Structural models for intergrowth structures in the phase system Al2O3-TiO2

The phase system Al₂O₃-TiO₂ was investigated in the compositional range from 48:52 to 62:38 mol% Al₂O₃:TiO₂. The samples were prepared by melting the binary oxides in an arc-imaging furnace and the obtained samples were examined by powder X-ray diffraction. The recorded powder patterns could be interpreted in terms of intergrowth structures consisting of two basic building blocks, which were deduced from the known crystal structures of β-Al₂TiO₅ and Al₆Ti₂O₁₃. The structure of a new ordered compound with the formula Al₁₆Ti₅O₃₄ is proposed. The thermal stability was estimated from DTA and tempering experiments and showed that all prepared samples decompose at temperatures around 800 °C into the binary oxides corundum and titania.     

Sm2O3Ga2O3 and Gd2O3Ga2O3 phase diagrams

Four definite compounds exist in the Sm₂O₃Ga₂O₃ binary phase diagram, namely: Sm₃GaO₆, Sm₄Ga₂O₉, SmGaO₃, and Sm₃Ga₅O₁₂. The $\text{3}\text{1}$ compound is orthorhombic (space group Pnna - Z.4) with the cell parameters: a = 11.40₀Å, b = 5.51₅Å, c = 9.07Å and belongs to the oxysel family. Sm₃GaO₆ and SmGaO₃ melt incongruently at 1715 and 1565°C; Sm₄Ga₂O₉ and Sm₃Ga₅O₁₂ have a congruent melting point at 1710 and 1655°C. With regard to the Gd₂O₃Ga₂O₃ system three definite compounds have been identified: Gd₃GaO₆, Gd₄Ga₂O₉, and Gd₃Ga₅O₁₂. Only the garnet melts congruently at 1740°C with the following composition: Gd_3.12Ga_4.88O₁₂. Gd₃GaO₆, and Gd₄Ga₂O₉ melt incongruently at 1760 and 1700°C. GdGaO₃ is only obtained by melt overheating which may yield an equilibrium or a metastable phase diagram.     

Garnet-perovskite transformations in the Sm2O3Ga2O3 and Gd2O3Ga2O3 systems

The effect of heating garnet melts to various temperatures has been investigated. The previously reported decomposition of the garnet phase due to loss of Ga₂O₃ was corroborated. However, it was also observed that when gallium oxide loss is prevented and the maximum temperature of the melt exceeds a critical value, phase separation of garnet to perovskite and β-gallium oxide occurs:

$\text{RE}{}_3\text{Ga}{}_5\text{O}{}_{12}\text{?3RE}\text{GaO}{}_3\text{+}\text{Ga}{}_2\text{O}{}_3$

.The reverse reaction will occur by reheating the two-phase mixture to the garnet melting point.     

Phase relations in the pseudobinary system TiO2Ga2O3

Phase relations and microstructures in the TiO₂-rich part of the TiO₂Ga₂O₃ pseudobinary system have been determined at temperatures between 1373 and 1623°K using X-ray diffraction and electron and optical microscopy. The phases occurring in the system are TiO₂ (rutile), β-Ga₂O₃, a series of oxides Ga₄Ti_m?4O_2m?2 (m odd) which exist above 1463°K, and Ga₂TiO₅, which exists above 1598°K. The width of the phase region occupied by the Ga₄Ti_m?4O_2m?2 phases varies with temperature. At 1473°K it is narrow, and has limits of Ga₄Ti₂₅O₅₆ to Ga₄Ti₂₁O₄₈ while at higher temperatures it broadens to limits of from Ga₄Ti₂₇O₆₀ to Ga₄Ti₁₁O₂₈ at 1623°K. These phases are often disordered and crystals frequently contain partially ordered intergrowths of oxides with various values of m. On the TiO₂-rich side of the phase region there is a continuous change in texture from rutile to the end members of the Ga₄Ti_m?4O_2m?2 structures. The findings are summarized on a phase diagram.     

Solubility isotherm and some properties of the phases of the Ga2O3SeO2H2O system

The solubility isotherm of the three-component system Ga₂O₃SeO₂H₂O at 100°C was studied within a wide concentration range of selenious acid.     

Structural studies in the BaO-B2O3-TiO2 system by XAS and B-NMR

The structure of barium-titanium-metaborate xBaO-xB₂O₃-yTiO₂ (y=0%, 4%, 8%, 16% and x=50-y/2) amorphous and crystallized powders, obtained using a polymeric precursor method, was investigated by Ti and B K-edge X-ray absorption spectroscopy (XAS) and ¹¹B-NMR high-resolution techniques. XANES study of amorphous samples shows that Ti⁴⁺ ions exist as ^[4]Ti species associated to ^[6]Ti and ^[5]Ti species in a practically equivalent amount. After crystallization, titanium environment is predominately composed by ^[6]Ti species. According to XANES results obtained at the B K-edge, the fraction of boron in tetrahedral sites (^[4]B) reduces as the amount of TiO₂ is increased from x=0% to 4%, with a consequent increase of boron in trigonal sites (^[3]B). By a combination of ¹¹B-NMR spin-echo and triple quantum magic angle spinning (3Q-MAS) techniques, the detailed borate speciation was determined as consisting in ^[4]B and two kind of trigonal sites, ^[3]B_A and ^[3]B_B, corresponding, respectively, to borates sharing three and two O atoms with other boron units. NMR results reveal not only the reduction in boron coordination also seen by XANES but also the simultaneous reduction in the condensation degree of trigonal units, when the Ti content is increased in the glass. In crystallized samples, β-BaB₂O₄ and BaTi(BO₃)₂ phases were identified and quantified by ¹¹B-NMR.     

Oxidative coupling of methane over BaCl2-TiO2-SnO2 catalyst

The performance of BaCl2-TiO2-SnO2 composite catalysts in oxidative coupling of methane reaction has been investigated. A series of BaCl2-TiO2, BaCl2-SnO2, TiO2-SnO2, and BaCl2-TiO2-SnO2 catalysts were prepared, and characterized by BET, XRD, XPS, CO2-TPD and H2-TPR, respectively. The synergistic effect among BaCl2, SnO2 and TiO2 compositions enhances the catalytic performance. The best C2 selectivity and ethylene yield are obtained on the catalyst with the equal molar amount of the three compositions (BaCl2 : TiO2 : SnO2 molar ratio of 1 : 1 : 1). The optimal reaction conditions are as follows: 800°C, 44 mL·min-1 for methane, 22 mL·min-1 for oxygen and a space velocity of 5000 mL·h-1 g-1, and the C2H4 yield over the catalyst is 20.1% with the CH4 conversion of 43.8% and C2 selectivity of 53.3%.     

Phase states in the Bi4Ti3O12-BiFeO3 section in the Bi2O3-TiO2-Fe2O3 system

Specific features of the thermal behavior of Bi _{m + 1}Fe _m−3Ti₃O_{3m + 3} layered perovskite-like compounds (where m takes integer and some fractional values between 3 and 9) were considered, and the temperature limits of stability of these compounds were determined. The phase diagram of the Bi₄Ti₃O₁₂-BiFeO₃ section through the Bi₂O₃-TiO₂-Fe₂O₃ system was constructed.     

Ion-beam irradiation of lanthanum compounds in the systems La2O3-Al2O3 and La2O3-TiO2

Thin crystals of La₂O₃, LaAlO₃, La_2/3TiO₃, La₂TiO₅, and La₂Ti₂O₇ have been irradiated in situ using 1 MeV Kr²⁺ ions at the Intermediate Voltage Electron Microscope-Tandem User Facility (IVEM-Tandem), Argonne National Laboratory (ANL). We observed that La₂O₃ remained crystalline to a fluence greater than 3.1×10¹⁶ ions cm⁻² at a temperature of 50 K. The four binary oxide compounds in the two systems were observed through the crystalline-amorphous transition as a function of ion fluence and temperature. Results from the ion irradiations give critical temperatures for amorphisation (T_c) of 647 K for LaAlO₃, 840 K for La₂Ti₂O₇, 865 K for La_2/3TiO₃, and 1027 K for La₂TiO₅. The T_c values observed in this study, together with previous data for Al₂O₃ and TiO₂, are discussed with reference to the melting points for the La₂O₃-Al₂O₃ and La₂O₃-TiO₂ systems and the different local environments within the four crystal structures. Results suggest that there is an observable inverse correlation between T_c and melting temperature (T_m) in the two systems. More complex relationships exist between T_c and crystal structure, with the stoichiometric perovskite LaAlO₃ being the most resistant to amorphisation.     

氧化镓纳米带的合成和发光性质研究

采用化学气相沉积法，以碳纳米管作还原剂还原Ga₂O₃粉末，生成的气态Ga₂O和载气Ar中的微量O₂反应，在多孔氧化铝模板上沉积得到了Ga₂O₃纳米带。用扫描电子显微镜，透射电子显微镜和选区电子衍射对产物的结构和形态进行表征，发现产物为β-Ga₂O₃纳米带，宽度在20～500 nm之间，厚度为5～100 nm，长度可达几十微米。产物中还有几微米宽的Ga₂O₃纳米片。光致发光谱结果表明β-Ga₂O₃纳米带能发射蓝光和紫外光。文中还简单推测了β-Ga₂O₃纳米带的形成机理。     

Some A6B5O18 cation-deficient perovskites in the BaO-La2O3-TiO2-Nb2O5 system

Some dielectric oxides have been synthesized and characterized in the BaO-La₂O₃-TiO₂-Nb₂O₅ system. Through Rietveld refinement of X-ray powder diffraction data, Ba₅LaTi₂Nb₃O₁₈ and Ba₄La₂Ti₃Nb₂O₁₈ are identified as the A_nB_n−1O_3n (n=6) type cation-deficient perovskites with space group and lattice constants , and for Ba₅LaTi₂Nb₃O₁₈; , and for Ba₄La₂Ti₃Nb₂O₁₈, respectively. Their ceramics exhibit high dielectric constant up to 57 and high quality factors (Qf) up to 21,273 GHz. The temperature coefficient of resonant frequency (τ_f) of these ceramics is decreased with the increase of B-site bond valence.     

Study of the acid-base properties of Ga2O3/Al2O3 by using test reactions

Two test reactions: conversions of propan-2-ol and 2-methyl-3-butyn-2-ol were applied for the investigation of the acid-base properties of Ga₂O₃-Al₂O₃ materials. Both test reactions lead to consistent conclusions on the overall acid-base properties of the materials studied. It has been found that the deposition of gallium oxide on the surface of γ-Al₂O₃ enhances the overall basicity.     

On the ammonolysis of Ga2O3: An XRD, neutron diffraction and XAS investigation of the oxygen-rich part of the system Ga2O3-GaN

We investigated the ammonolysis of β-Ga₂O₃ at elevated temperatures by means of ex situ X-ray diffraction, ex situ neutron diffraction and in situ X-ray absorption spectroscopy. Within the detection limits of these methods, we can rule out the existence of a crystalline or amorphous oxynitride phase that is not derived from wurtzite-type GaN. No evidence for a β-Ga₂O₃ related oxynitride phase was found, and the nitrogen solubility in β-Ga₂O₃ was found to be below the detection limit of about 2-3 at% in the anionic sublattice. These findings were obtained by monitoring the anionic occupancy factors and the lattice parameters of the β-Ga₂O₃ phase obtained from total diffraction pattern refinement with the Rietveld method and by linear combination fitting of the X-ray absorption spectra that were recorded during the ammonolysis.     

New open-framework in the uranyl vanadates A3(UO2)7(VO4)5O (A=Li, Ag) with intergrowth structure between A(UO2)4(VO4)3 and A2(UO2)3(VO4)2O

New uranyl vanadates A₃(UO₂)₇(VO₄)₅O (M=Li (1), Na (2), Ag (3)) have been synthesized by solid-state reaction and their structures determined from single-crystal X-ray diffraction data for 1 and 3. The tetragonal structure results of an alternation of two types of sheets denoted S for _∞²[UO₂(VO₄)₂]⁴⁻ and D for _∞²[(UO₂)₂(VO₄)₃]⁵⁻ built from UO₆ square bipyramids and connected through VO₄ tetrahedra to _∞¹[U(3)O₅-U(4)O₅]⁸⁻ infinite chains of edge-shared U(3)O₇ and U(4)O₇ pentagonal bipyramids alternatively parallel to a- and b-axis to construct a three-dimensional uranyl vanadate arrangement. It is noticeable that similar _∞[UO₅]⁴⁻ chains are connected only by S-type sheets in A₂(UO₂)₃(VO₄)₂O and by D-type sheets in A(UO₂)₄(VO₄)₃, thus A₃(UO₂)₇(VO₄)₅O appears as an intergrowth structure between the two previously reported series. The mobility of the monovalent ion in the mutually perpendicular channels created in the three-dimensional arrangement is correlated to the occupation rate of the sites and by the geometry of the different sites occupied by either Na, Ag or Li. Crystallographic data: 293 K, Bruker X8-APEX2 X-ray diffractometer equipped with a 4 K CCD detector, MoKα, λ=0.71073 Å, tetragonal symmetry, space group P4¯m2, Z=1, full-matrix least-squares refinement on the basis of F²; 1,a=7.2794(9) Å, c=14.514(4) Å, R1=0.021 and wR2=0.048 for 62 parameters with 782 independent reflections with I?2σ(I); 3, a=7.2373(3) Å, c=14.7973(15) Å, R1=0.041 and wR2=0.085 for 60 parameters with 1066 independent reflections with I?2σ(I).     

Thermal characteristics, Raman spectra and structural properties of new tellurite glasses within the Bi2O3-TiO2-TeO2 system

Within the Bi₂O₃-XO₂-TeO₂ (X=Ti, Zr) systems, a large glass-forming domain was found for X=Ti, but no glass formation was evidenced for X=Zr. Densities, glass transition (T_g), crystallization (T_c) temperatures and Raman spectra of the relevant glasses were studied as functions of the composition. The Raman spectra of the glasses were interpreted in terms of the structural transformations produced by the modifiers. It was established that the addition of Bi₂O₃ and TiO₂ content to TeO₂ glass influences the T_g temperature in a similar manner: this value progressively increases with the increase of the modifier concentration. However, the structural evolutions are different: (a) the addition of TiO₂ to TeO₂ glass keeps the polymerized framework structure in transforming a number of Te-O-Te bridges into the Te-O-Ti ones without producing any tellurite anions (i.e., the [TeO₃]²⁻ groups); (b) on the contrary, the addition of Bi₂O₃ destroys the glass framework by giving rise to the island-type [Te_nO_m]^2(m−2n)− complex tellurites anions, thus causing a depolymerization of the glass.     

一维Ga2O3/SnO2纳米纤维的制备及其气敏性能

通过静电纺丝法制备了一维Ga2O3/SnO2纳米纤维,采用X射线衍射(XRD)、扫描电镜(SEM)、紫外可见漫反射光谱(UV-Vis DRS)等方法对材料进行了表征,测试了不同Ga2O3质量分数(0、40%、50%、60%、70%、100%)的Ga2O3/SnO2纳米纤维(650℃,5 h)对应元件对三甲胺、丙酮、乙醛、乙酸、氨气、乙醇、甲醛7种气体的气敏性能。结果表明:在室温(25℃)时,60%(w/w)Ga2O3-40%(w/w)SnO2纳米纤维对三甲胺气体具有较高的灵敏度和较短的响应/恢复时间。对1000μL·L^-1三甲胺的灵敏度达到51;检出限达到0.8μL·L^-1,其灵敏度为1.3。     

Interaction in the Ga2S3-Y2O2S system

The phase diagram of the Ga₂S₃-Y₂O₂S system has been investigated by differential thermal, X-ray powder diffraction, microstructural, and thermodynamic analyses. It has been established that the system is eutectic, and solubility at 295 K from the Ga₂S₃ side reaches 3 mol % Y₂O₂S. The coordinates of the eutectic point are 14 mol % Y₂O₂S and 1320 K.     

Effect of inert components (Y2O3, Al2O3, and Ga2O3) on the chemistimulating effect of the Sb2O3 activator of GaAs thermal oxidation

Chemistimilated thermal oxidation of gallium arsenide was studied using Sb₂O₃ activator oxide in compositions with Ga₂O₃, Al₂O₃, and Y₂O₃ inert components. For Sb₂O₃-Y₂O₃ compositions, the thickness of the resulting oxide layer on GaAs was found to be a linear function of composition over the enter range of the compositions. For antimony oxide compositions with Ga₂O₃ and Al₂O₃ inert components, nonadditivities were observed near the component ordinates. For the Sb₂O₃-Ga₂O₃ system, the chemistimulating efficiency noticeably weakened at low concentrations of the inert component. The linear trend observed for this system within 0–60 mol % Sb₂O₃ is additively determined by the oxide layer thickness on GaAs in the presence of Sb₂O₃ and in the absence of activator. In the presence of inert Al₂O₃, the chemistimulating effect was enhanced near the Al₂O₃ ordinate and the resulting function was nonadditive with respect to the thicknesses reached in the presence of the individual components.