Spinel-type Crystals Research Articles

Mössbauer study and magnetic properties of MgFe2O4 crystallized from the glass system B2O3/K2O/P2O5/MgO/Fe2O3

An iron containing magnesium borate glass with the mol% composition 51.7 B2O3/9.3 K2O /1 P2O5/27.6MgO/10.4Fe2O3was prepared by the conventional melts quenching method followed by a thermal treatment process at temperatures in the range from 530 to 604°C.The thermally treated samples were characterized by X-ray diffraction, scanning and transmission electron microscopy. It was shown that superparamagnetic MgFe2O4 nanoparticles were formed during thermal treatment. The size of the spinel type crystals was in the range from 6 to 15nm. Mössbauer spectra of the powdered glass ceramic samples and the extracted nanoparticles after dissolving the glass matrix in diluted acid were recorded at room temperature. The deconvolution of the spectra revealed the crystallization of two spinel phases MgFe2O4 (as a dominant phase) and superparamagnetic maghemite, γ-Fe2O3 (as a secondary phase). Room temperature magnetic measurements showed that, increasing the crystallization temperature changed the superparamagnetic behavior of the samples to ferrimagnetic behavior. The Curie temperatures of the samples were measured and showed a higher value than that of the pure bulk MgFe2O4.

Chemical composition, crystal structure, and their relationships with the intrinsic properties of spinel-type crystals based on bond valences.

Spinel-type crystals may possess complex and versatile chemical composition and crystal structure, which leads to difficulty in constructing relationships among the chemical composition, crystal structure, and intrinsic properties. In this work, we develop new empirical methods based on bond valences to estimate the intrinsic properties, namely, compressibility and thermal expansion of complex spinel-type crystals. The composition-weighted average of bond force constants in tetrahedral and octahedral coordination polyhedra is derived as a function of the composition-weighted average of bond valences, which can be calculated according to the experimental chemical composition and crystal structural parameters. We discuss the coupled effects of tetrahedral and octahedral frameworks on the aforementioned intrinsic properties. The bulk modulus could be quantitatively calculated from the composition-weighted average of bond force constants in tetrahedral and octahedral coordination polyhedra. In contrast, a quantitative estimation of the thermal expansion coefficient could be obtained from the composition-weighted average of bond force constants in octahedral coordination polyhedra. These empirical methods have been validated by the results obtained for a new complex quaternary spinel-type oxynitride Mg0.268Al2.577O3.733N0.267 as well as MgAl2O4 and Al2.85O3.45N0.55 from the literature. Further, these empirical methods have the potential to be extensively applied in other types of complex crystals.

Spinel-type crystals based on LiFeSiO4 with high electrical conductivity for lithium ion battery formed by melt-quenching method

Spinel-type lithium-iron-silicate crystals were synthesized by using the melt-quenching method and the subsequent heat treatment for the sample with the composition of 25Li2O–25Fe2O3–50SiO2 (mol %), i.e., LiFeSiO4. It was found from X-ray diffraction analyses and transmission electron microscope observations that crystals formed in the heat treatment at 560°C for 3 h in Ar have the structure of Fd3m (A-227) with a lattice constant of a = 0.8317 nm and contain both Fe and Si elements. The heat-treated sample showed an electrical conductivity of 2.5 × 10−4 S cm−1 at room temperature and the activation energy is 0.21 eV, suggesting the presence of both Li+ ion conduction and hopping conduction between Fe2+ and Fe3+ ions. The charge–discharge performance of the heat-treated sample showed the total capacity (at 0.1C rate) of 95 mAh/g for 1.0–4.5 V cutoff ranges, demonstrating that a new spinel-type oxide synthesized works as cathode materials in LiB.

Radiative properties of α-ZnAl 2S 4:V spinel type single crystals

The absorption and luminescent properties of α-ZnAI 2S 4:V spinel type crystals in the temperature range 10–300 K are investigated. The spectra are assigned to the electronic transitions of trivalent vanadium ions located in octahedral sites. It is shown that at low temperatures the three main components of the revealed IR luminescence spectra are caused by the 1A 1g( 1G)→ 1E g( 1D), 1T 2g( 1D), 3T 2g( 3F)→ 3T 1g( 3F), and 1E g( 1D)→ 3T 1g( 3F) transitions. The observed dependencies of the emission components intensities on temperature are explained assuming that there is a phonon assisted tunnelling between 3T 2g( 3 F) and 1E g( 1D) states. On temperature rise, the 3T 2g( 3F)→ 3T 1g( 3F) vibronic transitions suppress other emission channels, which leads to the enhancement of the integral luminescence intensity and to the broadening of the spectrum centred at λ=1.4 μm.

Luminescent properties of cobalt-doped α-ZnAl 2S 4 spinel type single crystals

The absorption and luminescent properties of α-ZnAl 2S 4:Co spinel type crystals grown by chemical transport reactions method are investigated. The spectra are assigned to the electronic transitions of Co 2+ ions located in tetrahedral sites. It is shown, that the revealed four radiative spectral components are caused by the 2A( 2G)→ 4T 1( 4F), 4T 1( 4P)→ 4T 2( 4F), 2E( 2G)→ 4T 2( 4F) and 2E( 2G)→ 4T 1( 4F) transitions.

Magnetooptical activity of spinel type crystals with complex mixed-valence lattice irregularities

In this Letter we analyze the peculiarities of the Faraday rotation in the imperfect spinel type crystals for which the lattice irregularities are represented by the mixed valence clusters of transition metal ions. A such kind of complex system with the electronic configuration d 5 ( t 2 g 3 e g 2 ) − d 5 ( t 2 g 3 e g 2 ) − d 4 ( t 2 g 2 e g 2 ) (for example, Mn 2 + Mn 2 + Mn 3 + ) is considered in detail. The model takes into account the cubic and low symmetry crystal fields acting on the metal ions, migration of the “extra” hole as well as the orientational degeneracy (hidden optical anisotropy). The electric dipole transitions in these types of clusters with migrating hole coupled to the spin core through the double exchange mechanism are shown to lead to a significant magnetooptical activity in the visible range of spectrum. The intensity of these transitions can be comparable or higher than those for the magnetic dipole ones in the same range of frequencies. The electric dipole mechanism of the magnetooptical effects manifests itself in a highly anisotropic character of the spectra.

Spectroscopic characterization of Ti-dopedα-ZnAl2S4 spinel-type single crystals

The spectroscopic characteristics of theα-ZnAl2S4 wide bandgap semiconductor doped with Ti ions are investigated. It is shown, that theZnAl2S4:Ti spinel-type crystals exhibit luminescence in the IR spectral range 0.8–1.4 µm.The observed spectroscopic characteristics are assigned to the emission bands arising from the ligand-Ti4+ charge transfer for octahedral sites of titanium that is in agreement with the experimentalevidence for the absence of the EPR signal from Ti ions. A qualitative explanation of theexperimental data is given.

On the g factors of 2E state for 3d3ions at trigonal symmetry

In connection with local structure, the g factors of 2E state are studied by using complete diagonalization method （CDM）, with the spinel type crystals ZnAl2Ｏ4:Cr3＋, ZnGa2Ｏ4:Cr3＋ and MgAl2Ｏ4:Cr3＋ as the cases. The slight magnetic interactions, including spin-spin （SS）, spin-other-orbit （SOO） and orbit-orbit （OO） interactions are taken into account in our investigations. Theoretical explanations of g factors of 2E state for ZnAl2Ｏ4:Cr3＋ have been given. The results show that the sign of g‖（E′） for ZnGa2Ｏ4:Cr3＋ should be negative. And the value of g factors of 2E state for MgAl2Ｏ4:Cr3＋ have been predicted. It is shown that, the contributions to g factors of 2E state from slight magnetic interactions are appreciable. Compared with the g factors of ground state, the g factors of 2E state are sensitive to the local structure.

Jahn–Teller coupling in spinel-type crystals doped with transition metal ions

In this article, we report the theoretical evaluation of the vibronic constants for the spinel-type crystals ZnAl 2S 4, ZnAl 2Se 4, CdAl 2Se 4, and CdIn 2S 4 doped with the transition metal ions Cr 3+, Cr 4+, Ti 2+, and Ti 3+. In these crystals the ions Cr 3+, Ti 3+ and Cr 4+, Ti 2+ occupy positions with octahedral and tetrahedral surrounding correspondingly. In the case of a tetrahedral complex the interaction with full symmetric A 1, tetragonal E and two trigonal vibrations T 2 ( 1 ) and T 2 ( 2 ) is considered, while for the octahedral cluster along with the A 1 and E vibrations only one trigonal vibrational mode is taken into account. The examined semiconductor systems are mainly covalent and application of the crystal field point charge model loses proper exactity. For this reason, we employ the exchange charge model for the crystal field that accounts for the covalence effects and provides relatively simple expressions for the crystal field and vibronic parameters keeping at the same time a reasonable level of accuracy. The vibronic coupling constants are numerically calculated and for each case an appropriate multi-mode Jahn–Teller vibronic problem is found out.

Resource recovery of cupola dust: Study on sorptive property and mechanism for hydrogen sulfide

Properties and mechanism for sorption of hydrogen sulfide (H2S) by cupola dust, which is formed by the solidification of gas exhausted from cupola furnaces, have been investigated through the characterizations, aiming at recycling as the desulfurization. From the results, the sorptive property was found to be induced by spinel-type crystals such as (MnxZn1−x)(MnyFe1−y)2O4 solid solutions in spherical matrixes comprising of amorphous SiO2 at room temperature in existence of water vapor. The further examination on the spinel-type crystals extracted by NaOH treatments revealed that H2S was dissolved in water adsorbed on the surface of the spinel-type crystals, and reacted with metal ions released from them. In addition, the H2S sorptive property was indicated to depend on amounts of Mn and Zn in the spinel-type crystals.

Photoluminescence of the Cr 3+ ions in α-ZnAl 2S 4:Cr single crystals

Photoluminescence measurements have been performed on chromium doped α-ZnAl 2S 4 spinel-type crystals in the temperature range 2–540 K. a single configuration coordinate diagram for the Cr 3+ ions in the α-ZnAl 2S 4 host is constructed to interpret the absorption and emission spectra and to allow a determination of the energy gaps between the 2E g − 2T 1g and 2E g − 4T 2g states. A fit of the measured temperature dependence of the luminescence decay, using a simplified four level model gave the lifetimes of the 2 E g , 2 T 1 g and 4 T 2 g excited levels and confirmed the gap values obtained from steady state measurements.

Selective Oxide Films of a 5% Aluminum-Iron Alloy in a Low Oxygen Potential Atmosphere

Abstract Selective oxide films on a 5% Al-Fe alloy in the temperature range 700–900 C (1292–1652 F) for 1 hour in an atmosphere of hydrogen-water vapor mixture with a constant dewpoint as low as 20 C (68 F) were identical with the spinel type crystals, γ-Al2O3, containing small amounts of iron. Oxide films however, produced after heating at higher temperatures than 920 C (1688 F) in the same oxidizing atmosphere consisted only of corundum type crystals, α-Al2O3, having iron. It is probable that the iron ions contribute to the formation of the corundum type oxide at the temperature of the stable region of pure γ-Al2O3.

Identification of the Initial Oxide Films on 18-8 Stainless Steel in High Temperature Water

Abstract Initial oxide film formed on 18–8 stainless steel in high temperature water at 300 C (572 F) for 1 hour was identical with corundum type crystals like (Cr, Fe)2O3 alone or its coexistence with α-Fe2O3. On the contrary, the oxide film produced by prolonged oxidation (24 hours) consisted mainly of the spinel type crystals containing nickel.

Magnetic Field at the Nucleus in Spinel-Type Crystals

Measurements of the magnetic field at the ${\mathrm{Fe}}^{57}$ nucleus were obtained on powder samples at room temperature using the M\"ossbauer effect for the spinel-type ferrites $\ensuremath{\gamma}$-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$, "ordered" lithium ferrite ${(\mathrm{L}\mathrm{i}}_{0.5}$${\mathrm{Fe}}_{2.5}$${\mathrm{O}}_{4}$), and "disordered" lithium ferrite giving values of|496\ifmmode\pm\else\textpm\fi{}20|koe,|508\ifmmode\pm\else\textpm\fi{}20|koe, and|510\ifmmode\pm\else\textpm\fi{}20| koe. These compounds contain only trivalent and no divalent iron. The M\"ossbauer spectra of all of these compounds were very similar and no difference could be detected between the "ordered" and "disordered" compounds. Only one set of lines was observed indicating that the fields at the octahedral and tetrahedral sites are about the same value. The value of the hyperfine interaction constant $A$ obtained from electron paramagnetic resonance spectrum of the divalent ${\mathrm{Mn}}^{55}$ (isoelectronic with trivalent Fe) impurity in single crystals of the isomorphous spinel-type crystal "disordered" lithium aluminate (${\mathrm{Li}}_{0.5}$${\mathrm{Al}}_{2.5}$${\mathrm{O}}_{4}$) was found to be|77.2\ifmmode\pm\else\textpm\fi{}1.0| \ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The angular variation of the spectrum indicated that the divalent ${\mathrm{Mn}}^{55}$ ions were located substantially on octahedral sites. The corresponding magnetic field for the ${\mathrm{Mn}}^{55}$ nucleus is around 550 koe, which is close to the value obtained elsewhere for the ${\mathrm{Mn}}^{55}$ nucleus located on a tetrahedral site in a spinel-type aluminate. This is in good agreement with the M\"ossbauer results.