Spinel Block Research Articles

Anisotropic Rh3+ Diffusion in Layered Hexaaluminate Mitigates Thermal Deactivation of Supported Rhodium Catalysts

When exposed to a high-temperature oxidizing environment, Rh catalysts supported on Al2O3-based oxides lose their three-way catalytic activity as a result of unfavorable interface interactions that allow Rh3+ to diffuse into the support structure and occupy Al3+ sites. This study showed that the incorporated Rh3+ ions were not easily reduced to active Rh metal species and caused substantial thermal deactivation. The deactivation was most obvious for γ-Al2O3 and MgAl2O4 with a spinel-type structure but much less for hexaaluminate (LaMgAl11O19) with a layered structure consisting of alternative stacking of a spinel block and a La–O monolayer. After annealing at 900 and 1000 °C for 100 h in the air, Rh-deposited single crystals were studied by dynamic secondary ion mass spectrometry to analyze the Rh depth profile. The Rh depth profile in LaMgAl11O19 showed that the diffusion along the c axis (∥c) was significantly suppressed compared to that normal to the c axis (⊥c). The diffusion of Rh3+ was faster and nearly isotropic in a MgAl2O4 single crystal, which was used as a model crystal for γ-Al2O3. These results show that the layered structure of hexaaluminate influences the Rh3+ diffusion, i.e., the La–O interlayer between closely packed spinel blocks running at every approximately 1.1 nm interval is likely the diffusion barrier. This barrier effectively blocks further penetration of the incorporated Rh3+ ions from the surface and preserves their smooth reduction to active metallic Rh nanoparticles.

In Situ TEM Study of Structural Changes in Na-β″-Alumina Using Electron Beam Irradiation.

Real-time structural changes in Na-β″-alumina were observed in situ using transmission electron microscopy (TEM) with electron beam irradiation. Na-β″-alumina has been widely investigated as a solid electrolyte material for sodium–sulfur secondary batteries owing to its high ionic conductivity. This high conductivity is known to be due to the Na+ ions on the loosely packed conduction planes of Na-β″-alumina. In the present study, we acquired real-time videos of the generation of spinel blocks caused by the conduction of Na+ ions. In addition, by observing Na extraction during electron beam irradiation, we experimentally confirmed that spinel block generation originates from the Na+ ion conduction, which has been a subject of recent debate.

Composition and Antithermal Quenching of Noninteger Stoichiometric Eu2+-Doped Na-β-Alumina with Cyan Emission for Near-UV WLEDs.

Phosphors with high quantum efficiency and thermal stability play a key role in improving the performance of phosphor-converted white light-emitting diodes (pc-WLEDs). A near-UV-pumped LED shows a great advantage due to its reduction of the negative effect of blue light on human health. In this work, we propose a series of near-UV excitable cyan-emitting Eu2+-activated phosphors with a nominal composition of Na2-2xAl11O17+a:xEu2+ (x = 0.01-0.40), which crystallize in a sodium β-alumina phase with a composition close to Na1.22Al11O17.11. An excess amount of the sodium carbonate raw material makes up the volatile Na during the high-temperature process. The noninteger stoichiometric composition promotes the rigidity of the crystal structure with a slight excess of Na insertion into layers between spinel blocks of the NaAl11O17 matrix. The nonequivalent substitution of Na+ by Eu2+ generates intrinsic defects acting as carrier traps. As a result, the phosphor with an optimal nominal composition Na1.6Al11O17+a:0.20Eu2+, under the excitation at 365 nm, shows an asymmetric cyan emission band at 468 nm with internal and external quantum efficiencies of 81.3 and 56.9%, respectively. Remarkably, the phosphor exhibits antithermal quenching within 200 °C. A pc-WLED with a high color rendering index (87.2) suggests great potential of the phosphor in pc-WLEDs. Therefore, a combination of a rigid structure and deep trap level is an effective way in exploring new phosphors with high quantum efficiency and thermal stability.

Kahlenbergite KAl<sub>11</sub>O<sub>17</sub>, a new <i>β</i>-alumina mineral and Fe-rich hibonite from the Hatrurim Basin, the Negev desert, Israel

Abstract. Kahlenbergite, ideally KAl11O17, and Fe-rich hibonite, CaAl10Fe2O19, are high-temperature minerals found in “olive” subunits of pyrometamorphic rocks, in the Hatrurim Basin, the Negev desert, Israel. The crystal structures of both minerals are refined using synchrotron radiation single-crystal diffraction data. The structure of kahlenbergite (P63/mmc; a=5.6486(1) Å; b=22.8970(3) Å; Z=2) exhibits triple spinel blocks and so-called R blocks. The spinel blocks show mixed layers with AlO6 octahedra and (Al0.56Fe0.44)O4 tetrahedra and kagome layers with (Al0.92Fe0.08)O6 octahedra. One-dimensional diffuse scattering observed parallel to c* implies stacking faults in the structure. Also, in one of the investigated kahlenbergite crystals additional reflections can be identified, which obviously belong to a second phase with a smaller lattice parameter c: Fe3+-rich hibonite. The structure of hibonite contains the same spinel blocks as kahlenbergite. The R blocks in hibonite contain Ca atoms, AlO5 bipyramids, and AlO6 octahedra, whereas the R blocks in kahlenbergite contain potassium atoms and AlO4 tetrahedra.

Structural systematics of SFCA-I type solid-solutions in the system CaO\u2013Fe2O3\u2013FeO\u2013Al2O3

Effects of Fe ↔ Al substitution on triclinic SFCA-I-type compounds with general formula A40O56 (A: Ca, Al, Fe3+, Fe2+) have been studied using single-crystal X-ray diffraction. Crystals of sufficient quality and size were synthesized in the temperature range between 1200 and 1300 °C. Six samples with Al/FeTot ratios of 0.127, 0.173, 0.216, 0.310, 0.349 and 0.459 have been structurally characterized. SFCA-I can be described with a modular approach involving the stacking sequence < PSS > of “P” and “S” modules that can be imagined as being cut from the well-known pyroxene (P) and spinel (S) structure types. Furthermore, SFCA-I is related to the sapphirine supergroup of minerals. Within the present solid-solution series, the contents in calcium show only minor variations (≈ 6.7 a.p.f.u.). The twenty crystallographically independent tetrahedrally (T) and octahedrally (M) coordinated cation sites exhibit considerable differences concerning the Al uptake. Indeed, Al is preferentially incorporated into the tetrahedra belonging to the single-chains located in the pyroxene modules. Ferrous iron, on the other hand, is restricted to one of the T-positions within the spinel blocks. Most structural aspects from unit-cell parameters and cell volumes to site occupancies, tetrahedral chain kinking as well as polyhedral distortions are defined by linear or nearly linear trends when plotted against the Al/FeTot ratio. Analysis of the < T–O > and < M–O > distances showed a complex interplay between the different coordination polyhedra resulting in a contrasting behavior of these values with positive or negative change rates as a function of composition. Evaluation of the average chemical strain tensor derived from the sets of lattice parameters for the two samples of the abovementioned series showing the highest and lowest Al/FeTot ratios indicated, that the major contraction with increasing Al content is perpendicular to the pyroxene- and spinel modules. Furthermore, the pyroxene module seems to be more affected when compared with the spinel block. There is evidence that the SFCA-I-type solid-solution series is limited on both the Al- and Fe-rich sides. The present investigation provides—for the first time—a detailed crystallographic analysis on the impact of chemical variations on a compound that is of relevance to the field of applied mineralogy related to the technologically important process of iron-ore sintering.

Reverse Water Gas Shift by Chemical Looping with Iron-Substituted Hexaaluminate Catalysts

The Fe-substituted Ba-hexaaluminates (BaFeHAl) are active catalysts for reverse water-gas shift (RWGS) reaction conducted in chemical looping mode. Increasing of the degree of substitution of Al3+ for Fe3+ ions in co-precipitated BaHAl from 60% (BaFeHAl) to 100% (BaFe-hexaferrite, BaFeHF), growing its surface area from 5 to 30 m2/g, and promotion with potassium increased the CO capacity in isothermal RWGS-CL runs at 350–450 °C, where the hexaaluminate/hexaferrite structure is stable. Increasing H2-reduction temperature converts BaFeHAl to a thermally stable BaFeHF modification that contains additional Ba-O-Fe bridges in its structure, reinforcing the connection between alternatively stacked spinel blocks. This material displayed the highest CO capacity of 400 µmol/g at isothermal RWGS-CL run conducted at 550 °C due to increased concentration of oxygen vacancies reflected by greater surface Fe2+/Fe3+ ratio detected by XPS. The results demonstrate direct connection between CO capacity measured in RWGS-CL experiments and calculated CO2 conversion.

Structural confirmation and elucidation of spectral, optical and magnetic properties of cerium doped lead hexaferrite

Cerium ion doped M-type lead hexaferrite with chemical formula Pb1-xCexFe12O19 (where x = 0.0, 0.1, 0.2 and 0.3) were prepared using sol-gel auto combustion process. XRD patterns revealed that all diffraction peaks are in good agreement with magnetoplumbite mineral crystal structure. Unit cell ‘c’ value slightly decreases and ‘a’ value almost remains constant, when the concentration of Ce ion is increased. Magnetic properties were analysed using Vibrating Sample Magnetometer. It was observed that saturation magnetization (Ms), retentivity (Mr) decreases and coercivity (Hc) increases with increasing Ce concentration and the samples become more and more hard magnetic. Observed tetrahedral, octahedral and spinel block vibrations in Raman spectra establishes the molecular structure of synthesized samples. Optical transmission spectral analysis reveals that the samples are transparent in the entire visible region and the band gap are in the range 2.44–2.91 eV.

Effect of incorporation of nitrogen on calcium hexaaluminate

Ions doping is regarded as an efficient approach to improve the density of calcium hexaaluminate (CA6). In this work, different amounts of N3− have been tried to dope into CA6 (CaAl12O(19-1.5x)Nx, x = 0-0.8) by pressureless sintering under N2 at 1750 °C using CaO, Al2O3 and Al fine powder as the raw materials. The results of the synthesized specimens characterized by XRD in combination with SEM and EDS show that a small amount of N3- up to x = 0.30 can be incorporated into CA6, increasing the thickness of the CA6 grains. Besides, first-principle calculation and structure refinement analysis suggest that the structure formula of the N-doped CA6 is CaAl12O18.58N0.28, and it is preferential for N3− to substitute O(3) ions near Al(2) in the spinel block. Such a substitution occurs in the Al-O (Al2-O3) bond parallel to c axis, and the bond lengths for tetrahedral Al-N bonds are larger than those of Al-O bonds, showing a promising potential of N3− for increasing the c-axis crystallization thickness of CA6.

Microstructural evolution of Ti4+-doped calcium hexaaluminate ceramics

Calcium hexaaluminate (CaAl12O19 or CA6) ceramics were fabricated by solid-state reaction sintering of Al2O3 and CaCO3 powder mixtures. The influence of Ti4+ doping on microstructural development of CA6 ceramics was investigated. After doping Ti4+, the lengths and widths of the CA6 grains increased, and the aspect ratios decreased significantly. Al vacancies (VAl) in mirror planes of CA6 are created by substituting Ti4+ for Al3+. As more VAl generates in the mirror planes, vacancy concentration varies considerably between mirror planes and spinel blocks. Combined with the fact that the ionic radius of Ti4+ is larger than that of Al3+, the crystal growth of CA6 along c axis is more promoted than that along basal plane. CaTiO3 and residual Al2O3 were observed in Ti4+-doped samples because of the reactions between CaO and TiO2.

Tuning the Magnetic Order in Sc-Substituted Barium Hexaferrites

Polycrystalline BaFe12- x Sc x O19 hexaferrites ( x = 0.1, 0.6, and 1.2), produced using ceramic technology, have hexagonal structure with space group (P63/mmc) as determined by neutron powder diffraction. Magnetic properties were analyzed by vibrating sample magnetometry and Mossbauer spectroscopy. Tuning the magnetic parameters by Sc substitution is associated with several factors: first, ordering the Sc3+ ions in positions 2 b and 4 f 2 and the cation vacancies at the boundary of the spinel and hexagonal blocks; second, transfer of spin density from Fe ions localized at position 12 k to 3 d orbitals of Sc3+ ions; third, weakening the indirect exchange interactions between 2 b , 4 f 2, and 12 k sublattices; and finally, presence of a noncollinear magnetic order for Sc concentration x above 0.6.

Zn Incorporation in X and Z-Type Hexagonal Ferrites Ba(Zn,Fe)15O23 and Ba3(Zn,Fe)26O41

Interatomic distance considerations and site occupancy refinements indicate that Zn2+ is exclusively located on tetrahedral sites in both Z and X-type hexagonal ferrites. Zinc is more strongly enriched in four layer thick spinel blocks of X-type hexaferrite than in the two layer thick spinel blocks of both magnetic oxides. No zinc was incorporated in the tetrahedra of the T blocks of Z-type hexaferrite. Bond valence analyses reveal different local charge compensation requirements as origin of the unequal distribution of diamagnetic Zn2+ over the tetrahedral sites, which strongly modify the magnetic properties. Diamagnetic zinc is more strongly enriched on the tetrahedral sites in the four layer thick spinel blocks of X-type hexaferrite than in the two layer thick spinel blocks of both compounds which strongly modifies their magnetic properties.

Corrosion of MgAl&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4 &lt;/sub&gt;Spinel with Different Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; Contents in Iron-Containing Gasifier Slag

The MgAl2O4 spinel ceramic specimens with five different alumina contents ( w(Al2O3)=66%,72%,78%,85%,90%) were prepared using alumina micropowder and magnesia micropowder as raw materials, after mixing, shaping, drying, and heating at 1800 °C. The crucible specimens were tested for slag corrosion with commercial gasifier slag at 1500 °C for 2 h under oxidazing and reducing atmosphere. The specimens before and after slag test were studied by analyzing the microstructure and element distributions of corroded specimens with XRD, SEM and EDS. The results show that the slow dissolution of spinel into slag was observed because a low melting point material was formed by spinel reacting with CaO and SiO2 in slag on the surface of the spinel block. Meanwhile, FeOx in the slag was absorbed around the surface of spinel block to form the multiple solid solution, which was composed of MgO-Al2O3-FeOx and had a denser microstructure. The absorption of FeOx in slag had contributed to the resistance to slag penetration for the spinel. The relationship between the absorption capacity of spinel on FeOx and the chemical composition of spinel was discussed.

Thermometry of red nanoflaked SrAl12O19:Mn4+ synthesized with boric acid flux

This paper presents the luminescence properties and potential of red SrAl12O19:Mn4+ (SAO:Mn4+) phosphor for optical thermometry application. The SAO crystal consisted of a spinel block along with two mirror-like blocks. The Al3+/Sr2+ molar ratio of the precursor solution affected the crystalline-phases, morphology, and photoluminescence of the phosphor. The addition of flux H3BO3 promoted the growth of hexagonal-nanoflakes and enhanced the external quantum efficiency of phosphor 2.6-fold. The absolute sensitivity Sa and relative sensitivity Sr of SAO:Mn4+ showed a linear function of the temperature. The value of Sa was 4.17 × 10−3 K−1, and the maximum Sr was 2.70 × 10−3 K−1 at 393 K. A stable emission color was observed even with a change in temperature and a bright red light was seen in both daylight and a darkroom.

Effect of Magnetic Pulse Processing on the Structure and Magnetic Properties of Ferrites

The effect a pulsed magnetic field has on the crystal structure and macroscopic magnetic parameters of hexagonal ferrites BaFe12O19 and SrFe12O19 are studied. It is shown that changes in the physical properties of ferrites are due to the ordering of cation vacancies on the boundaries of hexagonal and spinel blocks that minimize local distortion of the oxygen polyhedrons. Violation of the collinear ordering of the magnetic moments of iron ions in the nonequivalent positions of SrFe12O19 ferrite is observed, due to the selective localization of such vacancies (and thus violations of the magnetic relationships in Fe–O–Fe).

Study of the features of the magnetic and crystal structures of the BaFe12-Х ALXO19 AND BaFe12-Х GaXO19 substituted hexagonal ferrites

The mechanism of formation of polycrystalline hexagonal barium ferrite with diamagnetic substitution with Al and Ga ions is considered. The localization of the dopants (Al and Ga) in hexagonal and spinel blocks is shown. It is found that the main Al – Fe and Ga – Fe substitutions occur in the 12k sublattice. The substitutions break the Fe–O–Fe exchange couples of the 12k sublattice with other sublattices, which leads to the emergence of non-equivalent positions of Fe3+ions, on the basis of which in the Mossbauer spectra with isomorphic aluminum, 7 sextets are identified and with isomorphic gallium – 6. This leads to a decrease in the magnetic parameters of ferrites, such as specific and residual magnetization, Curie temperature. It is shown that aluminum (x=2.1) entry increases the coercive force, magnetic hardness of the hexagonal ferrite, and gallium (x=0.6) entry lowers the coercive force and magnetic hardness. The angles θ between the magnetic moment and the γ-radiation direction in isotropic substituted hexagonal ferrite polycrystals and foil are determined.

Substitution of Ba for Ca in the Structure of CaAl12O19

The effect of Ba2+ on the crystallization thickness of calcium hexaaluminate (CA6) along the c axis was studied. The results show that after introduction of Ba2+, the CA6 flake crystals fired at 1600°C become thicker obviously. The structure refinement shows that the structure formula of the doped CA6 is (Ca0.973Ba0.027)O·6Al2O3, indicating Ba2+ has entered the lattices of CA6 and substituted Ca2+ in the mirror layers. The introduction of Ba2+ results in more defects VAl in spinel block layers in CA6, making ions easier to diffuse along c axis and fastening the growing of CA6 crystals along c axis. Thus, the introduction of Ba2+ is feasible and advantageous to increase the crystallization thickness of CA6 along c axis.

ChemInform Abstract: Hexaaluminates: A Review of the Structure, Synthesis and Catalytic Performance

AbstractReview: 138 refs.

Mechanism of CeMgAl11O19: Tb3+ alkaline fusion with sodium hydroxide

Comprehensive CeMgAl11O19: Tb3+ (CTMA) disintegration via alkaline fusion was discussed. The rare earth (RE) elements in CTMA were dissolved by HCl completely after alkaline fusion. Relationships between the alkaline fusion temperature and various properties of the compounds were examined by various techniques to elucidate their roles in the expected CTMA disintegration. X-ray diffraction (XRD) analysis indicates the phase transformation sequence. A scientific hypothesis of crystal structure disintegration presents that sodium ions substitute for the europium and barium ions in the mirror plane and magnesium ions in the spinel block successively, resulting in that more oxygen vacancies and interstitial sodium ions appear. The unit cell [P63/mmc (194)] breaks from the mirror plane. Then it is decomposed into NaAlO2, and magnesium, cerium, and terbium ions combine with free OH− into MgO, Tb2O3 and CeO2; Tb2O3 and CeO2 change into Ce0.6Tb0.4O2−x . In the end, the rare earth oxide is recycled easily by the acidolysis. The mechanism provides fundamental basis for recycling of REEs from waste phosphors.

Resonant X-ray magnetic scattering study of Ti-Mn-doped Ba ferrites

BaFe12O19 has M-type hexaferrite structure, giving strong uniaxial anisotropy of the magnetization along c axis. In doped BaTixMnxFe12-2xO19, the substitution of Fe3+ by a pair of Ti4+ and Mn2+ results in a weakening of the magnetic interactions due to the canting of magnetic moments. The crystal structure has a sequence of (Fe6O8) spinel and (BaFe6O11) hcp blocks, associated with five independent Fe sites: tetrahedral 4f1, bipyramidal 2b, and octahedral 2a, 4f2 and 12k sites. Single crystals synthesized by a flux method have cell dimensions of a = 5.8953(2) and c = 23.1887(8) Å and a = 5.9039(2) and c = 23.2047(8) Å for samples of x = 0.5 and 1.0, respectively. Each crystal was mounted with short glass fiber on rare-earth magnet and goniometer head. Conventional intensity measurements were made using a Rigaku AFC7 four-circle diffractometer with a graphite (002) monochromator for Mo Kα radiation. Site-occupancy experiments were performed by using a vertical-type four-circle diffractometer at PF-BL-10A. In this study the magnetic structure has been examined to use resonant X-ray magnetic scattering (RXMS) at the Fe K absorption edge in PF-BL-6C. Circularly polarized X rays produced by a diamond phase retarder were switched between right-handed (helicity=+1) and left-handed polarizations. An AFC-5u four-circle diffractometer was used in the geometry of horizontal scattering plane. Intensity measurements were made to collect as many as possible of Bragg reflections. Based on an asymmetrical ratio of the intensities between right- and left- handed circular polarizations, the intensity difference in the RXMS was estimated for each of reflections. A set of observed asymmetrical ratios was fitted to a model of spin orientation in crystal-structure calculations with the resonant magnetic scattering factors. A residual function in the least-squares fitting is useful to evaluate magnetic moments for individual sites, which is a multiplicity parameter refined in terms of atomic scattering factor. It gives a minimum in plot between site-multiplicity and residual function, assigning the multiplicity related to the canting of magnetic moment in the Fe sites. The magnetic structure obtained in this study is compared with those of M-type BaFe12O19 and BaTiCoFe10O19.

Thermal Evolution Crystal Structure and Fe Crystallographic Sites in LaFexAl12–xO19 Hexaaluminates

Compared with BaFe12O19 hexaferrite, La-magnetoplumbite (MP) hexaaluminates with a high-substituted concentration have rarely been investigated. LaFexAl12–xO19 catalysts at various Fe loadings (x = 0–12) were synthesized by coprecipitation. Thermal evolution of the samples from 700 up to 1400 °C has been followed by X-ray diffractometry. Fe distribution in different crystallographic sites in the final MP structure was clarified by means of Rietveld refinement and room-temperature and low-temperature 57Fe Mössbauer spectroscopy. The results showed that Fe doping (x = 1–9) lowered the threshold temperature of La-hexaaluminates. When x ≥ 6, the segregation of LaFeO3 intermediates essentially hindered the formation of monophasic MP phase and even led to the absence of the MP structure when x = 12. Compared with the unsubstituted sample, Fe-substituted La-hexaaluminates (x ≤ 11) were more stoichiometric. When x = 1–3, Fe preferentially entered the Al(2) sites within spinel blocks and the Al(3) and Al(5) sites in mirror planes. Increasing x to ≥4, Al(1) and Al(4) sites in the spinel block started to be systematically occupied up to x = 11. The reasonably constant turnover frequencies ((0.16 to 0.47) × 102 h–1 m–2) indicated a correlation between the N2O catalytic decomposition performance and Fe3+ ions in the mirror plane (Al(3), Al(5)).