Influence of inversion level on the optical absorption spectra of Ti-doped transparent MgGa2O4 ceramics

Ti-doped (0.08, 0.30, and 1.00 atomic% [at.%]) transparent MgGa₂O₄ ceramics (possessing a high inversion level; i up to 0.8) were fabricated by pulsed electric current sintering, at 950°C, under vacuum for 30–90 min. Optical transmission, emission, and electron paramagnetic resonance spectra were recorded. The maximal transmission level was ∼70% (820 nm), for a thickness of ∼1 mm, which, while not very high, permitted the observation of the optical absorption bands location and profile. Interpretation of the fluorescence spectra suggests that some Ti⁴⁺ cations (mostly hexacoordinated) were accommodated by the host despite the scarcity of oxygen in the atmosphere during the sintering process. The Ti³⁺ cations substitute native ions located in tetrahedral sites, distorting the original T_d symmetry toward a D_2d symmetry. Comparing the Ti-doped MgGa₂O₄ (high inversion) and MgAl₂O₄ (low inversion) spinels, spectral characteristics revealed that a significant increase in the inversion level drives Ti³⁺ cations from octahedral toward tetrahedral sites. This is reflected in the optical absorption spectra by the disappearance of the band at ∼20 000 cm⁻¹ (detectable in MgAl₂O₄) in MgGa₂O₄; the two d–d bands, of MgA₂O₄, in MgGa₂O₄ are reduced to a single one, located at 11 800 cm⁻¹. These results, for MgGa₂O₄, strongly support a similar assignment—of the strong band at 12 800 cm⁻¹, in Ti-doped MgAl₂O₄—to a tetracoordinated Ti³⁺. Thus, while in MgAl₂O₄, Ti³⁺ appears in both octahedral and tetrahedral coordination and in MgGa₂O₄ only the latter state is stable. In both spinels, Ti dopant speciates into Ti³⁺ and Ti⁴⁺ cations.     

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.     

Solid Solution Effects on the Thermal Properties in the MgAl2O4–MgGa2O4 System

Solid solution effects on thermal conductivity within the MgO–Al₂O₃–Ga₂O₃ system were studied. Samples with systematically varied additions of MgGa₂O₄–MgAl₂O₄ were prepared and the laser flash technique was used to determine thermal diffusivity at temperatures between 200°C and 1300°C. Heat capacity as a function of temperature from room temperature to 800°C was also determined using differential scanning calorimetry (DSC). Solid solution in the MgAl₂O₄–MgGa₂O₄ system decreases the thermal conductivity up to 1000°C. At 200°C thermal conductivity decreased 24% with a 5 mol% addition of MgGa₂O₄ to the system. At 1000°C, the thermal conductivity decreased 13% with a 5 mol% addition. Steady‐state calculations showed a 12.5% decrease in heat flux with 5 mol% MgGa₂O₄ considered across a 12 inch thickness.     

Sol-gel synthesis,characterization, and supercapacitor applications of MCo2O4 (M = Ni,Mn, Cu,Zn) cobaltite spinels

High performance MCo₂O₄spinels (M = Ni, Mn, Cu, Zn) were synthesized by the sol gel method (citrate) and their capacitive behavior was investigated in alkaline electrolyte. Their structural, morphological, functional groups and textural properties were characterized by TG/DSC, XRD, SEM, FTIR, EDS and BET. The capacitive properties of spinel MCo₂O₄ samples were thoroughly investigated in 1?M KOH aqueous electrolyte using cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The results revealed high stability of the samples and excellent electrochemical reversibility, and exhibited specific capacity depending on the nature of the transition metal ion M. A high specific capacitance of 285?F?g^?1 was measured for CuCo₂O₄ and a low capacitance of 158?F?g^?1 for ZnCo₂O₄.In addition, MCo₂O₄ spinels displayed good stability during long-term cycles with a cycling efficiency which exceeds75% after 1000 cycles. The obtained results classified MCo₂O₄ cobaltite spinels as most promising materials for their application in super capacitors.     

Revealing the effect of synergistic interaction between ZnO and ZnCr2O4 on the syngas aromatization

Zn-Cr-based catalysts are widely used as oxide catalysts for syngas aromatization, and it is difficult to study the synergistic effect of ZnO and Zn-Cr spinel due to the complex system of non-stoichiometric Zn-Cr spinels. In order to reveal the synergistic effect, we physically mixed ZnO and ZnCr₂O₄ with definite structure to avoid ambiguous structure of non-stoichiometric Zn-Cr spinels. The results showed that the introduced ZnO affected the oxygen vacancies generation and promoted the activation of CO and H₂, leading to an increase of oxygenates compared to the sole ZnCr₂O₄. Due to the synergy of ZnO and ZnCr₂O₄, the xZZC catalysts could produce more adsorbed species than the ZnCr₂O₄ catalyst, while the ZnCr₂O₄/ZSM-5 catalyst was more difficult to convert formate species. The ZnO in xZZC/ZSM-5 decreased the formate adsorption strength, which favored the continued conversion of formate and further realized the enhanced pulling effect on CO conversion.     

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.     

Effects of transition metal additives on redox stability and high-temperature electrical conductivity of (Fe,Mg)3O4 spinels

Magnetite-based spinels are considered as promising oxide materials to meet the requirements for ceramic consumable anodes in molten oxide pyroelectrolysis process, a breakthrough low-CO₂ steel technology aimed to overcome the environmental impact of classical extractive metallurgy. The present work focuses on the assessment of phase relationships, redox stability and electrical conductivity of Fe_2.6Me_0.2Mg_0.2O₄ (M = Ni, Cr, Al, Mn, Ti) spinel-type materials at 300–1773 K and p(O₂) from 10⁻⁵ to 0.21 atm. The oxidation state of substituting transition metal cation, affecting the fraction of Fe²⁺ in spinel lattice, was found to be a key factor, which determines the electronic transport and tolerance against oxidative decomposition, while the impact of preferred coordination of additives on these properties was less pronounced. At T > 650 K thermal expansion of Fe_2.6Me_0.2Mg_0.2O₄ ceramics exhibited complex behaviour, and, in highly oxidizing conditions, resulted in significant volume changes, unfavourable for high-temperature electrochemical applications.     

Thermodynamics and electronic structure characteristics of MFe2O4 with different spinel structures: A first-principles study

In recent years, spinel ferrites with chemical formula MFe₂O₄, have attracted much attention due to their impressive photocatalytic and electrocatalytic performances, which are significantly influenced by their spinel structures. However, it is still a big challenge to distinguish or predict spinel structures for spinel ferrites. As an attempt to address this issue, this paper presents a first-principles study of the thermodynamics and electronic structures for six spinel ferrites with different spinel structures. The configurational free energy of these spinel ferrites at different inversion degrees is calculated to determine the equilibrium inversion degree for each spinel, which successfully predicts the spinel structure type of these spinel ferrites. The partial density of states is obtained for six spinel ferrites assuming they are normal or inverse spinels. The electronic states close to the Fermi energy of each spinel ferrite are carefully examined, showing that normal spinels have weak interactions between M and Fe states, while strong interactions exist in mixed or inverse spinels. Our results offer an insightful understanding of different spinel structures, and provide a reliable approach to determine the spinel structure of spinel ferrites.     

Effect of Y2O3 content on densification,microstructure and mechanical properties of reaction sintered magnesium aluminate spinel

Dense magnesium aluminate (MgAl₂O₄) spinels were developed via single-stage solid-state reaction sintering method at 1550–1650^oC using combinations of varied commercial grade reactants-three different sources of alumina and two different sources of magnesia. The effect of Y₂O₃ doping in the concentration range of 1–4 wt % on different spinel batches was studied. Y₂O₃ addition was found to favour the densification of all the spinels at all dopant concentrations and maximum densification was found for the 2 wt % Y₂O₃ containing spinel batches. Phase analysis of the Y₂O₃ containing batches revealed the presence of yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) at all the sintering temperatures. Owing to similar crystal structure isotropic configuration of YAG (cubic) as that of spinel (cubic), Y₂O₃ doping was found to favour densification of spinel. Microstructural investigation revealed that Y₂O₃ containing batches have a controlled grain structure as compared to the without additive batches. Also, 2 wt % Y₂O₃ containing spinel batches sintered at 1650^oC revealed better mechanical properties such as cold modulus of rupture and strength retainment after thermal shock than that of the undoped spinel batches.     

Extremely high corrosion resistance of ZnxNi1–xCr2O4 spinel as sidewalls in the aluminum electrolyte

The sidewall material is a key component in new electrolytic cell with an inert electrode for the aluminum electrolysis industry. The continuous development of novel sidewall materials with excellent corrosion resistance in molten salts electrolyte is an important topic. Herein, a new system of sidewall material, spinel structured Zn_xNi_1–xCr₂O₄ (x = 0 – 1), is prepared by solid-phase reaction and the corrosion-resistance enhancement is investigated. The results prove that Zn²⁺ plays two roles in the Zn_xNi_1–xCr₂O₄ spinels. Firstly, Zn²⁺ tunes the surface energies of spinels resulting in the octahedral grains, which suppresses the cation diffusion in the corrosion process. Secondly, Zn²⁺ stabilizes the Cr³⁺ in the spinels. As a result, the Zn_0.5Ni_0.5Cr₂O₄ spinel displays an extremely low corrosion rate ～0.007 cm·a^–1 in NaF-KF-AlF₃ bath at 800 °C comparing with other sidewall materials. The as-obtained spinel shows great potential as a novel sidewall material for the new electrolytic cell.     

Friedel–Crafts type alkylation of benzene and other aromatic compounds using MgGa2O4 (spinel)–MgO based catalysts

The MgGa₂O₄ (spinel)–MgO catalyst (with Mg/Ga=2.0) requires a long induction period for exhibiting high catalytic activity in the benzylation of benzene, naphthalene, substituted benzenes and naphthalenes, depending upon the aromatic substrate. However, after its first use or pretreatment with gaseous HCl, the catalyst shows almost no induction period and also has very high benzylation activity, even in the presence of moisture in the reaction mixture. But its physico-chemical properties are changed drastically with a partial collapse of the spinel structure. The catalytically active species in this catalyst are expected to be mainly Ga₂O₃ and GaCl₃ dispersed on MgO.     

MCr2O4 (M=Co,Cu, and Zn) nanospinels for 2-propanol combustion: Correlation of structural properties with catalytic performance and stability

The correlation between structure and activity of MCr₂O₄ nanospinels (M=Co, Cu, and Zn) synthesized by a sol–gel combustion method was investigated for the oxidation of 2-propanol. The catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), N₂ adsorption/desorption, temperature programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Wide-angle XRD patterns show that the samples are pure spinel phases with cubic structure for CoCr₂O₄ and ZnCr₂O₄, and tetragonal structure for CuCr₂O₄. FTIR spectra confirmed the spinel structure of samples. The spinels were tested for total oxidation of 2-propanol as a model reaction for the catalytic combustion of oxygenated organic pollutants. ZnCr₂O₄ exhibited the highest activity and stability than the others toward the combustion of 2-propanol. The higher activity of ZnCr₂O₄ was ascribed to existence of excess surface oxygen on catalyst, active Cr³⁺–Cr⁶⁺ pair sites, and synergistic effect between ZnO and ZnCr₂O₄ confirmed by TPR and XPS techniques. The high stability of ZnCr₂O₄ and CuCr₂O₄ was explained by the existence of stable Cr⁶⁺ species on the surface of catalysts. The study showed that ZnCr₂O₄ could be used as a promising catalyst in the catalytic conversion of organic compounds.     

Theoretical study on composition-dependent properties of ZnO·nAl2O3 spinels. Part I: Optical and dielectric

Due to the transparency for both light and radio waves, Zn-Al spinels (ZnO·nAl₂O₃) may be applied as multi-mode windows. The rules of cation distribution in ZnO·nAl₂O₃ were determined with in-depth analysis of the local structure and then restricted by these rules, reasonable Zn_16–6xAl_32+4xO₆₄ (x = 0.0, 0.5, 1.0, 1.5, 2.0) supercells were constructed to explore the composition-dependence of properties via the first-principles calculations. Based on the insights into the crystal, bond, electronic, and phonon structures, the effect of the cation substitution on the light absorption in ultraviolet and infrared wavelengths, refractive index, and dispersion in the visible region as well as the dielectric constant and dielectric loss in the microwave band was disclosed. The d−p hybridization of Zn and O results in a lower refractive index and a higher Abbé number of ZnO·nAl₂O₃ at larger n. Since Al^IV ions are lighter and form stronger bonds with O²⁻ than Zn^IV ions, the ultraviolet and infrared cut-off of the transmission window are red-shifted and blue-shifted with rising n, respectively. Al^IV ions are insignificant contributors to the low-frequency vibration modes, therefore the dielectric loss weakens as n increases. The understandings are significant for not only customizing the composition of ZnO·nAl₂O₃ but also designing novel oxide spinels with desired optical and dielectric properties.     

Lithium-deficient LiYMn2O4 spinels (0.9 ≤ Y < 1): Lithium content, synthesis temperature, thermal behaviour and electrochemical properties

Lithium-deficient Li_YMn₂O₄ spinels (LD-Li_YMn₂O₄) with nominal composition (0.9 ≤ Y < 1) have been synthesized by melt impregnation from Mn₂O₃ and LiNO₃ at temperatures ranging from 700 °C to 850 °C. X-ray diffraction data show that LD-Li_YMn₂O₄ spinels are obtained as single phases in the range Y = 0.975-1 at 700 °C and 750 °C. Morphological characterization by transmission electron microscopy shows that the particle size of LD-Li_YMn₂O₄ spinels increases on decreasing the Li-content. The influence of the Li-content and the synthesis temperature on the thermal and electrochemical behaviours has been systematically studied. Thermal analysis studies indicate that the temperature of the first thermal effect in the differential thermal analysis (DTA)/thermogravimetric (TG) curves, T_C1, linearly increases on decreasing the Li-content. The electrochemical properties of LD-Li_YMn₂O₄ spinels, determined by galvanostatic cycling, notably change with the synthesis conditions. So, the first discharge capacity, Q_disch., at C rate increases on rising the Li-content and the synthesis temperature. The sample Li_0.975Mn₂O₄ synthesized at 700 °C has a Q_disch. = 123 mAh g⁻¹ and a capacity retention of 99.77% per cycle. This LD-Li_YMn₂O₄ sample had the best electrochemical characteristics of the series.     

Influence of the tetravalent cation on the high-voltage electrochemical activity of LiNi0.5M1.5O4 spinel cathode materials

The electrochemical properties of substituted LiNi_0.5Mn_1.5−xM_xO₄ spinels at high potential (>4 V vs Li⁺/Li) have been investigated for M = Ti and Ru, in order to determine the role of the tetravalent cation in such systems where nickel is a priori the only electroactive species. These systems are found to form extended solid solutions (up to x = 1.3 and x = 1.0 for Ti and Ru, respectively) that were characterized by X-ray diffraction and Raman spectroscopy. Titanium substitution induces a drastic decrease in high potential electrochemical capacity, whereas the capacity is maintained and the kinetics are even improved in the presence of ruthenium. These results are completed by new results on the Li_4−2xNi_3xTi_5−xO₁₂ spinel system, which shows not any high potential activity in spite of the presence of up to 0.5 Ni²⁺ per spinel formula unit on the octahedral site. Taking into account previous data on LiNi_0.5Ge_1.5O₄, we clearly show that even if the tetravalent cation does not participate in the overall redox reaction, electrochemical activity is only possible when nickel is surrounded by tetravalent cations able to accept a local variation of valence (Mn, Ru), whereas full-shell cations such as Ti⁴⁺ and Ge⁴⁺ block the necessary electron transfer pathways in the spinel oxide electrode.     

Spinels as cathodes for the electrochemical reduction of O2 and NO

Spinels were synthesised and investigated as electro-catalyst for the electrochemical reduction of oxygen and nitric oxide using cyclic voltammetry and cone shaped electrodes. The following four spinels were investigated; CoFe₂O₄, NiFe₂O₄, CuFe₂O₄ and Co₃O₄. The composition CuFe₂O₄ revealed the largest difference in activity between reduction of oxygen and the reduction of nitric oxide, the activity being highest for the reduction of nitric oxide. The material is probably not stable when polarised cathodically. However it seems that the electrode material can be regenerated upon oxidation. NiFe₂O₄ is also more active for the reduction of nitric oxide than for the reduction of oxygen, whereas the cobalt containing spinels have a higher activity for the reduction of oxygen than for the reduction of nitric oxide.     

Intrinsic correlation between structure,vibration spectrum and microwave dielectric properties of ferroelectric-dielectric composite ceramics

A novel series of (1-x)Ba_0.45Sr_0.55TiO₃-xMgGa₂O₄ (x = 0, 10, 30, 50, 70 wt%) ceramics was prepared by a solid-state method to investigate the relationship between their dielectric properties and ion diffusion, composition effect, and lattice vibration. XRD refinement and DFT calculations of Ba_0.45Sr_0.55TiO₃ (BST45) revealed that the substitution of Ga³⁺ and Mg²⁺, both of which have small polarizability for Ti⁴⁺, reached the saturation state at x = 10 wt%, thus decreasing the quality factor (Q value). In contrast, the addition of MgGa₂O₄ (MG) with x > 10 wt% significantly reduced the relative permittivity and improved the Q value owing to the compositional effect. The vibration spectra (Raman and FT-IR) confirmed that the Q value initially decreased owing to ion diffusion at x < 10 wt% and then increased with increasing MG content according to the composition effect. Therefore, the Q value was remarkably improved in the Ba_0.45Sr_0.55TiO₃-MgGa₂O₄ composites, with good tunability and low relative permittivity.     

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.