Hydrothermal synthesis of antimony oxychloride and oxide nanocrystals: Sb4O5Cl2, Sb8O11Cl2, and Sb2O3

We described herein a facile solution-phase route to three nanocrystals of antimony oxychlorides and oxides (Sb₄O₅Cl₂, Sb₈O₁₁Cl₂, and Sb₂O₃), whose morphologies and phases were varied with the pH value of a reaction mixture or composition of a mixed solvent. In particular, the solvent composition controlled the selective preparation of cubic Sb₂O₃ (senarmontite) and orthorhombic Sb₂O₃ (valentinite). Both cubic and orthorhombic Sb₂O₃ samples exhibited strong emission properties.     

Phase equilibrium and intermediate phases in the Eu–Sb system

Rapid heating rate thermal analysis, X-ray diffraction, fluorescence spectrometry, and differential dissolution method were used to study the high-temperature phase equilibrium in the Eu–Sb system within the composition range between 37 and 96 at% Sb. The techniques were effective in determination of the vapor–solid–liquid equilibrium since intermediate phases except Eu₄Sb₃ evaporated incongruently after melting. A thermal procedure was developed to determine the liquidus and solidus lines of the T−x diagram. Six stable phases were identified: two phases, EuSb₂ and Eu₄Sb₃, melt congruently at 1045±10 °C and 1600±15 °C, the Eu₂Sb₃, Eu₁₁Sb₁₀, Eu₅Sb₄, and Eu₅Sb₃ phases melt incongruently at 850±8 °C, 950±10 °C, 1350±15 °C, and 1445±15 °C, respectively. The exact composition shifting of Sb-rich decomposable phases towards Eu₄Sb₃, the most refractory compound, was determined. The topology of the Eu–Sb phase diagram was considered together with that of the Yb–Sb system.     

Phase transitions, electrical conductivity and chemical stability of BiFeO3 at high temperatures

The multiferroic perovskite BiFeO₃ is reported to display two first order structural phase transitions. The structural phase transition at 925±5 °C is demonstrated to be first order by calorimetry and dilatometry. Electrical conductivity measurements revealed that the high temperature phase above 925±5 °C is semiconducting, in disagreement with recent reports. The sign and magnitude of the volumes of transition reflect the sign and magnitude of the discontinuities in electrical conductivity across the two first order phase transitions. A high partial pressure of oxygen was demonstrated to stabilise BiFeO₃ towards peritectic decomposition. Finally, the origins of the commonly observed decomposition of BiFeO₃ at high temperatures are discussed.     

Crystal chemistry of Co-doped Zn7Sb2O12

Zn₇Sb₂O₁₂ forms a full range of Co-containing α solid solutions, Zn_7−xCo_xSb₂O₁₂, with an inverse-spinel structure at high temperature. At low temperatures for x<2, the solid solutions transform into the low temperature β-polymorph. For x=0, the β→α transition occurs at 1225±25 °C; the transition temperature decreases with increasing x. At high x and low temperatures, α solid solutions are formed but are non-stoichiometric; the (Zn+Co):Sb ratio is >7:2 and the compensation for the deficiency in Sb is attributed to the partial oxidation of Co²⁺ to Co³⁺. From Rietveld refinements using ND data, Co occupies both octahedral and tetrahedral sites at intermediate values of x, but an octahedral preference attributed to crystal field stabilisation, causes the lattice parameter plot to deviate negatively from the Vegard's law. Sub-solidus compatibility relations in the ternary system ZnO-Sb₂O₅-CoO have been determined at 1100 °C for the compositions containing ?50% Sb₂O₅.     

Temperature-dependent structural study of microporous CsAlSi5O12

CsAlSi₅O₁₂ crystals were synthesized at high temperature by slow cooling of a vanadium oxide flux. Single-crystal X-ray diffraction structure analysis and electron microprobe analyses yielded the microporous CAS zeolite framework structure of Cs_0.85Al_0.85Si_5.15O₁₂ composition. High-temperature single-crystal and powder X-ray diffraction studies were utilized to analyze anisotropic thermal expansion. Rietveld refined cell constants from powder diffraction data, measured in steps of 25 °C up to 700 °C, show a significant decrease in expansion above 500 °C. At 500 °C, a displacive, static disorder-dynamic disorder-type phase transition from the acentric low-temperature space group Ama2 to centrosymmetric Amam (Cmcm in standard setting) was found. Thermal expansion below the phase transition is governed by rigid-body TO₄ rotations accompanied by stretching of T-O-T angles. Above the phase transition at 500 °C all atoms, except one oxygen (O6), are fixed on mirror planes. Temperature-dependent polarized Raman single-crystal spectra between −270 and 300 °C and unpolarized spectra between room temperature and 1000 °C become increasingly less resolved with rising temperature confirming the disordered static-disordered dynamic type of the phase transition.     

The influence of hematite nano-crystals on the thermal stability of polystyrene

A synthetic procedure based on thermal hydrolysis of iron(III) chloride solutions for the preparation of hematite (α-Fe₂O₃) sol consisting of nano-crystals (NCs) is described. The α-Fe₂O₃ NCs were characterized by transmission electron microscopy and X-ray diffraction measurements. Incorporation of α-Fe₂O₃ NCs into polystyrene (PS) was based on the transfer of α-Fe₂O₃ NCs from the aqueous phase to the organic solvent. A significant shift in the glass transition temperature of PS by 17 °C towards higher temperatures was observed after incorporation of α-Fe₂O₃ NCs. Also, the thermal stability of PS was improved by about 100 °C in the presence of 3.6 wt% of α-Fe₂O₃ NCs.     

CTAB-Mn3O4 nanocomposites: Synthesis,NMR and low temperature EPR studies

We are reporting on the synthesis of Mn₃O₄ nanoparticles and CTAB-Mn₃O₄ nanocomposites via a sonochemical route using MnCl₂, ethanol, NaOH and CTAB. The crystalline phase was identified as Mn₃O₄. The crystallite size of the CTAB-Mn₃O₄ nanocomposite was identified as 13 ± 5 nm from X-ray line profile fitting and the particle size from TEM was 107.5 ± 1.4 nm. The interaction between CTAB and the Mn₃O₄ nanoparticles was investigated by FTIR and ¹H NMR spectroscopies. Two different magnetic phase transitions were observed for both samples below the Curie temperature (43 °C) by using a low temperature Electron Paramagnetic Resonance (EPR) technique. Also we determined the effect of the capping with CTAB on the reduction in absorbed power.     

A chemical route for the synthesis of cubic bismuth zinc niobate pyrochlore nanopowders

Cubic bismuth zinc niobate pyrochlore (base composition (Bi_1.5Zn_0.5)(Zn_0.5Nb_1.5)O₇) powders were successfully prepared by a chemical method. The formation mechanism of the pyrochlore phase was investigated by TG-DSC, FT-IR, Raman, and X-ray diffraction (XRD). The optical bandgap for the powders treated at temperatures ranging from 500 to 700 °C is 3.0-3.1 eV, indicating low crystallization temperature for the pyrochlore phase. No detectable intermediary phases as BiNbO₄ or a pseudo-orthorhombic pyrochlore were observed at any time and the cubic-BZN phase was already formed after thermal treatment at temperatures as low as 500 °C. The phase formation study reveals that a well-crystallized single-phased nanopowder is obtained after calcination at 700 °C, indicating that the chemical synthesis conferred a higher chemical homogeneity and reactivity on the powder, modifying the crystallization mechanism.     

Synthesis and thermal decomposition of antimony(III) oxide hydroxide nitrate

The thermal decomposition of the only known antimony nitrate antimony(III) oxide hydroxide nitrate Sb₄O₄(OH)₂(NO₃)₂, whose synthesis routes were reviewed and optimized was followed by TG-DTA under an argon flow, from room temperature up to 750°C. Chemical analysis (for hydrogen and nitrogen) performed on samples treated at different temperatures showed that an amorphous oxide hydroxide nitrate appeared first at 175°C, and decomposed into an amorphous oxide nitrate above 500°C. Above 700°C, Sb₆O₁₃ and traces of -Sb₂O₄ crystallized.Author to whom all correspondence should be addressed     

Correlation of optical properties and temperature-induced irreversible phase transitions in europium-doped yttrium carbonate nanoparticles

Nanophase europium-doped yttrium carbonate precursors are subjected to heat treatments, ranging from 300 °C to 1100 °C for dwell times of 5 min, 30 min, and 180 min. XRD, TEM, FT-IR, fluorescence, fluorescence excitation, and fluorescence lifetime measurements are used to characterize the materials. Upon heating, the material transitions through several amorphous stages until it reaches the crystalline cubic Y₂O₃ phase. DSC measurements show an exothermic transition at 665.7 °C, indicating the formation of crystalline Y₂O₃. The grain size development is fitted by the relaxation equation and yields an activation energy of 50.3 kJ/mol. The amorphous phases are characterized by inhomogenously broadened optical spectra. Heating up to 700 °C leads to an increased fluorescence lifetime (from about 1 ms to 2.4 ms). As the material is heated to higher temperatures and completes the formation of the crystalline cubic Y₂O₃ phase, the optical spectra become narrower and the fluorescence lifetime decreases to about 1.2 ms.     

Novel simple methods for the synthesis of single-phase valentinite Sb2O3

Several methods with solid and dissolved reactants were investigated as possible routes for synthesis of single-phase valentinite Sb₂O₃. The methods are based on simple chemical reaction between SbCl₃ and NaOH. The method with solid state reactants was established on self-propagating room temperature reaction (SPRT), while wet syntheses were based on the same chemical reaction, and performed in either distilled water or absolute ethanol. The prepared powders were characterized by X-ray powder diffraction, scanning electron microscopy and field emission scanning electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction (SAED) and UV/vis diffuse reflectance spectroscopy. SPRT and aqueous solution syntheses resulted in single-phase valentinite Sb₂O₃, but with significantly different morphologies. In the case of SPRT method the obtained powder contains well crystallized prismatic shaped submicronic particles, with hexagonal or lozenge basis typical for valentinite crystal structure, while aqueous solution synthesis resulted in powder containing micronic agglomerates. The ethanolic solution synthesis product was Sb₂O₃ with cubic senarmontite as predominant phase and traces of orthorhombic valentinite. It was confirmed that not only the aggregate state, but also the choice of solvent has a great influence on the structural and optical characteristics of synthesized Sb₂O₃ powders.     

Kinetics of reduction of iron oxides by H2: Part II. Low temperature reduction of magnetite

This study deals with the reduction of Fe₃O₄ by H₂ in the temperature range of 210-950 °C. Two samples of Fe₃O₄ produced at 600 and 1200 °C, designated as Fe₃O₄₍₆₀₀₎ and Fe₃O_4(1200), have been used as starting material.Reduction of Fe₃O₄₍₆₀₀₎ by H₂ is characterized by an apparent activation energy ‘E_a’ of 200, 71 and 44 kJ/mol at T < 250 °C, 250 °C < T < 390 °C and T > 390 °C, respectively. The important change of E_a at 250 °C could be attributed to the removal of hydroxyl group and/or point defects of magnetite. This is confirmed during the reduction of Fe₃O_4(1200). While transition at T ≈ 390 °C is probably due to sintering of the reaction products as revealed by SEM.In situ X-rays diffraction reduction experiments confirm the formation of stoichiometric FeO between 390 and 570 °C. At higher temperatures, non-stoichiometric wüstite is the intermediate product of the reduction of Fe₃O₄ to Fe.The physical and chemical modifications of the reduction products at about 400 °C, had been confirmed by the reduction of Fe₃O₄₍₆₀₀₎ by CO and that of Fe₃O_4(1200) by H₂. A minimum reaction rate had been observed during the reduction of Fe₃O_4(1200) at about 760 °C. Mathematical modeling of experimental data suggests that the reaction rate is controlled by diffusion and SEM observations confirm the sintering of the reaction products.Finally, one may underline that the rate of reduction of Fe₃O₄ with H₂ is systematically higher than that obtained by CO in the explored temperature range.     

Synthesis of transparent mixed vanadia/niobia gels and their decomposition to a new metastable VNb9O25 phase

Controlled hydrolysis and condensation of a mixture of vanadyl-tris-n-propoxide, VO(OPr)₃, and niobium pentaethoxide, [Nb(OEt)₅]₂, at 5 °C in propanol yields clear and transparent gels in which the ratio of V:Nb is 1:1, 1:4.5 or 1:9. Oxalic acid and low temperatures are used to slow down the rate of condensation processes. At 800 °C, the thermal decomposition of a gel with the composition 1:9 forms a thermodynamically metastable, new phase of the composition VNb₉O₂₅. At lower temperatures, metastable solid solutions with TT-Nb₂O₅ structure (600 °C) and M-Nb₂O₅ structure (700 °C) are formed from the amorphous xerogel. The new VNb₉O₂₅ phase is structurally related to M-Nb₂O₅. The solid solution with M-Nb₂O₅ structure acts structure directing, leading preferentially to a monoclinic low-temperature form of VNb₉O₂₅. The full transformation of this metastable phase to the well known tetragonal VNb₉O₂₅ requires a annealing temperature of about 1000 °C.     

High-temperature X-ray diffraction study of crystallization and phase segregation on spinel-type lithium manganese oxides

To study crystallization process of spinel-type Li_1+xMn_2−xO₄, in-situ high-temperature X-ray diffraction technique (HT-XRD) was utilized for the mixture consisting of Li₂CO₃ and Mn₂O₃ as starting material in the temperature range of 25-700 °C. In-situ HT-XRD analysis directly revealed that crystallization process of Li_1+xMn_2−xO₄ was significantly affected by the difference in the Li/Mn molar ratio in the precursor. Single phase of stoichiometric LiMn₂O₄ formed at 700 °C. The formation of single phase of spinel was achieved at the lower temperature than the stoichiometric sample as Li/Mn molar ratio in the precursor increased. Lattice parameter of the stoichiometric LiMn₂O₄ at 25 °C was 8.24 Å and expanded to 8.31 Å at 700 °C, which corresponds to the approximately 3% expansion in the unit cell volume. From the slope of the lattice parameter change as a function of temperatures, linear thermal expansion coefficient of the stoichiometric LiMn₂O₄ was calculated to be 1.2×10⁻⁵ °C⁻¹ in this temperature range. When the Li/Mn molar ratio in Li_1+xMn_2−xO₄ increased (x > 0.1), the spinel phase segregated into the Li_1+yMn_2−yO₄ (x > y) and Li₂MnO₃ during heating, which involved the oxygen loss from the materials. During the cooling process from 700 °C, and the segregated phase merged into Li_1+xMn_2−xO₄ with oxygen incorporation. Such trend directly observed by in-situ HT-XRD was supported by thermal gravimetric analysis as reversible weight (oxygen) loss/gain at higher temperature (500-700 °C).     

Investigations of new binary phosphate K4Ce2P4O15

The new potassium cerium(III) phosphate of formula K₄Ce₂P₄O₁₅ in the system Ce₂O₃-K₂O-P₂O₅ was prepared by solid state reactions and characterized by thermal analysis (DTA, TG, DSC), powder X-ray diffraction and IR spectroscopy. This compound exists only in the solid state (below 880 °C) and exhibits a polymorphic transition at 527 °C. The low-temperature form β-K₄Ce₂P₄O₁₅ of this compound crystallizes as a triclinic phase (space group P) with unit cell parameters: a=9.319(7), b=12.129(3), c=9.252(1) Å, α=106.875, β=100.086, γ=107.202°, V=916.276 Å³.     

Sb2O4 at high pressures and high temperatures

Investigations on Sb₂O₄ at high pressure and temperature have been performed up to 600 °C and up to 27.3 GPa. The so-called “high temperature” phase (β-Sb₂O₄) was obtained following pressure increase at ambient temperature and at relatively low temperatures. Thus, in contrast to previous perceptions, β-Sb₂O₄ is the modification more stable at high pressures, i.e., at low temperatures. The fact that the metastable α-form is typically obtained through the conventional way of preparation has to be attributed to kinetic effects. The pressure-induced phase transitions have been monitored by in-situ X-ray diffraction in a diamond anvil cell, and confirmed ex-situ, by X-ray diffraction at ambient conditions, following temperature decrease and decompression in large volume devices. Bulk modulus values have been derived from the pressure-induced volume changes at room temperature, and are 143 GPa for α-Sb₂O₄ and 105 GPa for the β-Sb₂O₄.     

Reactions of powdered silicon with some pyrotechnic oxidants

Thermogravimetry (TG) and differential scanning calorimetry (DSC) have been used to examine the thermal behaviour, in N₂ and in air, of the Si/Sb₂O₃, Si/KNO₃, Si/Fe₂O₃ and Si/SnO₂ pyrotechnic systems, in relation to the behaviour of the individual constituents.TG curves for Si powder, heated alone in air, showed that limited oxidation of Si occurred above 700°. In N₂, Sb₂O₃ sublimed completely between 500 and 900° and, in air, sublimation was accompanied by oxidation to Sb₂O₄. The Sb₂O₄ decomposed at higher temperatures. DSC curves for KNO₃ heated in N₂ showed the usual crystalline transition and melting endotherms followed by endothermic decomposition between 400 and 950°. DSC and TG curves of SnO₂and Fe₂O₃ revealed no thermal events when samples were heated to 1000° in either N₂ or air.For the Si/Sb₂O₃ system, the oxidation of Si by Sb₂O₃ between 590 and 700°, was complicated by sublimation of Sb₂O₃ in N₂ and also by the oxidation of Sb₂O₃ in air. No thermal events were observed for the Si/SnO₂and Si/Fe₂O₃ systems when heated under a variety of conditions in either N2 or in air, although these systems do sustain combustion on suitable ignition. In the Si/KNO₃ system, oxidation of Si occurs in a KNO₃ melt at temperatures above 560° in nitrogen and in air.

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Zusammenfassung Mittels TG und DSC wurde das thermische Verhalten der pyrotechnischen Systeme Si/Sb₂O₃, Si/KNO₃, Si/Fe₂O₃ und Si/SnO₂in N₂ und in Luft im Vergleich zum Verhalten der einzelnen Komponenten untersucht.TG-Aufnahmen über das Erhitzen von Si-Pulver in Luft zeigten eine begrenzte Oxidation von Silizium oberhalb 700°C. Sb₂O₃ sublimiert in Stickstoff vollständig zwischen 500 und 900°C, in Luft wird die Sublimation durch Oxidation zu Sb₂O₄ begleitet. Sb₂O₄ zersetzt sich bei höheren Temperaturen. DSC-Aufnahmen für KNO₃ in N₂ zeigen die gewohnten Umwandlungs- und Schmelzendothermen, gefolgt von einer endothermen Zersetzung zwischen 400 und 950°C. Die DSC- und TG-Kurven für SnO₂und Fe₂O₃ zeigen bei Erhitzen bis 1000°C weder in N₂ noch in Luft den Verlauf thermische Prozesse an.Bei dem System Si/Sb₂O₃ spielt sich neben der Oxidation von Si durch Sb₂O₃ zwischen 590 und 700°C auch eine Sublimation von Sb₂O₃ in N₂ sowie eine Oxidation von Sb₂O₃ in Luft ab. Für die Systeme Si/SnO₂und Si/Fe₂O₃ konnten durch Erhitzen unter einer Reihe von Bedingungen weder in Luft noch in N₂ Thermoprozesse nachgewiesen werden, obwohl diese Systeme nach geeigneter Zündung den Brennvorgang aufrechterhalten. Im System Si/KNO₃ erfolgt sowohl in N₂ als auch in Luft oberhalb 560°C die Oxidation von Si in der KNO₃-Schmelze.

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Dedicated to Professor Dr. H. J. Seifert on the occasion of his 60^th birthday     

Structural and thermal investigation of gadolinium gallium mixed oxides obtained by coprecipitation: Observation of a new metastable phase

Polycrystalline gadolinium gallium mixed oxides were prepared by coprecipitation and annealing at various temperatures below 1000 °C. The oxide materials appear to be X-ray amorphous after a heat treatment at 500 °C for 30 h, but after 30 h at 800 and 900 °C a major, unreported, hexagonal phase, isostructural with TAlO₃ compounds (where T=Y, Eu, Gd, Tb, Dy, Ho, Er) appears to crystallize. On the other hand, a highly energetic mechanical treatment of the amorphous powder previously annealed at 500 °C changes considerably the shape and position of exothermal events occurring in the range from 700 up to 900 °C. Subsequent annealing at 900 °C of the mechanically treated powder gives rise to the complete formation of the Gd₃Ga₅O₁₂ garnet structure at the expense of the hexagonal phase and of the minor Gd₄Ga₂O₉ oxide phase. However, a 7.0 wt% contamination is found to be due to tetragonal zirconia coming from vials and balls colliding media. The garnet phase may have strong deviations from the nominal stoichiometry of the garnet, as suggested by the refined lattice parameter obtained from the powder diffraction patterns and by the remarkable absence of intensity relative to the (220) Bragg peak position.     

Constitution of the Sr-Ni-O system

The constitution of the Sr-Ni-O system was studied experimentally for the first time. Samples were prepared either from SrCO₃ and NiO or from Sr(NO₃)₂ and Ni(NO₃)₂·6H₂O and characterized by high-temperature X-ray powder diffraction, scanning electron microscopy, thermogravimetric and differential thermal analyses. In the SrO-NiO quasibinary system an eutectic reaction: liquid?SrO+NiO was found to occur at 1396±5 °C, while the homogeneity range of terminal solid solutions is negligible. Thermodynamic calculations using the regular solution model for the liquid and rocksalt-type phases were employed to predict liquidus and solidus curves. Three ternary compounds, SrNiO_2.5, Sr₅Ni₄O₁₁, and Sr₉Ni₇O₂₁ were observed in the samples prepared from nitrate solutions, but only Sr₉Ni₇O₂₁ was proved to be thermodynamically stable in air up to 1030±6 °C. When heating in air, SrNiO_2.5 and Sr₅Ni₄O₁₁ were found to transform irreversibly into a mixture of Sr₉Ni₇O₂₁ and NiO. Isothermal section of the SrO-NiO-O subsystem, which represents phase equilibria at 950-1030 °C as well as an isobaric section of the Sr-Ni-O system in air were constructed.     

Morphology-controlled nonaqueous synthesis of anisotropic lanthanum hydroxide nanoparticles

The preparation of lanthanum hydroxide and manganese oxide nanoparticles is presented, based on a nonaqueous sol-gel process involving the reaction of La(OiPr)₃ and KMnO₄ with organic solvents such as benzyl alcohol, 2-butanone and a 1:1 vol. mixture thereof. The lanthanum manganese oxide system is highly complex and surprising results with respect to product composition and morphology were obtained. In dependence of the reaction parameters, the La(OH)₃ nanoparticles undergo a shape transformation from short nanorods with an average aspect ratio of 2.1 to micron-sized nanofibers (average aspect ratio is more than 59.5). Although not directly involved, KMnO₄ plays a crucial role in determining the particle morphology of La(OH)₃. The reason lies in the fact that KMnO₄ is able to oxidize the benzyl alcohol to benzoic acid, which presumably induces the anisotropic particle growth in [0 0 1] direction upon preferential coordination to the ±(1 0 0), ±(0 1 0) and ±(−110) crystal facets. By adjusting the molar La(OiPr)₃-to-KMnO₄ ratio as well as by using the appropriate solvent mixture it is possible to tailor the morphology, phase purity and microstructure of the La(OH)₃ nanoparticles. Postsynthetic thermal treatment of the sample containing La(OH)₃ nanofibers and β-MnOOH nanoparticles at the temperature of 800 °C for 8 h yielded polyhedral LaMnO₃ and worm-like La₂O₃ nanoparticles as final products.