Spinel Crystals Research Articles

Petrography and geochemistry of distal spherules from the K–Pg boundary in the Umbria–Marche region (Italy) and their origin as fractional condensates and melts in the Chicxulub impact plume

The impact spherules from the distal K–Pg boundary sections are considered to represent silicate droplets condensed and solidified from a laterally expanding, cooling vapor plume formed upon hypervelocity impact. In the present-day Cretaceous–Paleogene boundary (K–Pg) spherule population of the Umbria–Marche region in Italy, three texturally and compositionally distinct types of impact spherules can be identified that are dominantly composed of (1) goethite, (2) K-feldspar or (3) glauconite. Although these phases represent the products of diagenetic alteration, the remnant textural characteristics of the spherules and the type of alteration product are indicative of the spherules’ original compositions, which are important to constrain the physicochemical conditions prevalent throughout the impact vapor plume. The presence of relict ghost crystals and the identification of ‘iddingsite’ indicate that goethite likely represents pseudomorphic replacement after olivine. Goethite spherules contain numerous dendritic, euhedral and skeletal spinel crystals variably dispersed in the groundmass. In terms of textures, five types of goethite spherules can be distinguished, showing striking similarities to chondrules: (I) skeletal, (II) barred, (III) radial/barred, (IV) porphyritic and (V) relict/granular. The morphology of both spinel and olivine (pseudomorphs) is consistent with established formation conditions (peak temperature Tmax, degree of supercooling ΔT, cooling rate, presence of nucleation sites) for different chondrule textural types. As goethite spherules are anomalously enriched in moderately to highly refractory lithophile (Sc, V, Y, Zr, Nb, REE, Hf, Ta, Th) and siderophile (Cr, Co, Ni, W, Ir, Pt) elements, they are interpreted to represent (diagenetically altered) refractory (high-T) condensation products from a well-homogenized plume consisting of both vaporized target and projectile matter. Different from goethite spherules, K-feldspar spherules exhibit pseudomorphic textures after lower-liquidus silicates such as Ca-rich pyroxene and plagioclase. Furthermore, the K-feldspar spherules yield systematically lower abundances of the most refractory trace elements. This suggests that the pre-altered K-feldspar spherules are part of the same fractional condensation sequence of the target-impactor vapor plume, cooled during lateral expansion. Glauconite spherules are cryptocrystalline, exhibiting hemispherical lamellae that resemble the palagonite/smectite alteration layers of basaltic glasses and the K–Pg spherules (microtektites) found at proximal sites around the Gulf of Mexico region. Their trace element contents and REE patterns are strikingly similarity to those of (altered) K–Pg microtektites, suggesting that glauconite spherules represent former glass spherules without crystallites/microlites. Glauconite spherules are interpreted not to be part of the fractional condensation sequence of the impact vapor plume that led to the formation of the replaced goethite and K-felspar spherules. They were likely formed by entrainment of melt, expelled at the steepest angles and highest ejection velocities from the central melt sheet, within the edges of the vapor plume. This work is the first to create a geochemical foundation for vapor plume models, both at the major and trace element scale. In addition, it highlights the unique characteristics of the Chicxulub impact event and emphasizes the importance of its unusual target lithologies. The heterogeneous, layered, and volatile-rich target contributed significantly to the formation of a dust-rich environment, high oxygen fugacities and the preservation of some degree of heterogeneity in the vapor plume. In addition, striking similarities between distal (goethite-type) K–Pg spherules and chondrules may argue for a reevaluation of the impact model as a possible origin for the formation of certain types of cosmic chondrules.

Crystallization and nonlinear optical properties of transparent glass-ceramics with Co:Mg(Al,Ga)2O4 nanocrystals for saturable absorbers of lasers at 1.6–1.7 µm

Transparent glass-ceramics (GCs) containing nanocrystals of Co:Mg(Al,Ga)2O4 spinel and Co:magnesium gallium aluminotitanate solid solutions are synthesized by secondary heat-treatments of the magnesium aluminosilicate glass nucleated by TiO2 and doped with Ga2O3 and CoO. Optical spectroscopy confirms that Co2+ ions located in the initial glass in octahedral and tetrahedral positions, enter the Mg(Al,Ga)2O4 nanocrystals predominantly in tetrahedral sites and the fraction of tetrahedrally coordinated ions increases gradually with the heat-treatment temperature. The feature of these GCs is a long-wavelength shift of the absorption band related to the 4A2(4F)→4T1(4F) transition of Co2+ ions (up to ~1.67µm) as compared with that for Co:MgAl2O4 single crystals. An increase of the heat-treatment temperature from 800 to 950°C leading to an increase of the fraction and size of spinel crystals allowed increasing the peak absorption within this band while keeping the saturation intensity within the range 0.5…0.7J/cm2 and the recovery time in the range 240…335ns. The developed GCs are promising as saturable absorbers for erbium lasers emitting at 1.6–1.7µm.

Vitrification of hazardous Fe-Ni wastes into glass-ceramic with fine crystalline structure and elevated exploitation characteristics

Hazardous residues from ferronickel smelting plant are mixed with glass cullet and are vitrified for 2h at 1400°C. The obtained glass is characterized with a very high crystallization trend. As a result, it is inappropriate for sinter-crystallization, but a fine crystalline glass-ceramic can be obtained by bulk crystallization after short heat-treatment at low temperatures.Due to the presence of 1.5wt.% chromium oxide in the parent glass, together with high amounts of iron oxides and magnesium oxides, the crystallization process is peculiar. It starts during the melt cooling with the precipitation of preliminary Fe-Mg-Cr spinel crystals, which then act as centers for further epitaxial growth of pyroxene phase. It is also highlighted that, due to liquid–liquid immiscibility, the main amorphous phase is characterized by a nonhomogeneous binodal structure, which becomes finer after the nucleation treatment. As a result, pyroxenes with sizes below 1μm are formed as main crystal phase after the crystallization step.Notwithstanding of the high amount of hazardous wastes in the batch the obtained glass is characterized with high chemical durability. At the same time, the obtained glass-ceramics is characterized by a suitable coefficient of thermal expansion and attractive mechanical characteristics.

Compositional variations in optical characteristics of Mn doped spinel crystals

Mn doped spinel (MgAl2O4) crystals have been grown by floating zone (FZ) techniques with various conditions of O2 concentrations in the growth atmosphere from 0 to 75vol%, with a starting composition of molar ratio x=MgO/(MgO+Al2O3) from 0.3 to 1.0 and/or Mn concentrations from 1.0 to 6.0at%. Optical absorption spectra and photoluminescence spectra were evaluated using crystals grown under various growth conditions. The color of the crystals is found to vary with their composition and the O2 concentration in the atmosphere. Crystals grown under a 100vol% Ar atmosphere, were a pale green color and emitted a strong green luminescence at λ=520nm. Colors of stoichiometric crystals, x=1.0, and Mg-poor crystals grown under an oxidizing atmosphere are yellow and red, respectively. Both optical absorption spectra and photoluminescence spectra of Mg-poor, x<1.0 crystals grown under an oxidizing (O2–Ar) atmosphere are found to vary greatly from those of the stoichiometric crystals.

Aerosol sprayed Mn1.5Co1.5O4 protective coatings for metallic interconnect of solid oxide fuel cells

A stable suspension has been obtained by disbursing Mn1.5Co1.5O4 (MCO) particles (100–300 nm in diameter) in ethanol using a ball milling process. The suspension is used for deposition of a uniform MCO coating (5–8 μm thick) on a steel (T441) substrate preheated to 200 °C by aerosol spraying. It is found that the grain size of spinel crystals in the MCO coatings ranges from 100 nm to 1 μm, and remains uniform and stable when exposed to air at 800 °C for 1000 h. However, a Cr-rich oxide scale of approximately 0.75 μm thick appears between the MCO coating and the T441 substrate during the exposure. In contrast, a bare T441 sample (without the MCO coating) is oxidized significantly more under the same exposure, forming an oxide scale of 4 μm thick. The result indicates that the spinel coating had effectively improved the oxidation resistance of T441 by inhibiting the transport of Cr. Besides, the results also suggest that aerosol spraying is a simple and cost-effective method for deposition of thin and dense protective coating.

Optical properties of transparent cobalt-containing magnesium aluminosilicate glass-ceramics doped with gallium oxide for saturable absorbers

Transparent glass-ceramic materials based on glasses of the MgO–Al2O3–SiO2–TiO2 system doped with CoO and Ga2O3 are synthesized. The secondary heat treatment of the initial glasses at temperatures of 800–950°C leads to precipitation of nanosized (6–7 nm) crystals of magnesium aluminogallium spinel doped with cobalt ions and magnesium aluminotitanate solid solutions. The optical absorption spectra of the initial glass and glass-ceramic materials are studied. It is shown that the absorption band caused by the 4A2(4F)→ 4T1(4F) transitions of tetrahedrally coordinated Co2+ ions in glass-ceramics with nanosized Co:Mg(Al,Ga)2O4 crystals is shifted to longer wavelengths (up to ~1.67 µm) compared to the position of this band in materials with Co:MgAl2O4 crystals. The synthesized glass-ceramics are characterized by a relatively low saturation fluence FS ~ 0.5 ± 0.1 J/cm2 at a wavelength of 1.54 µm, as well as by a high radiation resistance to nanosecond laser pulses, which is no lower than ~15 ± 2 J/cm2. This explains their attractiveness as materials for saturable absorbers for erbium lasers emitting in the spectral range 1.5–1.7 µm.

Raman study of Yuan Qinghua porcelain: the highlighting of dendritic CoFe2O4crystals in blue decorations

Raman spectroscopy and scanning electron microscopy equipped with energy dispersive spectrometer (SEM-EDS) were used to analyze the decors of the Yuan blue and white porcelains (Qinghua porcelains). SEM-EDS carried out on the dark spots of the blue decors revealed the presence of dendritic crystals of a mix cobalt iron oxide. The Raman spectra of these crystals are close to the standard spectrum of cobalt ferrite (CoFe2O4). Comparing the Fe/Co ratios obtained by EDS to the corresponding Raman spectra, we have identified two main types of spinel crystals: one type richer in iron and another type richer in cobalt. Both types of spinel cause the blackening of the blue decors. It is the first time that spinel crystals with a composition close to cobalt ferrite were observed as the main constituent of the dark spots in blue decors of Qinghua porcelains. Copyright © 2016 John Wiley & Sons, Ltd.

Experimental investigation of the kinetics of the spinel-to-garnet transformation in peridotite: A preliminary study

To study the kinetics of the spinel-to-garnet transformation in peridotite, we conducted reaction experiments in the garnet peridotite stability field (3.2 GPa, 1020–1220 °C, for 0.6–30 h) using a single spinel crystal embedded in monomineralic orthopyroxene powder or in a mixture of powdered orthopyroxene and clinopyroxene. The growth textures observed in the reaction rim between the spinel crystal and the polycrystalline pyroxenes show that the reaction rim grew in both the spinel and pyroxenes directions, suggesting mobility of both SiO 2 and R 2 O 3 components (where R is a trivalent cation). Olivine grains formed only in the presence of monomineralic orthopyroxene and were present in some domains without forming reaction rims. Based on a diffusion-controlled growth model, the growth kinetics of the garnet reaction rim can be described by [ x ( t )] 2 = k 0 exp(− H */R T ) t , where x ( t ) is the rim width at time t , R is the gas constant, T is the absolute temperature, and H * is the activation enthalpy of reaction; k 0 and H * are, respectively, k 0 = 10 −19.8 ± 4.9 m 2 /s and H * = 171 ± 58 kJ/mol. The development of a garnet reaction rim around a spinel core has been observed in alpine-type peridotitic rocks and mantle xenoliths. The reaction rims experimentally produced in this study are characteristic of corona textures observed in natural rocks, and the experimentally measured growth rate of the rims places important constraints on dynamic transformation processes involving spinel and garnet in peridotite. However, to reconstruct the P-T-t history of the corona texture based on these elementary processes, additional detailed studies on the textural evolution and quantitative kinetics of the garnet-rim growth stage are required.

Magnetoreflection of light in CoFe2O4 magnetostrictive spinel

The reflection and magnetoreflection of natural light within the infrared spectral range is studied in single crystals of CoFe2O4 ferrimagnetic ferrite spinel. Correlation between the reflection of light and magnetoelastic characteristics of this spinel is found. It is shown that the most significant magnetic-field-induced changes in the magnetoreflection of the spinel occur near the fundamental absorption edge and within the range of the phonon spectrum.

Single-Crystal Based Diagnostics for Li-Ion Battery Cathode Development

Lithium transition-metal oxides (Li-TM oxides) are presently the most promising high energy density cathode materials for rechargeable lithium batteries. When operated at high voltages (>4.3 V), however, these oxides suffer from structural instability, extensive side reactions with the electrolyte, poor cyclability and severe thermal runaway reactions. [1, 2] In order to develop successful strategies addressing these issues, there is a clear need in fundamental knowledge of the relationships between the material’s specific physical properties/reaction mechanisms and its reactivity. This is difficult to obtain on conventional aggregated secondary particles because isolating specific physical properties of interest from less controlled ones, such as particle porosity, grain boundaries, primary particle size and size distribution, is often challenging if not impossible. To this end, we developed a unique diagnostic approach combining carefully prepared cathode model samples and the start-of-the-art analysis techniques with high spatial resolution and chemical specificity. In this presentation, we will discuss the synthesis of high-voltage Ni/Mn spinel (LMNO) and high-capacity Li-and Mn-rich layered oxide (LMR-NMC) crystals with various physical characteristics. [3] We demonstrate the effective use of complementary microscopy and spectroscopy techniques at multi-length scale, such as aberration corrected (scanning) transmission electron microscopy, electron energy loss spectroscopy, X-ray energy dispersive spectroscopy, surface sensitive soft X-ray absorption spectroscopy and X-ray photoelectron spectroscopy, full-field transmission X-ray microscopy and X-ray absorption near edge structure imaging, to determine elemental, chemical and atomic-level structural make-up of the entire bulk as well as the surface properties of the crystals. [4, 5] Combined with electrochemical analysis, insights on the phase transformation mechanism during Li extraction/reinsertion and the key factors influencing the reactions occurring at the cathode/electrolyte interface were obtained. [6] Future directions and research in addressing some of the pressing challenges facing Li-TM oxide cathode development will also be discussed.

Sub-2 nm Thick Fluoroalkylsilane Self-Assembled Monolayer-Coated High Voltage Spinel Crystals as Promising Cathode Materials for Lithium Ion Batteries.

We demonstrate herein that an ultra-thin fluoroalkylsilane self-assembled monolayer coating can be used as a modifying agent at LiNi0.5Mn1.5O4−δcathode/electrolyte interfaces in 5V-class lithium-ion batteries. Bare LiNi0.5Mn1.5O4−δ cathode showed substantial capacity fading, with capacity dropping to 79% of the original capacity after 100 cycles at a rate of 1C, which was entirely due to dissolution of Mn3+ from the spinel lattice via oxidative decomposition of the organic electrolyte. Capacity retention was improved to 97% on coating ultra-thin FAS17-SAM onto the LiNi0.5Mn1.5O4 cathode surface. Such surface protection with highly ordered fluoroalkyl chains insulated the cathode from direct contact with the organic electrolyte and led to increased tolerance to HF.

Spinel and Aspidolite from the Des Cèdres Dam Occurrence, Notre-Dame-du-Laus, Québec, Canada

The Parker mine in Notre-Dame-du-Laus, Quebec, is well known among collectors for having produced substantial amounts of well-formed, lustrous black spinel crystals (rarely up to 6 cm) associated w...

Non-isothermal Crystallization Kinetics of Spinels in Vanadium Slag with High CaO Content

This paper investigated the non-isothermal crystallization kinetics of the spinel crystals in vanadium slags containing high CaO content. Experiments were performed in combination with theoretical calculation to address this issue, and statistical analyses based on the Crystal Size Distribution theory. The results indicate that low cooling rate and high CaO content benefit the growth of spinel crystals. The growth mechanism is revealed to be controlled by interface reactions and diffusion at the cooling rates of 5 K/min and 15 K/min, respectively. However, at higher temperatures (>1673 K), the growth of spinel crystals is controlled by nucleation. While the temperature is decreased to 1523 K at the cooling rate of 5 K/min, the mean diameter of spinel crystals could reach 36.44 μm. Experimental results combining with theoretical reveal that low cooling rate benefits spinels growth, especially for the interval of 1523 K–1200 K.

New perspectives on the origin of olivine-rich troctolites and associated harrisites from the Ligurian ophiolites (Italy)

The mid-ocean ridge (MOR)-type gabbroic sequences from the Internal Ligurian ophiolites include olivine-rich troctolite lenses up to tens of metres thick. For one of these lenses, a portion of metre-scale thickness characterized by skeletal to dendritic olivines (harrisites) was observed near the contact with host gabbros. Spinels from the olivine-rich troctolites locally include Ti-pargasite to kaersutite frequently associated with phlogopite to Na-phlogopite. Spinel-hosted amphibole displays rare earth element (REE) patterns characterized by a negative Eu anomaly, thereby recording a magmatic process associated with plagioclase segregation. The amphiboles show variable depletion of light REE (LREE) relative to middle REE (MREE), and heavy REE (HREE) that are weakly enriched to depleted with respect to MREE. Crystallization of the inclusion-bearing spinels is attributed to cooling of hybrid melts that originated by interaction between primitive melts and gabbro-related melts relatively rich in SiO 2 and incompatible elements. Clinopyroxene and amphibole from the harrisites show extensive variability for trace element compositions, albeit characterized by subparallel incompatible element patterns. This chemical variability was most probably acquired in response to rapid crystal growth related to undercooling of the harrisite parental melt. We propose that the melt undercooling was related to interaction of a primitive melt batch with a gabbroic crystal mush. Supplementary material : Details of the analytical technique are available at http://doi.org/10.6084/m9.figshare.c.3284312

Color-center production and recovery in electron-irradiated magnesium aluminate spinel and ceria

Single crystals of magnesium aluminate spinel (MgAl2O4) with (1 0 0) or (1 1 0) orientations and cerium dioxide or ceria (CeO2) were irradiated by 1.0 MeV and 2.5 MeV electrons in a high-fluence range. Point-defect production was studied by off-line UV–visible optical spectroscopy after irradiation. For spinel, regardless of both crystal orientation and electron energy, two characteristic broad bands centered at photon energies of 5.4 eV and 4.9 eV were assigned to F and F+ centers (neutral and singly ionized oxygen vacancies), respectively, on the basis of available literature data. No clear differences in color-center formation were observed for the two crystal orientations. Using calculations from displacement cross sections by elastic collisions, these results are consistent with a very large threshold displacement energy (200 eV) for oxygen atoms at room temperature. A third very broad band centered at 3.7 eV might be attributed either to an oxygen hole center (V-type center) or an F2 dimer center (oxygen di-vacancy). The onset of recovery of these color centers took place at 200 °C with almost full bleaching at 600 °C. Activation energies (~0.3–0.4 eV) for defect recovery were deduced from the isochronal annealing data by using a first-order kinetics analysis. For ceria, a sub-band-gap absorption feature, which peaked at ~3.1 eV, was recorded for 2.5 MeV electron irradiation only. Assuming a ballistic process, we suggest that the latter defect might result from cerium atom displacement on the basis of computed cross sections.

Effect of temperature and dissolved hydrogen on oxide films formed on Ni and Alloy 182 in simulated PWR water

Alloy 182 is a nickel-based weld metal, which is susceptible to stress corrosion cracking in PWR primary water. It shows a peak in SCC susceptibility at a certain temperature and hydrogen concentration. This peak is related to the electrochemical condition where the Ni to NiO transition takes place. One hypothesis is that the oxide layer at this condition is not properly developed and so the material is not optimally protected against SCC. Therefore the oxide layer formed on Alloy 182 is investigated as a function of the dissolved hydrogen concentration and temperature around this Ni/NiO transition. Exposure tests were performed with Alloy 182 and Ni coupons in a PWR environment at temperatures between 300 °C and 345 °C and dissolved hydrogen concentration between 5 and 35 cc (STP)H2/kg. Post-test analysis of the formed oxide layers were carried out by SEM, EDS and XPS. The exposure tests with Ni coupons showed that the Ni/NiO transition curve is at a higher temperature than the curve based on thermodynamic calculations. The exposure tests with Alloy 182 showed that oxide layers were present at all temperatures, but that the morphology changed from spinel crystals to needle like oxides when the Ni/NiO transition curve was approached. Oxide layers were present below the Ni/NiO transition curve i.e. when the Ni coupon was still free of oxides. In addition an evolved slip dissolution model was proposed that could explain the observed experimental results and the peak in SCC susceptibility for Ni-based alloys around the Ni/NiO transition.

Spinel in the allivalites of Malyi Semyachik Volcano, Kamchatka

A microprobe study of Ol–An nodules (allivalites) as sampled from the basaltic andesites of Malyi Semyachik Volcano in Kamchatka revealed small inclusions of a mafic mineral in crystals of olivine and anorthite and in the intergranular mass. The mineral was later identified as spinel. A microprobe analysis of the spinel showed that the grains that were found in minerals are not zonal, with spinel inclusions of different chemical compositions occurring in a single crystal. In contrast to this situation, the spinel crystals in the intergranular basis are zonal and exhibit a reaction rim. The spinel compositions form a long and well-pronounced trend that occurs in the solvus zone of solid solutions that has not been adequately described in the literature. The existence of the resulting spinel trend in the solvus zone can be explained by an early capture of spinel grains by growing silicate crystals and by their rapid cooling after the eruption, which chilled the metastable solution. Spinel grains occasionally exhibited initial phases in the decomposition of a solid solution owing to their long residence in a cooling lava flow at temperatures below the solvus. It is supposed that these spinels were formed somewhat earlier or simultaneously with the crystallization of the silicate phase. This diversity in the compositions of non-zonal spinel can be explained by thermo diffusion that led to homogeneous compositions in the initially zonal spinel crystals after they had been included in the host mineral crystals.

Storage and Evolution of Mafic and Intermediate Alkaline Magmas beneath Ross Island, Antarctica

We present the results of phase equilibrium experiments carried out on basanite and phonotephrite lavas from Ross Island, Antarctica. Experiments were designed to reproduce the P–T–X–f O 2 conditions of deep and intermediate magma storage and to place constraints on the differentiation of each of the two predominant lava suites on the island, which are thought to be derived from a common parent melt. The Erebus Lineage (EL) consists of lava erupted from the Erebus summit and the Dry Valley Drilling Project (DVDP) lineage is represented by lavas sampled by drill core on Hut Point Peninsula. Experiments were performed in internally heated pressure vessels over a range of temperatures (1000–1150°C) and pressures (200–400 MPa), under oxidized conditions (NNO to NNO + 3, where NNO is the nickel–nickel oxide buffer), with XH2O of the H 2 O–CO 2 mixture added to the experimental capsule varying between zero and unity. The overall mineralogy and mineral compositions of the natural lavas were reproduced, suggesting oxidizing conditions for the deep magma plumbing system, in marked contrast to the reducing conditions (QFM to QFM – 1, where QFM is the quartz–fayalite–magnetite buffer) in the Erebus lava lake. In basanite, crystallization of spinel is followed by olivine and clinopyroxene; olivine is replaced by kaersutitic amphibole below ∼1050°C at intermediate water contents. In phonotephrite, the liquidus phase is kaersutite except in runs with low water content ( X H 2 O fluid < 0·2) where it is replaced by clinopyroxene. Experimental kaersutite compositions suggest that the amphibole-bearing DVDP lavas differentiated below 1050°C at 200–400 MPa and NNO + 1·5 to NNO + 2. Olivine- and clinopyroxene-bearing EL lavas are consistent with experiments performed above 1050°C and pressures around 200 MPa. The plagioclase liquidus at <1–2 wt % H 2 O suggests extremely dry conditions for both lineages ( X H 2 O fluid approaching zero for EL, ∼0·25 for DVDP), probably facilitated by dehydration induced by a CO 2 -rich fluid phase. Our results agree with previous studies that suggest a single plumbing system beneath Ross Island in which DVDP lavas (and probably other peripheral volcanic products) were erupted through radial fractures associated with the ascent of parental magma into the lower crust. The longer travel time of the DVDP lavas through the crust owing to lateral movement along fractures and the lack of a direct, sustained connection to the continuous CO 2 -rich gas flux that characterizes the main central Erebus conduit is probably responsible for the lower temperatures and slightly wetter conditions and hence the change in mineralogy observed.

Competitive growth of crystals in vanadium–chromium slag

Based on the classic kinetic theory of crystallization, we have developed a mathematical model to describe the crystallization behavior of crystals in complex vanadium–chromium slag systems. The results show that the crystallization rates of spinels, olivines and augites are dominated by the component with the highest melting point. The sequence of the crystallization ability of crystals in vanadium–chromium slag is as follows: FeCr2O4 > Mg2SiO4 > FeV2O4 > MgSiO3 > Fe2TiO4 > Mn2SiO4 > MnSiO3 > Fe2SiO4. The optimum temperature ranges for the crystallization of these crystals are 1497–1547 K, 1380–1430 K, 1301–1351 K, 1192–1242 K, 1098–1148 K, 1069–1119 K, 1038–1088 K and 985–1035 K, respectively. We also find that with the increase in Cr2O3/V2O3 and decrease in SiO2/FeO, the total crystallization rate of spinels increases remarkably, while the TiO2 content has little effect on the crystallization of spinels in vanadium–chromium slag.

High-quality spinel LiCoTiO4single crystals with co-exposed {111} and {110} facets: flux growth, formation mechanism, magnetic behavior and their application in photocatalysis

Novel spinel LiCoTiO4 single crystals with co-exposed {111} and {110} facets were grown for the first time using a facile LiCl flux method. The obtained cubic-phase LiCoTiO4 crystals exhibit high crystallinity and regular octahedron-like geometric morphology. In this method, LiCl was used not only as an effective lithium source without any additives but also as the optimum molten medium compared with LiCl–NaCl and LiCl–KCl eutectic mixtures. The formation mechanism responsible for the flux-grown spinel LiCoTiO4 crystals with co-exposed {111} and {110} facets was also proposed. Moreover, the spinel LiCoTiO4 crystals showed a weak ferrimagnetic behavior with some antiferromagnetic effects. Photocatalytic tests revealed that the pure LiCoTiO4 sample had a relatively low photocatalytic activity although it had a relatively narrow band-gap energy (1.58 eV) corresponding well to the broad visible spectrum. However, the spinel LiCoTiO4 crystal could serve as an efficient visible-light sensitizer, and when coupled with graphitic carbon nitride (g-C3N4) nanosheets, the novel LiCoTiO4/g-C3N4 composites exhibited higher activity than pure g-C3N4 for the photocatalytic degradation of organic pollutants under simulated sunlight. A possible mechanism for the improved photocatalytic performance was also proposed. Overall, our present work opens a new approach to fabricating other lithiated transition-metal oxide spinel crystals for broad industrial applications.