Inversion,chemical complexity,and interstitial transport in spinels

Spinels with the generic chemical formula AB₂O₄ have potential applications in nuclear energy and batteries. In both cases, their functionality is related to mass transport through the crystal. Here, using long-time atomistic simulations, we examine the impact of the cation structure on interstitial transport in two spinel chemistries, inverse MgGa₂O₄ and double MgAlGaO₄. We emphasize two aspects of the transport properties: the unit mechanisms that are described by individual barriers, for which we introduce pole-figure-like plots, and the aggregate behavior of those unit mechanisms. Compared to previous work on normal spinels, we find that inversion significantly reduces the rate of interstitial transport in these structures and has an impact on the stability of defects as they move through the lattice. In particular, B cation interstitials are found to be kinetically stable only in the inverse MgGa₂O₄. These results provide new insight into relationship between structure, chemistry, and transport in spinels.     

Comprehensive study of ferromagnetic MgNd2X4 (X = S,Se) spinels for spintronic and solar cells device applications

Full potential LAPW + lo method is used for exploring electronic, structural and thermoelectric properties for MgNd₂X₄ (X = S, Se) spinels that are found to show ferromagnetic-semiconductor behaviour in the spinel structure. The investigated negative value of formation energy and positive value of phonon spectra computed using PBEsol GGA indicates the energetic and dynamical stability of the studied cubic ferromagnetic-semiconductors. We have used TB-mBJ potential functional for electronic and magnetic properties, which lead to a reliable account of electronic structure, demonstrating band occupancy in the spinels along with a clear explanation of density of states. The stability of ferromagnetic state in the studied materials is because of the exchange splitting of Nd cations based on p-d hybridization which is in accordance with the results obtained for electronic band structure and density of states. The exchange splitting of bands can be justified by the spin magnetic moment between anions and cations, and sharing of charge. The computed values of dielectric constant and their associated optical parameters are used to explain the optical active behaviour of the spinels under investigation; indicating that the two spinels studied in the present work are suitable for solar cells device applications. The calculation of thermoelectric properties is very useful for determining a material's potential use in waste energy recovery systems and many other innovative applications.     

Role of Ti and Sn ions in spinel formation and reactive sintering of magnesia-rich ceramics

The solid solubility of magnesia in magnesium aluminate spinel and magnesium aluminate spinel in magnesia does not change with temperature thus not creating bonds or precipitation over periclase grains in a single stage sintering process. In comparison, the precipitated spinels in magnesia-chromia refractories form complex spinel due to inversion in the position of bivalent and trivalent cations within the structure, making them more stable at high temperature than either normal or inverse spinel. Additives form low-temperature compounds that diffuse into the spinel structure and create defects that change the properties of spinel solid solution. In the present study, magnesia and alumina powders along with tetravalent oxide additives were analyzed for their role in reactive densification of spinel in a single stage firing process in order to achieve a better binding system for magnesia-based refractories. These tetravalent oxides on reaction with magnesia form spinel solid solution with MgAl₂O₄ as they have similar crystal structure. The spinel solid solution formed using oxide additives is expected to have higher solubility in magnesia than magnesium aluminate spinel, resulting in improvement of the bonding during sintering through increased in solid solubility at elevated temperatures followed by precipitation of secondary spinel phases, similar to the complex spinel in magnesia-chrome refractories. The formation of spinel during firing remains as a second phase that retards the grain growth of periclase. The changes in unit cell dimensions with temperature and amount of additive were analyzed using Reitveld method and correlated with the densification behaviour at different temperatures.     

Dependence of the synthetic strategy on the thermochemical energy storage capability of CuxCo3-xO4 spinels

Cu_xCo_3-xO₄ spinels have been prepared through three different synthetic approaches to obtain oxide materials where the cationic distribution along the octahedral and tetrahedral sites is modified. The correlation of the spectroscopic characterization techniques and XRD patterns allowed to identify the relative concentration of the cationic species in the coordination environments. The aqueous routes favor the intimate contact of the cations and then, the migration of Co(III) to a tetrahedral coordination with Cu incorporation is promoted. This conversion to a partially inverse spinel structure leads to a decrease of the thermochemical capability of the prepared materials, in terms of energy balance but the O₂ release from the structure is favored with the increase in the number of cations occupying the tetrahedral sites. The endothermic reaction associated to the redox transition to CoO occurs in two differentiated steps which can be correlated with the degree of the inversion of the spinel structure.     

Influence of mixing time during glycine–nitrate process on the structural properties and reducibility of a dual-phase Ni–Cu–Mn spinel catalyst

The potential of Ni–Cu–Mn spinels as methane reforming catalysts for hydrocarbon-fueled solid oxide fuel cell (SOFC) applications is highly dependent on its catalytic properties, particularly reducibility. The reducibility of a spinel-structured catalyst is often correlated with its structural properties and fabrication processes. In this work, the structural properties and reducibility of a Ni–Cu–Mn spinel catalyst was evaluated on the basis of mixing time during the glycine–nitrate process. Phase analysis results showed that Ni_0.4Cu_0.6Mn_2.0O₄ and (Cu, Mn)₃O₄ in normal or inversed spinel structures were observed in GNP-produced Ni–Cu–Mn spinel catalyst powders. Distortion in inverse spinel structures enhanced the reducibility of the spinel catalyst. Morphological analysis results showed that complete nitrate binding occurred at a minimum mixing time of 24 h and resulted in homogenous particle size distribution and uniform elemental distribution. Furthermore, the Ni–Cu–Mn spinel catalyst produced after 24 h of mixing was fully reduced at 450 °C. The reducing pattern of the Ni–Cu–Mn spinel catalyst produced after 24 h of mixing time showed strong metal–support interaction and the fast adsorption of reactants. These effects were due to either the distribution of divalent cations in octahedral sites or large amounts of bulk pores. In conclusion, a minimum mixing time of 24 h is sufficient to produce the desired structural properties and reducibility of Ni–Cu–Mn spinel catalysts for SOFC applications.     

The role of the modifying chromium ion in the structure of magnesian spinel

The factors determining the principles of filling the polyhedrons in spinels with cation components are considered. An attempt is made to explain the variation in the color range of magnesian spinel modified with chromium ions. A hypothesis for two coordination states of chromium (four and six coordination) in the structure of magnesian spinel is put forward and justified.     

Influence of gamma nonstoichiometry on spinel inversion

The nonequivalence between the constituent oxides of spinels, or gamma nonstoichiometry, arising at high temperatures is examined. It is shown with reference to aluminum-magnesium, aluminum-nickel, and aluminum-zinc spinels that this process is accompanied by a redistribution of the positions occupied by cations in the lattice and partial inversion of the spinel, where the degree of inversion is dependent on the gamma-nonstoichiometry and temperature. Models are proposed for disordering and ordering in the spinel lattice with allowance for the specific arrangement of the spinel crystal. Translated from Steklo i Keramika, No. 8, pp. 12–17, August, 1997.     

Sol-gel synthesis of substoichiometric cobalt ferrite (CoFe2O4) spinels: Influence of additives on their stoichiometry and magnetic properties

Spinel cobalt ferrites (CoFe₂O₄) with varying levels of substoichiometry were prepared via sol-gel synthesis with different combinations of citric acid, dextrose and PVP (polyvinylpyrrolidone). The gels, prepared from the metal nitrates, were dried at 110?°C, further treated at 850?°C and finally subjected to thorough structural and magnetic characterization in order to correlate the composition of the gel to the structural and magnetic properties displayed by the ferrites. The materials synthesized in the present work have shown to be rather iron-depleted, reaching over 50% deficiency of the metal, which leaves the spinel lattice and forms Fe₂O₃ instead. The fuel/oxidant ratio and the thermal behavior of the xerogels exert a direct influence on the compositional variation of the prepared spinels, which could in turn be correlated to the magnetic properties displayed by the particles. A maximum in coercivity of 2154.4?Oe was achieved without the application of any additive to the nitrate precursors. On the other hand, the magnetic remanence displayed by the ferrites shows a linear relationship to the iron content in their chemical formula. The results open up the possibility of fine-tuning the structural and magnetic properties displayed by the spinel product via careful control of the composition of the reaction medium.     

Preparation and characterization of novel nonstoichiometric magnesium aluminate spinels

Magnesium aluminate spinel is of great importance for nuclear industry, and its structure, showing a great impact on properties, is sensitive to the composition. In order to explore the stoichiometric effect on structure and properties of spinels, several different spinel compositions with MgO·nAl₂O₃ (n?=?0.5–2.4) were synthesized via solid state reaction. Synthetic samples were characterized by X-ray diffraction, scanning electron microscope and nanoindentation tests. The results of XRD and SEM indicate that the single-phase magnesia alumina spinels have been prepared successfully for the first time ranging from n?=?0.667 to n?=?1.5, which is beyond the previous reported ranges of n$\ge$ 0.91. The hardness and modulus decrease with increasing n, implying further that the nonstoichiometric spinel crystal structures are likely to exhibit superior mechanical properties.     

Properties of spinels formed in highly fired periclase — Spinel refractories

Conclusions In high-temperature firing of periclase-spinel refractories one of the principal aims of a rational technology is achieved: the formation of spinels of magnochromite composition, with the crystal structure of a normal spinel and a high melting point.In such products the modified chrome spinel relates to magnochromite in composition whilst having the structure of normal spinel. The melting point of the modified chrome spinel exceeds 2100°C. The secondary spinels correspond in composition, or are close to, magnochromite. They have the structure of the mixed spinel with a very low degree of rotation, and a melting point of 2050–2070°C.Translated from Ogneupory, No. 8, pp. 41–44, August, 1972.     

Effect of the thermal treatment on the magnetic and structural properties of cobalt ferrite particles

We herein report a study on the sol-gel synthesis of CoFe₂O₄ and the effect of thermal treatment on the product outcome. Xerogels treated at 750, 800 and 850 °C had their structural and magnetic properties thoroughly studied, in order to correlate their synthesis conditions to the positions in which the cations are inserted in the spinel structure. X-ray diffractograms exhibit reflections representative of the spinel structure and demonstrate that the thermal treatment does not affect the lattice parameters of the material. Mossbauer spectroscopy studies indicate a very low inversion degree in the synthesized spinels, which is very unusual for CoFe₂O₄. A maximum in coercivity of 1405.2 Oersted was achieved for the sample treated at 800 °C.     

The effect of the inner particle structure on the electronic structure of the nano-crystalline Li-Ti-O spinels

The effect of the inner particle structure on Li insertion activity and electronic structure of the nano-crystalline Li-Ti-O spinels was studied on materials prepared by solid state and solvothermal synthesis. The high temperature prepared materials of composition corresponding to Li₄Ti₅O₁₂ feature particles with characteristic size of ca. 200 nm with randomly distributed defects. The products of solvothermal synthesis with composition Li_1.1Ti_1.9O_4+δ, feature cubic particles of characteristic dimension of ca. 50 nm; the characteristic particle size differs from that of the coherent domain determined by X-ray diffraction. The reduction of the solvothermal and high temperature synthesized nano-crystalline spinels in Li containing solutions leads according to ⁶Li MAS NMR spectra to Li insertion into tetrahedral 8b and octahedral 16c position, respectively. Additional broad NMR signal attributable to a Knight shift was observed in spectra of partially reduced high temperature spinels. In the case of solvothermal spinels is the Knight shift signal less pronounced and appears only in spectra of samples in which the phase transition occurs on the local level. The UV-vis-NIR spectra of the partially reduced Li-Ti-O spinel samples correspond to expected semiconductor character of Li-Ti-O spinels. Both types of materials are characterized by band gap of 3.8 eV (high temperature spinel) and 3.5 eV (solvothermal material). Partial reduction accompanied with Li insertion causes additional optical transition in the visible to near infrared region, which can be attributed to formation of trivalent Ti, character of which changes with degree of reduction. The behavior observed for partially reduced high temperature spinels is similar to that reported for TiO₂ (anatase). The spectral behavior of the partially reduced solvothermal spinels is more complex and reflects suppressed phase transition.     

Scanning tunneling microscopy and spectroscopy of electron-irradiated thin films of C60 molecules

We have used scanning tunneling microscopy and spectroscopy to determine the effect of electron irradiation on thin films of C₆₀ molecules. Upon the irradiation of electrons with an energy of 100 eV or 2 keV, clusters of molecules are created in the C₆₀ films as the initial products. The molecules in the clusters exhibit different electronic structures from that of pristine C₆₀ molecules; the molecule at the center of the cluster exhibits a metallic or semimetallic nature and the other molecules exhibit an additional density of states in the filled states. C₆₀ thin films are covered with clusters upon continuous electron irradiation for a longer duration, changing the electronic structure of the films. We consider that the electron-induced polymerization between C₆₀ molecules is responsible for the observed alteration of the C₆₀ thin films.     

Equilibrium Order-Disorder in Spinels

The distribution of cations in different coordinations in the spinel structure as a function of equilibrium temperature was quantitatively determined for several phases. The spinels were synthesized and/or reacted at various temperatures and pressures. The relative X-ray intensities of various reflections of the quenched phases were measured and compared with intensities calculated for various models of cation distribution. In NiAl₂O₄ the cation arrangements at 600° and 1550°C are (Al_1.0)^IV(Ni_1.0 Al_1.0)^VIO₄ and (Ni_0.25Al_0.75)^IV(Ni_0.75Al_1.25)^VIO₄, respectively. In Ni₂GeO₄, between 610° and 1400°C, the concentration of Ni²⁺ ions in tetrahedral sites increases from 0 to 23% and in octahedral sites it decreases from 100 to 77% of total nickel present. All these transitions were reversible under equilibrium conditions and can be classified as second-order “reconstructive transition of disorder.” Many spinels cannot be classified as normal or inverse but have a cation distribution which is an equilibrium function of the temperature.     

The effect of particle size and morphology on the rate capability of 4.7 V LiMn1.5+δNi0.5−δO4 spinel lithium-ion battery cathodes

The electrochemical impact of lithium-ion diffusivity on the discharge rate capabilities of cation ordered (P4₃32) and disordered (Fd3m) LiMn_1.5+δNi_0.5−δO₄ spinels were studied. Potentiostatic Intermittent Titration (PITT) measurements revealed up to 2 orders-of-magnitude lower lithium diffusion coefficient for the ordered spinel polymorph. The optimum structure of the high voltage spinel is resolved with respect to these ionic studies and our previous electronic transport studies. Modification of the morphology and pore dimensions was accomplished through the tuning of the ethylene glycol used for the synthesis of nanostructured spinel in the modified Pechini process. Glycol will be shown to play a major role on the energy and power density of LiMn_1.5+δNi_0.5−δO₄ electrodes.     

Ammonia gas sensing and magnetic permeability of enhanced surface area and high porosity lanthanum substituted Co–Zn nano ferrites

This work reports magnetic permeability and ammonia gas sensing characteristics of La³⁺ substituted Co–Zn nano ferrites possessing chemical formula Co_0.7Zn_0.3La_xFe_2-2xO₄ (x = 0–0.1) synthesized by a sol-gel route. Refinement of X-ray diffraction (XRD) patterns of the ferrite powders by the Rietveld technique has revealed the creation of single-phase spinel structure. The tenancy of constituent cations at tetrahedral/octahedral sites was obtained from the refinement of XRD. The crystallite sizes calculated from the W–H method vary from 20 to 24 nm. The scanning electron microscope (SEM) profiles of the ferrite samples were analyzed for the morphological details. The energy dispersive X-ray analysis (EDAX) patterns of the samples were obtained to test the elemental purity of the ferrites within their stoichiometry. The transmission electron microscope (TEM) image of the ferrite (x = 0.1) exhibits the spherical and oval shaped particles with a mean size of 20 nm. Fourier transform infra-red (FTIR) spectra were analyzed to confirm the superseding of La³⁺ cations at octahedral sites. The Brunauer-Emmett-Teller (BET) analysis of nitrogen adsorption-desorption isotherms of the ferrites was performed to investigate the porous structure and to determine the surface area of the nanocrystalline ferrites. The oxidation states of the constituent ions were confirmed by means of X-ray photoelectron spectroscopy (XPS). The complex permeability as a function of frequency was studied to explore the effects of structural parameters on the magnetic behaviour of the ferrites. Analysis of gas sensing properties of the ferrites have proved that the Co–Zn–La ferrite with controlled La composition can be utilized as an effective ammonia gas sensing material in commercial gas sensors.     

Structural,magnetic and magnetostrictive properties of Co1-xLixFe2O4 synthesized by cathode materials of spent Li-ion batteries

In this study, we investigated the effects of substituting Li⁺ for Co²⁺ at the B sites of the spinel lattice on the structural, magnetic and magnetostrictive properties of cobalt ferrites. The Li⁺ substituted cobalt ferrites, Co_1-xLi_xFe₂O₄, with x varying from 0 to 0.7 in 0.1 increments, were synthesized with a sol-gel auto-combustion method using the cathode materials of spent Li-ion batteries. X-ray diffraction analysis revealed that all the Co_1-xLi_xFe₂O₄ nanopowders had a single-phase spinel structure and the lattice parameters decreased with increasing Li⁺ content, which can be proved by slight shifts towards higher diffraction angle values of the (311) peak. Field emission scanning electron microscopy was used to observe the fractured inner surface of the sintered cylindrical rods and the increased porosity resulted in a decreased magnetostriction. The oxidation states of Co and Fe in the cobalt ferrite samples were examined by X-ray photoelectron spectroscopy. High resolution transmission electron microscopy micrographs showed that most particles were roughly spherical and with sizes of 25–35?nm. Li⁺ substitution had a strong effect on the saturation magnetization and coercivity, which were characterized with a vibrating sample magnetometer. The Curie temperature was reduced due to the decrease in magnetic cations and the weakening of the exchange interactions. The magnetostrictive properties were influenced by the incorporation of Li⁺ at the B sites of the spinel structure and correlated with the changes in porosity, magnetocrystalline anisotropy and the cation distribution.     

Magnetic CoFe2O4 nanoparticles doped with metal ions: A review

Ceramic-magnetic nanoparticles (CMNPs) are attracting attention due to their various applications, especially in biomedical industries. Among them, spinel ferrite CMNPs have received considerable deliberations among different spinel metal oxides due to their fascinating characteristics. Spinel ferrite CMNPs are used for enhancement of the applicability of CMNPs without affecting the intrinsic advantages of iron oxide CMNPs. Spinel ferrites with doping agents have useful electrical and magnetic properties in various fields. Moreover, the replacement of metallic atoms in ferrites is promising to manipulate physical characteristics and improve their performance. Among different spinel ferrites, CoFe₂O₄ nanoparticles are the most investigated CMNPs. Furthermore, they are used as permanent magnets, magnetic recorders in high-density and micro-wave devices, and magnetic fluids. This study reviews the CoFe₂O₄ nanoparticles doped with various elements and their applications in various fields.     

Microstructural effects on the sliding wear of transparent magnesium-aluminate spinel

Grain size effects have been investigated in the lubricated sliding wear of three transparent magnesium aluminate (MgAl₂O₄) spinel materials with different grains sizes identified as: Nano, Fine, and Coarse. Only Fine spinel shows classical wear behavior, which is characterized by initial mild wear followed by a sharp transition to severe fracture-controlled wear. Worn surfaces of Fine spinel show extensive grain pullout, consistent with intergranular mode of fracture found in that spinel. Nano and Coarse spinels both show gradual transition from mild wear to severe wear, and both have significantly lower overall wear rates compared to Fine spinel. Worn surfaces in both Nano and Coarse spinels show transgranular fracture and material removal, which is reminiscent of lateral-crack induced chipping. The transgranular fracture mode in Nano spinel can be attributed to stronger grain boundaries in that spinel, which could be due to the Y₂O₃ sintering additive used for grain refinement. Whilst the large scale of the grains in Coarse spinel could be responsible for the transgranular fracture observed in that spinel.