 198 Hz). This is the result of the presence of competing interactions (ferromagnetic between nearest-neighbours and antiferromagnetic between higher order neighbours) and of the disorder of their distribution due to the substitution between non-magnetic zinc and cadmium ions in the tetrahedral sites of the spinel lattice.