Role of Fuel on Cation Disorder in Magnesium Aluminate (MgAl2O4) Spinel Prepared by Combustion Synthesis

Magnesium aluminate spinel (MgAl₂O₄) forms an interesting system having tetrahedral and octahedral voids filled with near similar sized divalent Mg²⁺ and trivalent Al³⁺ cations. Structural disorder (e.g., Mg–Al antisite defects) can be tuned by synthetic conditions. This study reports the evolution of Mg/Al disorder in MgAl₂O₄ prepared by combustion synthesis using different types of fuels. The effect of nature of fuel and the final calcination temperature (600°C–900°C for 9 h) on degree of cation ordering has been investigated combining powder X‐ray (XRD) and neutron (NPD) diffraction. The results indicate very high degree of inversion in the samples crystallized at low annealing temperature, which on further annealing reduces toward the thermodynamically stable values. Raman spectroscopy, probing MgO₄, and AlO₄ tetrahedral bonds, confirmed the results at a local level.     

Composition and properties of chrome spinels separated by acids from kimpersai rocks

Conclusions We investigated compositions, properties and structure of elementary nuclei for Kimpersai chrome spinels, separated by acids from rocks of 6 new sources in Kazakhstan.The chemical composition of chrome spinels is relatively constant and close to the stoichiometric, with R₂O₃: RO changing from 0.9 to 1.2.According to the classification given in [13]they are related to magnesia chromites (Mg, Fe) Cr₂O₄ The properties of chrome spinels change with their composition almost linearly. The melting temperature of the chrome spinel is 2050–2060°, that is, they are highly refractory materials and valuable raw materials for the production of new types of refractories.The x ray defraction method established that in the Kimpersai chrome spinels the ordinary crystalline structure is predominant. The bivalent cations of magnesium and iron are located in the tetrahedral form and the trivalent cations of chromium, aluminum and iron in the octahedral. In the elementary nucleus of such chrome-spinels the cations are distributed in the following manner: 5–6 ions Mg²⁺ and 2–3 ions Fe²⁺ are located in the tetrahedral, and 12–13 ions Cr³⁺, 2–3 ions Al³⁺, and 1 ion Fe³⁺ in the octahedral. The oxygen parameter for Kimpersai chrome spinels is somewhat greater than the value for the ideal spinel structure, which can be explained by the expansion of the tetrahedral gaps and the compression of the octahedral.Translated from Ogeupory, No.8, pp. 29–36, August, 1966.     

Influence of Ln3+ and B3+ Ions Co‐Substitution on Thermophysical Properties of LnMB11O19‐type Magnetoplumbite LaMgAl11O19 for Advanced Thermal Barrier Coatings

Lanthanum hexaaluminate is a promising competitor to establish yttria partially stabilized zirconia as a thermal barrier coating material for Ni‐based superalloy due to its relative low intrinsic thermal conductivity and low sinterability at temperatures exceeding 1100°C. Sr²⁺ and Ti⁴⁺ were selected as two dopants to partially substitute the La³⁺ and Al³⁺ in LaMgAl₁₁O₁₉, respectively. The variation in thermal conductivity with Sr²⁺ and Ti⁴⁺ fractions was analyzed based on structure information provided by X‐ray diffraction and Raman spectroscopy. The average crystal size of LaMgAl₁₁O₁₉ sintered at 1600°C for 10 min by spark plasma sintering is in nanoscale. The fully dense La_1?xSr_xMgAl_11?xTi_xO₁₉ solid solution showed a minimum thermal conductivity value (λ = 1.12 W/(m K)^?1,T = 1273 K) at the composition of La_0.5Sr_0.5MgAl_10.5Ti_0.5O₁₉,which possibly reduces from the enhanced phonon scattering due to mass and strain fluctuations at the Ln³⁺ and B³⁺ sites.     

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.     

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.     

Sintering behavior and microstructure of (1-x)MgAl2O4-xMg2TiO4 spinel solid solutions prepared by isostructural heterogeneous nucleation

Magnesium aluminate (MgAl₂O₄) spinel has grasped considerable attention in high-temperature application by right of its excellent properties. However, the poor sintering behavior of MgAl₂O₄ is detrimental to its further development. In the present work, the application of isostructural heterogeneous nucleation method provides a novel idea for optimizing the sintering behavior of refractory materials. A series of (1-x)MgAl₂O₄-xMg₂TiO₄ (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) spinel solid solutions with a present ration of components were fabricated from light calcined magnesia, reactive alumina and pre-preparation Mg₂TiO₄. The effect of Mg₂TiO₄ heterogeneous nucleating agent on the crystalline phase, densification, and microstructure evolution of MgAl₂O₄–Mg₂TiO₄ spinel solid solutions was studied. The XRD, XPS, and EDS results showed that Mg₂TiO₄ entered the lattice of MgAl₂O₄ to form a spinel solid solution, and the heterovalent substitution process was identified, where Ti⁴⁺ and Mg²⁺ ions of larger radius in the Mg₂TiO₄ replaced the Al³⁺ of smaller radius in the MgAl₂O₄. For the sample at x = 0.08, the spinel solid solutions exhibited the optimized densification with a relative density of 93.3%, an apparent porosity of 1.2%, and a compressive strength of 84.5 MPa. A significant increase in densification was related to the lattice distortion induced by ion size mismatch during the heterovalent substitution, thus accelerating the diffusion rate of Mg²⁺ and Al³⁺ ions in the spinelisation state. Moreover, the solid solubility content of Ti⁴⁺ in the MgAl₂O₄–Mg₂TiO₄ spinel solid solutions had a significant effect on the grain morphologies. The Mg₂TiO₄ heterogeneous nucleating agent significantly increased the spinelisation rate of MgAl₂O₄ spinel with negligible effect on densification.     

Preparation and properties of MgAl2O4 spinel ceramics by double-doped CeO2 and La2O3

Magnesium aluminate spinel (MgAl₂O₄) ceramics are high-performance and carbon-free materials widely used in both military and civilian fields. However, it is usually challenging to densify during the solid-state sintering process. The excellent properties of some rare earth oxides have been proved to promote the densification of MgAl₂O₄ spinel ceramics. But the mechanism of promoting sintering is not clear. In the present work, MgAl₂O₄ spinel ceramics have been successfully fabricated by co-doping CeO₂ and La₂O₃ via a single-stage solid-state reaction sintering. The effects of addition amounts of CeO₂ and La₂O₃ on phase compositions, microstructures, sintering characteristics, cold compressive strength, and thermal shock resistance of as-prepared MgAl₂O₄ spinel ceramics were systematically investigated. The results show that by co-doping CeO₂ and La₂O₃ can increase the defect concentration due to the lattice distortion. This could promote the movement of Al³⁺ and Mg²⁺ at high temperature, which is beneficial to the formation of more secondary MgAl₂O₄ spinel. t-ZrO₂ with more Ce⁴⁺ filling between spinel grains could prevent the growth of grains and promote the densification, besides the new-formed LaAlO₃ that was mainly distributed along the grain boundary of the MgAl₂O₄ phase, both of which were favorable for the formation of dense microstructure of MgAl₂O₄ spinel materials. At the same time, the formation of more secondary MgAl₂O₄ spinel and sintering densification also improve the mechanical properties of spinel ceramics. La³⁺ will segregate to the spinel grain boundary, preventing grain boundary movement and absorbing the main crack's fracture energy. With 3 wt% CeO₂ and 3 wt% La₂O₃ co-doping, the bulk density of the sample increased from 3.02 g∙cm⁻³ to 3.55 g∙cm⁻³; the apparent porosity decreased from 12.21% to 9.97%; the cold compressive strength increased from 172.88 MPa to 189.54 MPa; and the residual strength retention ratio after thermal shock increased from 84.92% to 89.15%.     

Co2+:MgAl2O4 saturable absorber transparent ceramics fabricated by high-pressure spark plasma sintering

Single-stage processing of high-quality transparent functional polycrystalline ceramics is desirable but challenging. In the present work, spark plasma sintering (SPS) was employed for fabrication of Co²⁺:MgAl₂O₄ saturable absorbers for laser passive Q-switching. Densification of commercial MgAl₂O₄ powders, doped via co-precipitation, was carried out by conventional SPS and high-pressure SPS (HPSPS) under pressures of 60 and 400 MPa, respectively. The presence of LiF, a common sintering additive, was detrimental to optical properties as it promoted reaction of cobalt with sulfur impurities and the formation of Co₉S₈ inclusions. Densification by HPSPS without LiF allowed to obtain highly transparent Co²⁺:MgAl₂O₄. The optical properties of samples, with doping concentrations varying between 0.01 and 0.1 at.% Co²⁺, were assessed and saturable absorption was demonstrated at ~1.5 µm wavelength, exhibiting ground-state (σ_gs) and excited (σ_es) cross-sections of 3.5×10^-19 and 0.8×10^-19 cm², respectively. Thus, it was established that HPSPS is an effective method to fabricate transparent Co²⁺:MgAl₂O₄ ceramics.     

Preparation and optical properties of highly transparent MgAl1.9Ga0.1O4 ceramics via aqueous gel-casting method

The aqueous gel-casting technology has been widely used to prepare high-quality green body for various transparent ceramics with large dimension and complex shape. However, owing to the severe hydrolysis of MgAl₂O₄ powder, it is challenging to obtain thick aqueous slurry with high homogeneity and flowability. In this paper, the surface chemical state of MgAl₂O₄ powder was modified by introducing Ga³⁺, and stable MgAl_1.9Ga_0.1O₄ aqueous slurry with high solid-phase loading (52 vol. %) and low viscosity (136 mPa·s, at a shear rate of 50 s^-1) was successfully prepared. After pressureless presintering and hot isostatic pressing, the gel-cast sample exhibited much higher optical transmittance and more homogeneous microstructure than the dry-pressed sample, which is mainly derived from the improved homogeneity and densification of the green bodies and ceramics. The optical band gap, infrared cutoff wavelength, static refractive index and dispersion of both MgAl_1.9Ga_0.1O₄ and MgAl₂O₄ transparent ceramics were systematically compared. It is indicated that the transparent MgAl_1.9Ga_0.1O₄ ceramic has the increscent static refractive index of 1.695, the decrescent direct band gap energy of 6.15 eV and absorption coefficient of 0.49 cm^-1 at 5 µm, which could be ascribable to the fact that Ga³⁺ has different electronic structure, higher electronic polarizability and larger ionic radius in comparison with Al³⁺. This work provides a dependable solution for preparation of spinel oxide ceramics with superior optical properties and large dimension.     

Fabrication and properties of Co:MgAl2O4 transparent ceramics for a saturable absorber from coprecipitated nanopowder

The 0.05 at.% Co:MgAl₂O₄ precursor was synthesized by the coprecipitation method from a mixed solution of magnesium, aluminum, and cobalt nitrates using ammonium carbonate as the precipitant. 0.05 at.% Co:MgAl₂O₄ transparent ceramics were successfully obtained via vacuum sintering and hot isostatic pressing (HIP) of 0.05 at.% Co:MgAl₂O₄ nanopowder calcined at 1100°C for 4 hours. The properties of powder and ceramics were comprehensively investigated. X-ray diffraction (XRD) results showed that Co:MgAl₂O₄ nanopowder had a pure spinel phase. Also, the in-line transmittances of the HIP posttreated Co:MgAl₂O₄ ceramics with the thickness of 1.2 mm were 82% at 400 nm and 84.7% at 900 nm. The average grain sizes of 0.05 at.% Co:MgAl₂O₄ ceramics before and after the HIP posttreatment were 11 and 28 μm, respectively. The calculated ground state absorption cross section of 0.05 at.% Co:MgAl₂O₄ ceramics was 2.9 × 10⁻¹⁹ cm², indicating that this ceramics is a promising material applied as a saturable absorber for passive laser Q-switches in the 1.3-1.7 μm domain.     

Fabrication and characterization of IR-transparent Fe2+ doped MgAl2O4 ceramics

Initial investigations on the preparation of highly transparent Fe²⁺:MgAl₂O₄ ceramics using nanopowders synthesized in a laser plume were carried out. For the first time, dense Fe²⁺:MgAl₂O₄ ceramics with high transmission in the mid-IR range were fabricated at a temperature as low as 1300°C and with a short sintering time (1 hour). The obtained Fe²⁺:MgAl₂O₄ ceramics contain a secondary (MgO)_0.91(FeO)_0.09 phase with a low wt% content, causing a substantial decrease in transmittance in the visible range. The transmittance increases with an increase in wavelength due to a decrease in Rayleigh scattering and reaches 85.6% at λ = 4 μm, which is close to the theoretical value. The absorption cross section of divalent iron ions was estimated to be σ = (1.66 ± 0.14) × 10⁻²⁰ cm².     

Nonlinear Absorption of Submicrometer Grain‐Size Cobalt‐Doped Magnesium Aluminate Transparent Ceramics

Transparent cobalt‐doped magnesium aluminate spinel (Co:MgAl₂O₄) ceramics with a submicrometer grain size were prepared by spark plasma sintering. For the first time, the nonlinear absorption of Co:MgAl₂O₄ transparent ceramics was experimentally demonstrated. Both ground state absorption (σ_GSA) and excited state absorption (σ_ESA) were estimated using the solid‐state slow saturable absorber model based on absorption saturation measurements performed at 1.535 μm. σ_GSA and σ_ESA for 0.03 at.% Co:MgAl₂O₄ were found to be 4.1 × 10^?19 cm² and 4.0 × 10^?20 cm², respectively. In the case of 0.06 at.% Co:MgAl₂O₄ ceramics, σ_GSA = 2.6 × 10^?19 cm² and σ_ESA= 5.3 × 10^?20 cm² were determined.     

The structure evolution and microwave dielectric properties of MgAl2-x(Mg0·5Ti0.5)xO4 solid solutions

In the present work, MgAl_2-x(Mg_0·5Ti_0.5)_xO₄ (x = 0.02, 0.04, 0.06, 0.08, 0.10) solid solutions were synthesized via the traditional solid-state reaction route. The valence state of Ti ions, crystal structural characteristics, and microwave dielectric properties were discussed. A solid solution with spinel structure was revealed by the Rietveld refinement results. The partial substitution of (Mg_0·5Ti_0.5)³⁺ for Al³⁺ lowered the sintering temperature and improved the Q × f value of MgAl₂O₄ ceramic. The MgAl_2-x(Mg_0·5Ti_0.5)_xO₄ solid solutions with x = 0.06 can be well sintered at 1425 °C in an oxygen atmosphere for 8 h and exhibits excellent microwave dielectric properties with ε_r = 9.1, Q × f = 98,000 GHz, τ_f = −61.36 ppm/°C. The sintering temperature of MgAl_1·94(Mg_0·5Ti_0.5)_0.06O₄ microwave dielectric ceramics was approximately 200 °C lower than that of conventional MgAl₂O₄ ceramics.     

X-ray phase,structural, dielectric and magnetic studying of BiFe1-х(M1/2Ti1/2)хO3 (M = Co,Ni, Zn, х = 0–0.11) multiferroics

Perovskite-like rhombohedral distorted solid solutions of BiFe_1-х(M_1/2Ti_1/2)_хO₃ (M = Co, Ni, Zn, x = 0–0.11) were obtained by solid-phase synthesis. An indicator of the solid solution formation is the change of unit cells parameters, that corresponds to the ionic radii of mixed cations (M1/2Ti1/2)3+ (M = Co, Ni, Zn. Solid solutions of BiFe_1-х(M_1/2Ti_1/2)_хO₃ (M = Co, Ni), in contrast to BiFe_1-x(Zn_1/2Ti_1/2)_xO₃ demonstrate ferromagnetic hysteresis pels at room temperature. The x growth in the range from 0.01 to 0.11 for the BiFe_1-х(M_1/2Ti_1/2)_хO₃ system leads to, the saturation magnetization M_S and the remanent magnetization M_R increase from ∼0.1 and ∼2.4⋅10⁻³ emu/g to ∼0.4 and ∼0.038 emu/g respectively. In the same time the coercive force H_c decreases from ∼120 to ∼80 Oe. For the BiFe_1-х(Co_1/2Ti_1/2)_хO₃ system, a noticeably higher magnetic properties with a more complex dependence on x are observed. The maximum parameter values are observed at x = 0.04–0.05: M_S = 0.83 and M_R = 0.24 emu/g, H_c = 1.8 kOe. It is suggested that the detected anomalies of Co-containing solid solutions behavior are related to the one-ionic magnetocrystalline anisotropy of Co²⁺ cations. The BiFe_1-х(M_1/2Ti_1/2)_хO₃ (M = Co, Ni) samples demonstrate piezoelectric constant d₃₃ up to 7 pC/N. Due to the set of properties the materials obtained can be classified as high-temperature multiferroics.     

Study of the changes in the structure and optical properties of MgxCo1-xCr2-yAlyO4 spinel caused by A/B site ion substitution

In this work, nanosubmicron blue-green pigment powder based on the composition of Mg_xCo_1-xCr_2-yAl_yO₄（0 = x ≤ 1, 0 = y ≤ 2）was prepared by a gel casting method. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Rietveld refinement with GSAS (General Structure Analysis System), and UV–Vis absorption spectroscopy were used to study the phase composition, grain size, morphology, cation distribution in the crystal structure and spectral absorption of the samples. Colour parameters were also studied by using a colour measurement spectrophotometer. The studies demonstrate that the distribution of cations in the crystal structure is disordered and that divalent and trivalent cations are mixed to occupy tetrahedral and octahedral sites. Furthermore, the substitution of ions at the A/B site leads to a change in the cation distribution ratio at the tetrahedral and octahedral sites. With increasing Mg²⁺ doping concentration, the inversion parameter of the spinel increases, while with increasing Al³⁺ doping concentration, the inversion parameter of the spinel decreases. In addition, changes in the calcining atmosphere lead to a change in the oxygen vacancy content in the structure. Under the condition of a reductive atmosphere, the oxygen vacancy content significantly increases, and the inversion parameter also increases. The colour difference for the synthesized Mg_xCo_1-xCr_2-yAl_yO₄ spinel powder is related to the proportion of chromophore ions occupying tetrahedral and octahedral sites and the number of oxygen vacancies.     

Microwave dielectric properties and crystal structures of spinel-structured MgAl2O4 ceramics synthesized by a molten-salt method

The microwave dielectric properties and crystal structures of MgAl₂O₄ ceramics synthesized using either molten-salt (MA-M) or solid-state reaction (MA-S) methods were characterized in this study. Raman and ²⁷Al solid-state nuclear magnetic resonance spectra indicated that Al³⁺ cations primarily occupied in tetrahedral sites of the MA-M ceramic. The degree of inversion x, i.e., degree of cation disorder in tetrahedral and octahedral sites, of the MA-M was higher than that of MA-S; such the preferential site occupation of Al³⁺ cations enhanced the covalency of the MO bonds in the MO₄ tetrahedra (M = Mg and Al), leading to a decrease in the lattice parameters. The Q·f of the MA-M fired at 1600 °C was 201,690 GHz, while the MA-S synthesized at 1600 °C exhibited a Q·f of just 85,100 GHz. Based on these results, an intermediate spinel structure with a greater x evidently has a higher Q·f, and therefore the cation distribution is closely related to the Q·f of the ceramic.     

Luminescence of MgAl2O4 and ZnAl2O4 spinel ceramics containing some 3d ions

The luminescence properties of ceramic phosphors based on two spinel hosts MgAl₂O₄ and ZnAl₂O₄ doped with manganese ions have been studied. It has been found that the spectral properties of these phosphors can be strongly varied by changing synthesis conditions. Both types of doped ceramic spinel can serve as efficient Mn²⁺ green-emitting phosphors having peak emissions at 525 and 510 nm, respectively. Mn-doped MgAl₂O₄ spinel can also be prepared as an efficient Mn⁴⁺ red-emitting phosphor having peak emission at ~651 nm by using specific temperatures of heat treatment in air. It has also been shown that the conversion of Mn²⁺ to Mn⁴⁺ and viсe versa, as well as the coexistence of Mn²⁺ green and Mn⁴⁺ red emissions, can be accomplished by properly chosen annealing conditions of the same initially synthesized MgAl₂O₄:Mn sample. Manganese doped MgAl₂O₄ spinel with an optimal intensity ratio of green and red emissions can be a promising single-phase bicolor phosphor suitable for the development of warm white phosphor-converted LED lamps. On the other hand, it has been determined that perfectly normal ZnAl₂O₄ spinel cannot be doped with Mn⁴⁺ ions in contrast to partially inverse MgAl₂O₄ spinel. However, ZnAl₂O₄ samples unintentionally doped with impurity Cr³⁺ ions show emission spectra in the far-red region with well pronounced R, N and vibronic lines of Cr³⁺ luminescence due to the perfect normal spinel structure of synthesized ZnAl₂O₄ ceramics. Also, by partially substituting Al³⁺ cations for Mg²⁺ in ZnAl₂O₄ there is an opportunity to obtain Mn⁴⁺ doped or Mn⁴⁺/Cr³⁺ codoped far-red emitting phosphors which can be suitable for indoor plant growth lighting sources.     

Impact on aggregate/matrix bonding when a refractory contains zinc aluminate instead of spinel and magnesia-chrome

The high hot strength of MgO–Cr₂O₃ refractory is often ascribed to its intimate aggregate/matrix bonding. For a fundamental comparison with it, ∼2 mm aggregates of MgO and Al₂O₃ were separately embedded in ZnAl₂O₄ and MgAl₂O₄ matrices, sintered at 1600°C, and examined. It was found that similarity of thermal expansion coefficient (TEC) between the aggregate and the matrix is critical to achieve good bonding and this is more important than the extent of interdiffusion. The TEC mismatch of ≥5.7 × 10⁻⁶ K⁻¹ caused significant undesirable debonding in MgO aggregate/MgAl₂O₄ matrix sample and MgO/ZnAl₂O₄ despite >736 μm Zn²⁺ diffusion depth in the latter. Direct bonding, as inferred from a thicker interfacial reaction layer and a greater shift of the aggregate/matrix interface before and after firing, was better in MgAl₂O₄/ZnAl₂O₄ combination, followed by tabular Al₂O₃/ZnAl₂O₄ and Al₂O₃/MgAl₂O₄. Powder X-ray diffraction indicated that the volatilization of ZnAl₂O₄ at 1600°C in air was negligible compared to MgO–Cr₂O₃.     

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.     

Preparation,characterisation, and growth mechanism of mesoporous petal-like MgAl2O4 spinel

Petal-like MgAl₂O₄ spinel was successfully prepared using a novel inorganic salt-assisted nonhydrolytic sol-gel method without a template and was employed as absorbent in the removal of the Congo red (CR). The effects of the inorganic salt type, heat-treatment temperature, and dwelling time on the morphology and phase composition of the petal-like MgAl₂O₄ spinel were investigated systematically. Results indicated that when Na₂MoO₄ was employed as the salt and the heat-treatment temperature and dwelling time were 600 °C and 5 h, respectively, the as-obtained petal-like MgAl₂O₄ spinel exhibited a highly uniform morphology with a thickness of 19–23 nm and a length of 240–280 nm. The N₂ adsorption-desorption results revealed that the petal-like MgAl₂O₄ exhibited a large BET specific surface area of 161 m²g^-1 with a pore volume of 0.24 cm³g^-1. The growth mechanism of the petal-like MgAl₂O₄ is believed to be the formation of a two-dimensional layered network structure by the coordination between the condensation product of the magnesium aluminium bimetallic alkoxides and the ions in the salt. The as-prepared MgAl₂O₄ petal exhibited an effective adsorption capacity toward anionic dyes CR. The maximum adsorption capacity of CR onto the mesoporous MgAl₂O₄ petal was found to be 572.01 mg/g, it is showed the petal-like MgAl₂O₄ exhibit huge potential of application in the field of environmental remediation.