Heat capacity, enthalpy and entropy of strontium niobate Sr2Nb2O7 and calcium niobate Ca2Nb2O7

The heat capacity and the enthalpy increments of strontium niobate Sr₂Nb₂O₇ and calcium niobate Ca₂Nb₂O₇ were measured by the relaxation time method (2–300 K), DSC (260–360 K) and drop calorimetry (720–1370 K). Temperature dependencies of the molar heat capacity in the form C_pm = 248.0 + 0.04350T − 3.948 × 10⁶/T² J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and C_pm = 257.2 + 0.03621T − 4.434 × 10⁶/T² J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇ were derived by the least-square method from the experimental data. The molar entropies at 298.15 K, S_m°(298.15 K) = 238.5 ± 1.3 J K⁻¹ mol⁻¹ for Sr₂Nb₂O₇ and S_m°(298.15 K) = 212.4 ± 1.2 J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇, were evaluated from the low-temperature heat capacity measurements.     

Defect and dopant properties of the Aurivillius phase Bi4Ti3O12

Computer modelling techniques have been used to investigate the defect and oxygen transport properties of the Aurivillius phase Bi₄Ti₃O₁₂. A range of cation dopant substitutions has been considered including the incorporation of trivalent ions (M³⁺=Al, Ga and In). The substitution of In³⁺ onto the Bi site in the [Bi₂O₂] layer is predicted to be the most favourable. The calculations suggest that lanthanide (Ln³⁺) doping at the dilute limit preferentially occurs in the [Bi₂O₂] layer, with probable distribution over both the [Bi₂O₂] and the perovskite A-site at higher dopant levels. It is predicted that the reduction process involving Ti³⁺ and oxygen vacancy formation is energetically favourable. The energetics of oxide vacancy migration between various oxygen sites in the structure have been investigated.     

Evolution and characterization of fluorite-like nano-SrBi2Nb2O9 phase in the SrO-Bi2O3-Nb2O5-Li2B4O7 glass system

Transparent glasses of various compositions in the system (100−x)Li₂B₄O₇−x(SrO-Bi₂O₃-Nb₂O₅) (where x=10, 20, 30, 40, 50 and 60, in molar ratio) were fabricated via splat quenching technique. The glassy nature of the as-quenched samples was established by differential thermal analyses. X-ray powder diffraction (XRD) and transmission electron microscopic studies confirmed the amorphous nature of the as-quenched and crystallinity in the heat-treated samples. Fluorite phase formation prior to the perovskite SrBi₂Nb₂O₉ phase was analyzed by both the XRD and high-resolution transmission electron microscopy. Dielectric and the optical properties (transmission, optical band gap and Urbach energy) of these samples have been found to be compositional dependent. Refractive index was measured and compared with the values predicted by Wemple-Didomemenico and Gladstone-Dale relations. The glass nanocomposites comprising nanometer-sized crystallites of fluorite phase were found to be nonlinear optic active.     

Structure and antiferroelectric properties of cesium niobate, Cs2Nb4O11

The compound cesium niobate, Cs₂Nb₄O₁₁, is an antiferroelectric, as demonstrated by double hysteresis loops in the electric field versus polarization plot. The crystal structure refinement by X-ray diffraction at both 100 and 297 K shows it to have a centrosymmetric structure in point group mmm and orthorhombic space group Pnna, which is consistent with its antiferroelectric behavior. The 100-K structure data is reported herein. The lattice is comprised of niobium-centered tetrahedra and octahedra connected through shared vertices and edges; cesium atoms occupy channels afforded by the three-dimensional polyhedral network. Antiferroelectricity is produced by antiparallel displacements of niobium atoms along the c-axis at the phase transition temperature of 165 °C. The critical field for onset of ferroelectric behavior in a single-crystal sample is 9.5 kV/cm at room temperature.     

Synthesis and structural studies of cation-substituted Aurivillius phases ASrBi2Nb2TiO12

We report here a novel method to prepare high-quality samples of the three layered oxides ASrBi₂Nb₂TiO₁₂ACa, Sr or Ba using the pre-formed intermediates ABi₂Nb₂O₉ and SrTiO₃. The room temperature structures were refined using synchrotron X-ray and Neutron powder diffraction data in the orthorhombic space group B2cb. This symmetry arises as a consequence of cooperative tilting of the BO₆ octahedra in the [ASrNb₂TiO₁₀]²⁻ perovskite-like slabs and a polar displacement of the cations. The structure is characterized by extensive cation disorder but lacks appreciable oxygen vacancies.     

钙钛矿型La1+X/2Sr1-x/2Co1-xCuxO3催化CO氧化活性与表征

The catalytic activity and the reactive properties of perovskite-type oxides catalysts La(1+x/2)Sr(1-x/2)Co1-xCuxO3 for CO oxidation reaction were investigated. Results showed that the catalytic activity for CO oxidation reached to a maximum when x＝0.4. The temperature for complete CO oxidation under atmospheric and experimental conditions was 168℃. According to the stoicheometry of catalyst, all catalysts were oxygen defect compounds. The active oxygen species on this catalyst was the adsorbed oxygen which was adsorbed on the surface lattice oxygen defect. It was also found that Co4+ existed in the catalysts and the sufrace active oxygen species was caused by the Co4+. It was concluded that CO oxidation reaction on this catalyst was carried out by the valence change between Co3+ and Co4+ which was adjusted by the adsorbed oxygen.     

Transport and optical properties of heavily hole-doped semiconductors BaCu2Se2 and BaCu2Te2

Experimental and theoretical studies of the electronic and optical properties of orthorhombic BaCu₂Se₂ and BaCu₂Te₂ are reported. Experimental data include the electrical resistivity, Hall coefficient, Seebeck coefficient, thermal conductivity, and lattice constants for , and optical transmission and diffuse reflectance data at room temperature. Nominally stoichiometric, polycrystalline samples form with hole concentrations inferred from Hall measurements of 2×10¹⁸ and 5×10¹⁹ cm⁻³ near room temperature for the selenide and telluride, respectively. The corresponding mobilities are near 15 cm² V⁻¹ s⁻¹ for both materials. Optical measurements reveal a transition near 1.8 eV in BaCu₂Se₂, while no similar feature was observed for BaCu₂Te₂. First principles calculations indicate both materials are direct or nearly direct gap semiconductors with calculated gaps near 1.0 eV and 1.3 eV for the telluride and selenide, respectively, and predict weak absorption below about 2 eV. Transport properties calculated from the electronic structure are also presented.     

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.     

Some studies on the solid solutions in the systems Sr2Nb2O7Eu2Nb2O7 and Sr2Ta2O7Eu2Ta2O7

Preparation of new solid solutions containing divalent europium have been tried in the systems Eu₂Nb₂O₇Sr₂Nb₂O₇ and Eu₂Ta₂O₇Sr₂Ta₂O₇. These solid solutions described as Eu_2xSr_2(1?x)M₂O₇ (M = Nb and Ta) exist in a pure orthorhombic phase in a limited region of x from 0 to about 0.5. The compounds with compositions close to Eu₂M₂O₇ exist but techniques have not been found to prepare them in pure form.     

Defect processes in MgAl2O4 spinel

In perfect normal MgAl₂O₄ spinel the Mg²⁺ ions occupy tetrahedral 8a sites and Al³⁺ ions occupy octahedral 16d sites. In reality some cations are exchanged between the cation sublattices forming pairs of antisite defects and thus a degree of “inversion”. Here atomic simulation is used to investigate the influence that antisite defects have on the populations of other intrinsic defects, those associated with Schottky and Frenkel reactions. One consequence is that the total magnesium interstitial concentration is increased substantially over the aluminium interstitial concentration and the magnesium vacancy concentration is increased over the aluminium vacancy concentration but to a much smaller extent. The split structures of isolated interstitial defects and the stability of various defect clusters are also discussed.     

Preparation and comparison of the photoelectronic properties of Sr2Nb2O7 and Ba0.5Sr0.5Nb2O6

The two alkaline earth niobates Sr₂Nb₂O₇ and Ba_0.5Sr_0.5Nb₂O₆ have been prepared, their electronic properties measured, and their photoresponses compared. The indirect band gap in Sr₂Nb₂O₇ is 3.86 eV compared with 3.38 eV for Ba_0.5Sr_0.5Nb₂O₆. Hence, photoanodes composed of Sr₂Nb₂O₇ respond to much less of the “white” light spectrum than those made from Ba_0.5Sr_0.5Nb₂O₆. Nevertheless, their electrical outputs at an anode potential of 0.8 eV with respect to SCE in 0.2 M sodium acetate under “white” xenon arc irradiation of 1.25 W/cm² are comparable.     

钡改性对Cr_2O_3/Al_2O_3催化剂异丁烷脱氢性能的影响

采用浸渍法制备了不同BaO负载量的Cr_2O_3/Al_2O_3催化剂,利用XRD、BET、NH3-TPD和O2-TPO等表征方法对钡改性前后的催化剂进行了分析;在反应温度560℃、反应压力0.1MPa、催化剂用量10mL、GHSV=400h-1的条件下,在固定床反应器上考察了其异丁烷脱氢反应的性能。结果表明,钡的引入影响了活性组分Cr2O3的分散,导致了催化剂比表面积的降低,但影响并不显著;钡的引入降低了催化剂表面的酸性、减少了反应过程中催化剂的积炭,改善了催化剂的选择性,提高了催化剂的稳定性。当BaO负载量为3%时,催化剂的性能最佳。     

Improved adhesion of copper on Al2O3

Summary Improved methods for Al₂O₃ metallization by Cu are described. Good adhesion between Cu and Al₂O₃ substrate depends on the formation of chemical bonds between the substrate and the metallic layer. The temperature needed for the formation of a CuAl₂O₄ spinel interface is reduced from 1050°C to 900°C by the addition of various oxides. The adhesion between the CuAl₂O₄ interface and deposited Cu is stronger then the tensile strength of pure Cu. Plasma techniques for the formation of a Cu containing interface are also described. Bombardment of a Cu film with Xe⁺ ions in a rf-glow discharge implants Cu atoms into the substrate to a depth of 5 nm, as determined by SIMS depth profiling. Methods for reduction of the CuAl₂O₄ surface for subsequent metallization are also presented.     

层状类钙钛矿结构新铌酸盐KSr2Nb3O10

A new niobate compound KSr₂Nb₃O₁₀ was synthesized for the first time. The chemical compositions, crystal structure, optical property, density and melting point of the new compound were characterized by EPMA, TEM, XRD, DTA and so on. KSr₂Nb₃O₁₀ crystallizes the orthorhombic system with unit cell parameters a=0.7816(1) nm, b=0.7764(2) nm, c=2.9995(2) nm, V=1.8114(4) nm³, and space group P2₁2₁2₁, Z=8. The structure may be described as treble perovskite sheets ［Sr₂Nb₃O₁₀］^- interleaved with K⁺. Further, it was found that KSr₂Nb₃O₁₀ has intercalation phenomenon. Na⁺, Li⁺, H⁺, NH⁺₄ could exchange the interlayer cations K⁺ of KSr₂Nb₃O₁₀, and n-hexylamine also could intercalate into the place between the layers of ［Sr₂Nb₃O₁₀］^-.     

Wet chemical synthesis and photocatalytic activity of potassium niobate K6Nb10.8O30 powders

The nanometer potassium niobate powders with tungsten bronze (TB)-type structure were synthesized by a wet chemical method and characterized by X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM). X-ray photoelectron spectroscopy (XPS) analysis confirmed the niobium with mixed valence states exists in the crystal structure of the photocatalyst, which may be advantage for increasing the photocatalytic activity. The band gap of K₆Nb_10.8O₃₀ powders was estimated to be about 2.92 eV and shows a markedly blue-shift as compared to that of the sample obtained by the solid-state reaction. The photocatalytic activity of the samples was evaluated by degradation of acid red G under UV irradiation and the photocatalytic reaction follows first-order kinetics. The photocatalytic activity of the as-prepared sample is much higher than that of sample synthesized by solid-state reaction, and slightly higher than that of P25-TiO₂.     

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.     

Effect of sintering on the ionic conductivity of garnet-related structure Li5La3Nb2O12 and In- and K-doped Li5La3Nb2O12

Garnet-structure related metal oxides with the nominal chemical composition of Li₅La₃Nb₂O₁₂, In-substituted Li_5.5La₃Nb_1.75In_0.25O₁₂ and K-substituted Li_5.5La_2.75K_0.25Nb₂O₁₂ were prepared by solid-state reactions at 900, 950, and 1000 °C using appropriate amounts of corresponding metal oxides, nitrates and carbonates. The powder XRD data reveal that the In- and K-doped compounds are isostructural with the parent compound Li₅La₃Nb₂O₁₂. The variation in the cubic lattice parameter was found to change with the size of the dopant ions, for example, substitution of larger In³⁺(r_CN6: 0.79 Å) for smaller Nb⁵⁺ (r_CN6: 0.64 Å) shows an increase in the lattice parameter from 12.8005(9) to 12.826(1) Å at 1000 °C. Samples prepared at higher temperatures (950, 1000 °C) show mainly bulk lithium ion conductivity in contrast to those synthesized at lower temperatures (900 °C). The activation energies for the ionic conductivities are comparable for all samples. Partial substitution of K⁺ for La³⁺ and In³⁺ for Nb⁵⁺ in Li₅La₃Nb₂O₁₂ exhibits slightly higher ionic conductivity than that of the parent compound over the investigated temperature regime 25-300 °C. Among the compounds investigated, the In-substituted Li_5.5La₃Nb_1.75In_0.25O₁₂ exhibits the highest bulk lithium ion conductivity of 1.8×10⁻⁴ S/cm at 50 °C with an activation energy of 0.51 eV. The diffusivity (“component diffusion coefficient”) obtained from the AC conductivity and powder XRD data falls in the range 10⁻¹⁰-10⁻⁷ cm²/s over the temperature regime 50-200 °C, which is extraordinarily high and comparable with liquids. Substitution of Al, Co, and Ni for Nb in Li₅La₃Nb₂O₁₂ was found to be unsuccessful under the investigated conditions.     

不同烧结温度对PMN基铁电陶瓷B位有序度的影响

本文研究了不同烧结温度下,CuO添加对0.94PMN-0.06PT(PMN-6PT)弛豫铁电陶瓷B位化学有序度的影响。拉曼和XRD分析结果表明,不同的烧结温度下,离子取代不同,对B位有序度的影响不同。烧结温度在950℃时,主要是Cu2+进入晶格,不影响B位化学有序度;烧结温度在1 080℃时,主要是Cu1+进入晶格,B位化学有序度改变。介温测试所表现出来的弛豫特性与拉曼分析有序度的结果相一致。     

Structural changes resulting from doping Ti2O3 with Sc2O3 or Al2O3

The crystal structures of (Ti_1?xSc_x)₂O₃, x = 0.0038, 0.0109, and 0.0413, and of (Ti_0.99Al_0.01)₂O₃, have been determined from X-ray diffraction data collected from single crystals using an automated diffractometer, and have been refined to weighted residuals of 25–34. Cell constants have also been determined for x = 0.0005, 0.0019, and 0.0232. The compounds are rhombohedral, space group $\text{R}\text{3}\text{c}$, and are isomorphous with α-Al₂O₃. The hexagonal cell dimensions range from a = 5.1573(2)Å, c = 13.613(1)Å for (Ti_0.9995Sc_0.0005)₂O₃ to a = 5.1659(4)Å, c = 13.644(1)Å for (Ti_0.9587Sc_0.0413)₂O₃, and a = 5.1526(2)Å, c = 13.609(1)Å for (Ti_0.99Al_0.01)₂O₃. Sc and Al substitution cause similar increases in the short near-neighbor metal-metal distance across the shared octahedral face; for Sc doping the increase is from 2.578(1) Å in pure Ti₂O₃ to 2.597(1) Å in (Ti_0.9587Sc_0.0413)₂O₃. By contrast, changes in the metal-metal distance across the shared octahedral edge appear to be governed by ionic size effects. The distance increases from 2.994(1) Å in Ti₂O₃ to 3.000(1) Å in (Ti_0.9587Sc_0.0413)₂O₃ and decreases to 2.991(1) Å in (Ti_0.99Al_0.01)₂O₃.     

不同烧结温度对PMN基铁电陶瓷B位有序度的影响

本文研究了不同烧结温度下, CuO添加对0.94PMN-0.06PT(PMN-6PT)弛豫铁电陶瓷B位化学有序度的影响。拉曼和XRD分析结果表明, 不同的烧结温度下, 离子取代不同, 对B位有序度的影响不同。烧结温度在950℃时, 主要是Cu²⁺进入晶格, 不影响B位化学有序度;烧结温度在1 080℃时, 主要是Cu¹⁺ 进入晶格, B位化学有序度改变。介温测试所表现出来的弛豫特性与拉曼分析有序度的结果相一致。