On the electronic and structural properties of aluminum diboride Al0.9B2

Single crystals of aluminum diboride (space group P6/mmm, No. 191) a=3.0050(1) Å, c=3.2537 (8)  Å; Z=1) were prepared by the aluminum flux method. Crystal structure refinement shows defects at the aluminum site and resulted in composition Al_0.894(9)B₂≈Al_0.9B₂. The defect structure model is confirmed by the measured mass density ρ_exp=2.9(1) g/cm³ in comparison with a calculated value ρ_x=3.17 g/cm³ for full occupancy of the aluminum position. The results of ¹¹B NMR measurements support the defect model and are in agreement with the structure obtained by X-ray diffraction methods. Electrical resistivity measured on a single crystal parallel to its hexagonal basal plane with ρ(300 K)−ρ(2 K)=2.35 μΩ cm shows temperature dependence like a typical metal. Charge is dominantly carried by holes (Hall-coefficient R=+2×10⁻¹¹ m/C). Respective, p-type conductivity is confirmed by theoretical calculations. Chemical bonding in aluminum diboride is discussed using the electron localization function.     

Crystal structure of Eu-doped magnetoplumbite-type lanthanum aluminum oxynitride with emission site splitting

Eu-doped lanthanum aluminum oxynitride (LaAl₁₂(O,N)₁₉) with magnetoplumbite structure was prepared by nitridation of the oxide precursor obtained from aluminum glycine gel and subsequent post-annealing. Eu-doped lanthanum aluminum oxynitride exhibited blue light emission at 440 nm with a shoulder at 464 nm under excitation at 254 nm. Isostructural Eu-doped calcium aluminum oxide (CaAl₁₂O₁₉) exhibited a single emission peak at 415 nm. Structural refinement using neutron powder diffraction indicated that the lanthanum site occupied partially by Eu²⁺ splits into 2d and 6h sites in the aluminum oxynitride. The longer emission and the shoulder peak in the former aluminum oxynitride were observed in relation to the increasing covalency as well as crystal field splitting around doped Eu²⁺ induced by site splitting involved with the two kinds of anions.     

Crystal-chemistry of mullite-type aluminoborates Al18B4O33 and Al5BO9: A stoichiometry puzzle

Orthorhombic Al₂O₃-rich aluminoborate is an important ceramic material for which two slightly different compositions have been assumed: Al₅BO₉ (5Al₂O₃:B₂O₃) and Al₁₈B₄O₃₃ (9Al₂O₃:2B₂O₃). The formula Al₁₈B₄O₃₃ (=Al_4.91B_1.09O₉) was derived from results of chemical analyses when crystal structure data were not yet available. Subsequent structural investigations indicated Al₅BO₉ composition. Nevertheless, Al₁₈B₄O₃₃ was still accepted as the correct stoichiometry assuming that additional B replaces 9% Al.Powder samples of both compositions and ones with excess boron were prepared by solid state reactions between α-Al₂O₃ and B₂O₃/H₃BO₃ at temperatures above 1100 °C and single-crystals were grown from flux at 1100 and 1550 °C. Products were investigated by single-crystal and powder XRD, ¹¹B and ²⁷Al solid-state MAS-NMR, Raman and FTIR spectroscopy as well as Laser-ablation ICP-MS. No indication of the predicted 9% B→Al substitution was found. LA ICP-MS indicated 12.36(27) wt% B₂O₃ corresponding to Al_4.97B_1.03O₉. Hence, the suggested Al₁₈B₄O₃₃ stoichiometry can be excluded for all synthesized samples. A very low amount of Al vacancies at a five-fold coordinated site are likely, charge balanced by an additional nearby three-fold coordinated B site. All evidences indicate that the title compound should be reported as Al_5−xB_1+xO₉ with x<0.038(6), which is close to Al₅BO₉.     

The formation and physicochemical characterization of Al2O3-doped manganese ferrites

Mn/Fe mixed oxide solids doped with Al₂O₃ (0.32-1.27 wt.%) were prepared by impregnation of manganese nitrate with finely powdered ferric oxide, then treated with different amounts of aluminum nitrate. The obtained samples were calcined in air at 700-1000 °C for 6 h. The specific surface area (S_BET) and the catalytic activity of pure and doped precalcined at 700-1000 °C have been measured by using N₂ adsorption isotherms and CO oxidation by O₂. The structure and the phase changes were characterized by DTA and XRD techniques. The obtained results revealed that Mn₂O₃ interacted readily with Fe₂O₃ to produce well-crystallized manganese ferrite (MnFe₂O₄) at temperatures of 800 °C and above. The degree of propagation of this reaction increased by Al₂O₃-doping and also by increasing the heating temperature. The treatment with 1.27 wt.% Al₂O₃ followed by heating at 1000 °C resulted in complete conversion of Mn/Fe oxides into the corresponding ferrite phase. The catalytic activity and S_BET of pure and doped solids were found to decrease, by increasing both the calcination temperature and the amount of Al₂O₃ added, due to the enhanced formation of MnFe₂O₄ phase which is less reactive than the free oxides (Mn₂O₃ and Fe₂O₃). The activation energy of formation (ΔE) of MnFe₂O₄ was determined for pure and doped solids. The promotion effect of aluminum in formation of MnFe₂O₄ was attributed to an effective increase in the mobility of reacting cations.     

Magnetron sputtering of silver nanowires using anodic aluminum oxide template: a new active substrate of surface enhanced Raman scattering and an investigation of its enhanced mechanism

A high quality anodic aluminum oxide (AAO) template with ordered apertures about 50-80 nm was fabricated by anodizing aluminum in electrolytes through a two-step method, and silver nanowires with diameters from 40nm to 70nm were prepared on this AAO template by magnetron sputtering. On the glass covered with silver nanowires, high quality surface enhanced Raman scattering (SERS) spectra of sudan II (C₁₈H₁₆N₂O) with enhancement factors of 10⁵ were obtained. And comparison of SERS spectra on silver nanowires with the SERS spectra of silver colloids indicates that main enhanced mode is lightning rod effect of nanorods on the Sudan II/silver nanowires system.     

Synthesis and structural characterization of a new aluminum oxycarbonitride, Al5(O, C, N)4

A new aluminum oxycarbonitride, Al₅(O_xC_yN_4−x−y) (x∼1.4 and y∼2.1), has been synthesized and characterized by X-ray powder diffraction, transmission electron microscopy and electron energy loss spectroscopy (EELS). The title compound was found to be hexagonal with space group P6₃/mmc, Z=2, and unit-cell dimensions a=0.328455(6) nm, c=2.15998(3) nm and V=0.201805(6) nm³. The atom ratios O:C:N were determined by EELS. The final structural model, which is isomorphous with that of (Al_4.4Si_0.6)(O_1.0C_3.0), showed the positional disordering of one of the three types of Al sites. The maximum-entropy method-based pattern fitting (MPF) method was used to confirm the validity of the split-atom model, in which conventional structure bias caused by assuming intensity partitioning was minimized. The reliability indices calculated from the MPF were R_wp=6.94% (S=1.22), R_p=5.34%, R_B=1.35% and R_F=0.76%. The crystal was an inversion twin. Each twin-related individual was isostructural with Al₅C₃N (space group P6₃mc, Z=2).     

Thermal behavior of aluminum powder and potassium perchlorate mixtures by DTA and TG

In this work the thermal decomposition characteristics of micron sized aluminum powder + potassium perchlorate pyrotechnic systems were studied with thermal analytical techniques. The results show that the reactivity of aluminum powder in air increases as the particle size decreases. Pure aluminum with 5 μm particle size has a fusion temperature about 647 °C, but this temperature for 18 μm powder is 660 °C. Pure potassium perchlorate has an endothermic peak at 300 °C corresponding to a rhombic-cubic transition, a fusion temperature around 590 °C and decomposes at 592 °C. DTA curves for Al₅/KClO₄ (30:70) mixture show a maximum peak temperature for thermal decomposition at 400 °C. Increasing the particle size of aluminum powder increases the ignition temperature of the mixture. The oxidation temperature increased by enhance in the aluminum content of the mixture.     

A series of aluminum tungsten oxides crystallizing in a new ReO3-related structure type

A series of new aluminum tungsten oxides with the general formula Al₄W_2nO_6n+2 (n=4-7) was found and structurally characterized by electron diffraction, high-resolution transmission electron microscopy (HRTEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The structural model for Al₄W₁₀O₃₂ (I4/mmm (space group no. 139); a≈0.375, c≈3.95 nm) consists of slabs of [5×∞×∞] corner-sharing WO₆ octahedra that are connected via edge-sharing to AlO₆ octahedra. Simulated HRTEM images agree well with the experimental ones and thus support the proposed structural model. The connection between adjacent slabs of WO₃ via AlO₆ octahedra represents a novel variant of crystallographic shear operation for ReO₃-type structures. The crystallites display a wide range of stacking sequences that are frequently intergrown with each other.     

A new phase in the system lithium-aluminum: Characterization of orthorhombic Li2Al

Investigation of the Li rich part of the binary Li-Al system revealed the existence of a new phase, orthorhombic Li₂Al, which is isostructural to Li₂Ga and Li₂In. The crystal structure was determined from single crystal X-ray diffraction data (Cmcm, a=4.658(2) Å, b=9.767(4) Å, c=4.490(2) Å, Z=4). Refinement of atomic position site occupancies yielded a composition Li_1.92Al_1.08 (64 at% Li) indicating a small homogeneity range, Li_2−xAl_1+x. Li₂Al is the peritectic decomposition product of the stoichiometric compound Li₉Al₄, which is stable below 270±2 °C. Li₂Al itself decomposes peritectically to Li₃Al₂ and Li rich melt at 335±2 °C. The discovery of Li₂Al (Li_2−xAl_1+x) settles a long standing inconsistency in the Li-Al phase diagram which was based on the assumption that Li₉Al₄ possesses a high temperature modification.     

Silica gel modified with lumogallion for aluminum determination by spectroscopic methods

Simple, easy to use and selective method of Al(III) sorption-spectroscopic (SS) determination was proposed. For this purpose, silica modified with tridecyloctadecylammonium chloride(SGII) using adsorption technique and glass slide modified with thin silica-poly(dimethyldiallyl-ammonium chloride) (SGI) composite film obtained by sol-gel technique were worked out. It was shown that lumogallion (LG) easily absorbs on SGI and SGII. Obtained sorbents SGIII and SGIV, respectively, were used for aluminum(III) determination by diffuse reflectance and spectrophotometric methods. The ranges of determination were (mg L⁻¹): (0.08-0.54), s_r ≤ 0.13, n = 4 for SGIII and (0.05-2.0), s_r ≤ 0.11, n = 4 for SGIV. The detection limits (blank + 3σ) for aluminum were 70 and 30 μg L⁻¹ using SGIII and SGIV, respectively, where σ is the standard deviation of blank estimation. The accuracy of the developed spectrophotometric method was examined by the determination of standard addition of aluminum in alcohol-free beverages. The relative error did not exceed 9%. SGIII can be regenerated by 0.05 M EDTANa₂H₂ solution and reused. SGIV was shown to be perspective for determination of aluminum in solution in the range of 0.01-0.13 mg L⁻¹ by solid phase luminescent technique.     

Enhancement of photocatalytic activity in UV-illuminated tin dioxode/aluminum oxide system in aqueous 4-nitrophenol

A modified tin dioxide/aluminum oxide was prepared by co-precipitation, and its photocatalytic activity was measured. Tin dioxide modified by adding aluminum oxide had a higher photocatalytic activity than other photocatalysts in this study, especially at pH 7. A mass balance for carbon with UV-254 nm/SnO₂/Al₂O₃ system at pH 7 indicated that the amount of the intermediate increased with reaction time, but decreased after 120 min of photodegradation. The results reveal that the photocatalytic carrier, Al₂O₃, not only increased the amount of 4-nitrophenol removed but also promoted mineralization and destroyed 4-nitrophenol, which was degraded to other simple organic substances or inorganic substances.     

High-temperature X-ray diffraction and specific heat studies on GdAlO3, Gd3Al5O12 and Gd4Al2O9

Gadolinium aluminates, GdAlO₃, Gd₃Al₅O₁₂ and Gd₄Al₂O₉ were synthesized by the solution combustion method. Very fine particles in the nanoparticle range of ∼10-20 nm could be prepared by this method as evidenced by surface area measurement by multipoint BET method. Thermal studies on these compounds were carried out using high-temperature X-ray diffraction (HT-XRD) and differential scanning calorimetry (DSC) methods. The thermal expansion coefficients of GdAlO₃, Gd₃Al₅O₁₂ and Gd₄Al₂O₉ were calculated from the lattice parameter data and specific heats were calculated from DSC data. The lattice parameters of GdAlO₃ and Gd₃Al₅O₁₂ were found to increase linearly with temperature whereas Gd₄Al₂O₉ did not show a linear trend. The specific heats of these compounds show an increasing trend with increase in aluminum atom fraction. Based on the thermodynamic data available in the literature and the specific heat data obtained in this study, oxygen potential diagram was constructed at 1000 K.     

Synthesis, structural characterization and optical properties of a new cesium aluminum borate, Cs2Al2B2O7

A new borate, Cs₂Al₂B₂O₇, was synthesized by solid-state reaction. It crystallizes in the monoclinic space group P2₁/c with a=6.719(1) Å, b=7.121(1) Å, c=9.626(3) Å, β=115.3(1)°, and Z=2. In the structure, two AlO₄ tetrahedra and two BO₃ planar triangles are connected alternately by corner-sharing to from nearly planar [Al₂B₂O₁₀] rings, which are further linked via common O1 atom to generate layers in the bc plane. These layers then share the O3 atoms lying on a center of inversion to form a three-dimensional framework with Cs atoms residing in the channels. The IR spectrum confirms the presence of both BO₃ and AlO₄ groups and the UV-vis-IR diffuse reflectance spectrum indicates a band gap of about 4.13(2) eV.     

A novel gaseous pinacolyl alcohol sensor utilizing cataluminescence on alumina nanowires prepared by supercritical fluid drying

A cataluminescence (CTL) sensor using Al₂O₃ nanowires as the sensing material was developed for the determination of trace pinacolyl alcohol in air samples based on the catalytic chemiluminescence (CL) of pinacolyl alcohol on Al₂O₃ nanowires. Eight catalysts were examined and the CL intensity on Al₂O₃ nanowires prepared by supercritical fluid drying was the strongest. This novel CL sensor showed high sensitivity and selectivity to gaseous pinacolyl alcohol at optimal temperature of 340 °C. Quantitative analysis was performed at a wavelength of 460 nm. The linear range of CTL intensity versus concentration of gaseous pinacolyl alcohol was 0.09 × 10⁻⁶ to 2.56 × 10⁻⁶ g mL⁻¹ (r = 0.9983, n = 6) with a detection limit (3σ) of 0.0053 × 10⁻⁶ g mL⁻¹. None or only very low levels of interference were observed while the foreign substances such as water vapor, ethanol, ammonia, chloroform, benzene, nitrogen dioxide, methylbenzene, hydrochloric acid, methanol and butanol were passing through the sensor. The response time of the sensor is less than 100 s, and the sensor had a long lifetime more than 60 h. The sensor would be potentially applied to analysis of the nerve agents such as Soman.     

High-pressure transitions in MgAl2O4 and a new high-pressure phase of Mg2Al2O5

Phase transitions in MgAl₂O₄ were examined at 21-27 GPa and 1400-2500 °C using a multianvil apparatus. A mixture of MgO and Al₂O₃ corundum that are high-pressure dissociation products of MgAl₂O₄ spinel combines into calcium-ferrite type MgAl₂O₄ at 26-27 GPa and 1400-2000 °C. At temperature above 2000 °C at pressure below 25.5 GPa, a mixture of Al₂O₃ corundum and a new phase with Mg₂Al₂O₅ composition is stable. The transition boundary between the two fields has a strongly negative pressure-temperature slope. Structure analysis and Rietveld refinement on the basis of the powder X-ray diffraction profile of the Mg₂Al₂O₅ phase indicated that the phase represented a new structure type with orthorhombic symmetry (Pbam), and the lattice parameters were determined as a=9.3710(6) Å, b=12.1952(6) Å, c=2.7916(2) Å, V=319.03(3) Å³, Z=4. The structure consists of edge-sharing and corner-sharing (Mg, Al)O₆ octahedra, and contains chains of edge-sharing octahedra running along the c-axis. A part of Mg atoms are accommodated in six-coordinated trigonal prism sites in tunnels surrounded by the chains of edge-sharing (Mg, Al)O₆ octahedra. The structure is related with that of ludwigite (Mg, Fe²⁺)₂(Fe³⁺, Al)(BO₃)O₂. The molar volume of the Mg₂Al₂O₅ phase is smaller by 0.18% than sum of molar volumes of 2MgO and Al₂O₃ corundum. High-pressure dissociation to the mixture of corundum-type phase and the phase with ludwigite-related structure has been found only in MgAl₂O₄ among various A²⁺B³⁺₂O₄ compounds.     

Combined DTA and XRD Study of Sintering Steps Towards YAl3(BO3)4

Sintering processes in the Y₂O₃–Al₂O₃–B₂ O₃ system and its subsystems (Y₂O₃–B₂O₃ and Al₂ O₃–B₂O₃) have been investigated by using combined DTA and XRD measurements to get a better understanding of solid state chemical changes resulting in the formation of yttrium aluminum borate (YAl₃(BO₃)₄, YAB) phase and to study the possible role and contribution of various simple borates formed also in the former processes. Two new exothermic heat effects of YBO₃ formation have been detected by DTA in the Y₂O₃–B₂O₃ system between 720 and 980°C. In the Al₂O₃–B₂O₃ system a new experimental XRD profile of Al₄B₂O₉ was observed. Formation of these borates seems to promote the nucleation of double borate YAB below 1000°C. Conversion of Al₄B₂O₉ to Al₁₈B₄ O₃₃ was observed after a long term (10 h) sintering at 1050°C. Similarly, an increased formation of YAB has been observed as a product of the sintering reaction between YBO₃ and Al₁₈B₄O₃₃ at 1150°C. The two latter single borates are found to be identical with the high temperature decomposition products of YAB. This revised version was published online in July 2006 with corrections to the Cover Date.     

Alcoholysis of Al2(OtBu)6 – Synthesis and Crystal Structure of Al9O3(OEt)21

When Al₂(OtBu)₆ is treated with ethanol, Al₉O₃(OEt)₂₁ ( 1 ) is obtained, which is a missing link in the series of polynuclear aluminum alkoxides. Alcoholysis of Al₂(OtBu)₆ in 2‐propanol yields the well‐known homoleptic compound Al₄(OiPr)₁₂ ( 2 ). As recently published, similar reactions with Fe₂(OtBu)₆ gave different structures. However, there are recurring structural patterns from alkoxide chemistry found. For a deeper understanding of this hardly predictable chemistry, compounds have to be correlated by such common structural motifs. We briefly report the syntheses of 1 and 2 and the crystal structure of 1 . In addition, we provide an improved synthetic procedure for the preparation of the precursor Al₂(OtBu)₆. The structure of the new compound 1 is comprehensively compared to related structures from literature.     

Synthesis and structural characterization of Al4Si2C5-homeotypic aluminum silicon oxycarbide, (Al6−xSix)(OyC5−y) (x∼0.8 and y∼1.6)

We have prepared a new layered oxycarbide, [Al_5.25(5)Si_0.75(5)][O_1.60(7)C_3.40(7)], by isothermal heating of (Al_4.4Si_0.6)(O_1.0C_3.0) at 2273 K near the carbon-carbon monoxide buffer. The crystal structure was characterized using X-ray powder diffraction, transmission electron microscopy and energy dispersive X-ray spectroscopy (EDX). The title compound is trigonal with space group R3?m (centrosymmetric), Z=3, and hexagonal cell dimensions a=0.32464(2) nm, c=4.00527(14) nm and V=0.36556(3) nm³. The atom ratios Al:Si were determined by EDX, and the initial structural model was derived by the direct methods. The final structural model showed the positional disordering of one of the three types of Al/Si sites. The reliability indices were R_wp=4.45% (S=1.30), R_p=3.48%, R_B=2.27% and R_F=1.25%. The crystal is composed of three types of domains with nearly the same fraction, one of which has the crystal structure of space group R3¯m. The crystal structure of the remaining two domains, which are related by pseudo-symmetry inversion, is noncentrosymmetric with space group R3m.     

Dependence of size and size distribution on reactivity of aluminum nanoparticles in reactions with oxygen and MoO3

The oxidation reaction of aluminum nanoparticles with oxygen gas and the thermal behavior of a metastable intermolecular composite (MIC) composed of the aluminum nanoparticles and molybdenum trioxide are studied with differential scanning calorimetry (DSC) as a function of the size and size distribution of the aluminum particles. Both broad and narrow size distributions have been investigated with aluminum particle sizes ranging from 30 to 160 nm; comparisons are also made to the behavior of micrometer-size particles. Several parameters have been used to characterize the reactivity of aluminum nanoparticles, including the fraction of aluminum that reacts prior to aluminum melting, heat of reaction, onset and peak temperatures, and maximum reaction rates. The results indicate that the reactivity of aluminum nanoparticles is significantly higher than that of the micrometer-size samples, but depending on the measure of reactivity, it may also depend strongly on the size distribution. The isoconversional method was used to calculate the apparent activation energy, and the values obtained for both the Al/O₂ and Al/MoO₃ reaction are in the range of 200-300 kJ/mol.     

Preparation of Mn3O4 nanocrystallites by low-temperature solvothermal treatment of γ-MnOOH nanowires

The preparation of trimanganese tetroxide (Mn₃O₄) nanocrystallites from γ-MnOOH nanowires under mild conditions has been achieved by two steps: first, γ-MnOOH nanowires with a mean diameter of about 12 nm and lengths of up to several micrometers were directly prepared via hydrothermal reaction between KMnO₄ and toluene in water at 180°C for 24 h; then, pure Mn₃O₄ nanocrystallites could be obtained by solvothermal treatment of the γ-MnOOH nanowires in ethylenediamine (EDA) and ethylene glycol (EG) at 150°C for 24 h. It was found that the Mn₃O₄ product obtained in EDA comprised well-defined nanocrystallites with the size in the range of 15-35 nm, while the one obtained in EG consisted of aggregated nanoparticles with the size of less than 18 nm.The possible formation mechanism of nanocrystalline Mn₃O₄ in EDA and EG and reasons for the different effects of various solvents on the products were also proposed.