Octahedral Fe3 Research Articles

Origin of the Iron Bands in Supernovae Spectra

THE visible-region crystal-field spectra of Fe2+ and Fe3+ ions located in silicate minerals show several similar features, in particular a number of relatively sharp bands marking field-independent transitions, which are difficult to assign unless attendant chemical analyses are available. Three absorption bands at 6200 A, 4430 A and 3800 A in optical spectra of Type I supernova were recently assigned1,2 to transitions within octahedral Fe3+ ions in andradite garnet, while two other bands at 5100 A (19,600 cm−1) and 5700 A (17,500 cm−1) were attributed to Fe2+ ions in almandine garnet. Runciman3 also feels that the supernova bands are due mainly to Fe, but he suggests that they may mark transitions in Fe2+ ions in pyroxene minerals. Many stars have appreciable excess emission at 10 and 20 µm, the origin of which has been attributed to circumstellar silicates4–7, possibly orthopyroxenes and olivines.

Compositions of Garnets in Interstellar Dust

HUFFMAN1 has shown that absorption bands in the optical spectra of the supernova in NGC 4496 correlate well with d–d bands of Fe3+ in yttrium iron garnet and Fe2+ in terrestrial almandine, Fe2+3 Al3+2Si3O12. I have suggested2 that the 4430 A band in supernova spectra is a result of Fe3+ in a silicate matrix, the band being reminiscent of that marking the 6A1(S)→4A14E(G) transition in octahedral Fe3+ in andradite garnet, Ca2+3Fe3+2Si3O12. It is likely that interstellar garnets are silicate-based, and here I propose approximate formulae for these garnets based on the identification of two interstellar absorption bands, the energies of which can be related to garnet compositions.

Transport Properties of Iron-nickel Ferrites

The mechanism of electrical conduction in the magnetic semiconductors NixFe3-xO4 with 0.6 < x ≤ 1 was investigated. The electrical properties of these compounds are extremely sensitive to the presence of α-Fe2O3 as a second phase. The exponential temperature dependence of the electrical resistivity ρ and the temperature-independent thermoelectric power θ are in good agreement with an electron-hopping model. The electrical conduction occurs by thermally activated electron hopping between octahedral Fe2+ and Fe3+ ions with an activation energy q. The values of ρ, q and θ depend on the Fe2+ concentration only, and are not sensitive to small deviations from stoichiometry due to the presence of cation or anion vacancies.

Effect of next-nearest neighbour interaction on oscillator strengths in garnets

The oscillator strength of the octahedral Fe3+6A1(t3e2) to 4A1, 4E(t3e2) transition in the garnet andradite Ca3Fe23+Si3O12, is f=3.5*10-6. In schorlomite where 25% of the tetrahedral Si4+ ions are replaced by Al3+ or Ti4+ the oscillator strength is an order of magnitude greater. Substitution in the tetrahedral site destroys the inversion symmetry at the next-nearest neighbour octahedral site, and it is suggested that this primarily accounts for the observed increased absorption intensity for schorlomite.

Infrared Spectra of Cubic and Tetragonal Manganese Ferrites

AbstractThe infrared absorption spectra in the range from 200 to 700 cm−1 of cubic and tetragonal manganese ferrites, MnxFe3–xO4, are reported. An analysis of the spectra is made with a model by which the lattice vibrations are classified into species of the point group D2 d on the analogy of the results for gas molecules of equivalent symmetry. For cubic spinels with manganese concentrations x ≦ 1.0 the classification can be made according to the model of Waldron into the species of the point group Td. However, at 335 cm−1 an absorption has been observed which is out of the scope of the Td or D2 d classification, and which is caused by the substitution of a part of the octahedral Fe3+ ions by Mn2+ ions. The IR spectra of manganese ferrites with compositions 1 &lt; x ≦ 3 can be analysed with the D2 d classification. The spectra of these materials do not change suddenly with the manganese concentration or with the crystal structure. From the splitting of the absorption bands in cubic ferrites with x &gt; 1 it is concluded that there are local tetragonal distortions originating from the Jahn‐Teller effect of the octahedral Mn3+ ions.

Erratum: Magnetically Induced Electric Field Gradients in Octahedral Fe2+:RbFeF3

Erratum: Magnetically Induced Electric Field Gradients in Octahedral Fe2+:RbFeF3

The optical absorption spectra of certain transition metal ions in muscovite, lepidolite, and fuchsite

The unpolarized optical absorption spectra of the sheet silicate minerals muscovite, lepidolite, and fuchsite have been examined. The spectra, interpreted on the basis of ligand field theory, indicate that all three minerals contain Fe3+ and possibly Ti3+ in octahedral sites. In addition, muscovite is shown to contain octahedrally coordinated Fe2+, lepidolite octahedral Fe2+ and Mn2+, and fuchsite octahedral Cr3+. No evidence was found for transition metals in tetrahedral sites.Some comments are made on the similarity of the spectra of certain of the transition metal ions in several classes of minerals. This leads to the speculation that the cations Fe3+, Mn2+, and Cr3+ may tend to 'mold' their immediate surroundings.

Estimation of ligand-field parameters from Mössbauer spectra

Computer programmes are described for the calculation of the temperature-dependence of the Mossbauer quadrupole splitting of complexes containing octahedral and tetrahedral high-spin Fe2+ and octahedral low-spin FeIII. The derivation of ligand-field parameters from experimental data is critically considered, and contributions from the crystal lattice and covalency are also discussed. An iterative method for fitting trial models to experimental data is described and tested, and can be generally applied to the minimisation of any matrix solution.

Magnetic Anisotropy of Some Nickel Zinc Ferrite Crystals

Single crystals of Ni1−pZnpFe2O4 (p=0.20 and 0.65) were prepared from a melt in a PbO:B2O3 flux. The anisotropy constant K1 was deduced from torque measurements in a (100) plane. | K1 | increases linearly with p at low temperatures. Combining the results with data on p=0 and p=0.33 from the literature, one can represent K1 at 4°K as the sum of contributions from tetrahedral Fe3+ (+0.04 cm−1 per ion) and octahedral Fe3+ (−0.07 cm−1) in agreement with data on other iron spinels. The temperature, dependence of K1 for p=0.65 agrees with Wolf's expectations for ferrimagnets with a weak exchange. Above room temperature the torque curves for this sample show a spurious uniaxial term, increasing with temperature. Both K1 and this extra torque are reduced by increasing the external field.

Coercivity of Fe2+in octahedral sites of Fe-Ti spinels

High coercivities (∼1.8 to 8.1 kOe) have been observed in polycrystalline titanium substituted magnetites at 77°K. This property and unusually large ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\sim10^{7}</tex> ergs. cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> ) anisotropy constants observed by others in single crystals of the same compound are interpreted on the basis of a new model for cation distribution. Electrical resistivity and low temperature saturation magnetization measurements confirm this model, according to which new Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions occupy tetrahedral sites for most part of the solid solution series Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-2x</inf> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1+x</inf> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> +Ti <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-</sup> . This causes a negative trigonal field to be superimposed on octahedral Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions, hence a twofold degeneracy of the orbital ground state resulting in an unquenched orbital angular momentum and a large anisotropy. Mössbauer absorption spectra provide direct confirmation of the trigonal distortion in the shape of a large quadrupole splitting. For definite proof of the twofold degeneracy, however, such polycrystalline specimens with disordered octahedral cations and ultrabroad magnetic spectra are not useful. It is hoped that single crystal work would prove more definitive.

Magnetic and Crystallographic Studies of Substituted Gadolinium Iron Garnets

Magnetic data from measurements at moderate to high magnetic fields in the temperature range 1.4° to 300°K, lattice constants, and preparative data are reported for the systems {Gd3}[ScxFe2−x](Fe3)O12, {Gd2Y}[ScxFe2−x](Fe3)O12, {Gd3−xCax}[Fe2] (Fe3−xSix)O12, {Gd3−xCax} Fe5−xGexO12, {Gd3}Fe5−xAlxO12, {Gd3}[MgxFe2−x] (Fe3−xSix)O12, and for the three garnets {Gd2Ca}[ZrFe](Fe3)O12, {Gd0.6Y2.4}[ScFe](Fe3)O12, and {GdCa2}[Zr2](Fe3)O12. The following model for the general features of the 0°K magnetic structures of the substituted gadolinium iron garnets is proposed. At 0°K, gadolinium iron garnet itself is an ideal Néel ferrimagnet. The moments of the octahedral Fe3+ ions and of the dodecahedral Gd3+ ions are strictly antiparallel to the moments of the tetrahedral Fe3+ ions. Replacement of some of the octahedral Fe3+ ions by nonmagnetic ions such as Sc3+ ions as in {Gd3}[ScxFe2−x](Fe3)O12 causes random canting of the tetrahedral Fe3+ ions. Because the strong interactions of the Gd3+ ion moments are with those of the tetrahedral Fe3+ ion moments, this leads also to random canting of the Gd3+ ion moments thereby reducing the contribution of the dodecahedral sublattice to the net spontaneous moment of the garnet. Replacement of the tetrahedral Fe3+ by nonmagnetic ions as in {Gd3−xCax}[Fe2] (Fe3−xSix)O12 causes random canting of the octahedral Fe3+ ion moments, but this is not expected to affect the alignment of the Gd3+ ion moments. Such substitution appears to have the effect of removing from contribution to the net moment, those Gd3+ ions which are not linked through oxygen atoms to at least one tetrahedral Fe3+ ion. A comparison of the results on the systems {Gd3}[ScxFe2−x](Fe3)O12 and {Gd2Y}[ScxFe2−x](Fe3)O12 indicates that at or very near 0°K, the interaction in the dodecahedral sublattice appears to have a ferromagnetic component. Also the garnet {GdCa2}[Zr2](Fe3)O12 appears to be a weak ferromagnet.