Perovskite solid solutions in the BaO-CuO-Y2O3-Nb2O5 system

Freeze dried complex carboxylates are highly reactive precursors for complex perovskite solid solutions in the system BaO-CuO-Y₂O₃-Nb₂O₅ On thermal decomposition of the amorphous precursors the formation of complex crystalline phases begins at 600 °C. In most cases the themodynamically controlled phase composition is reached after a reaction time of two hours at about 900 °C. Beginning from the perovskite compound Ba₂YNbO₆ a partial substitution of Y by Cu or by a combination 2/3 Cu,1/3 Nb leads to extended fields of solid solutions with cubic perovskite structure. Substitution according to Y_0,5xBa₂(Y_1-0,5xCu_xNb)O_6+x is limited to x ≤ 0,4. A compound LBa₂Cu₂NbO₈ (x=2), well characterized for L=La, does not exist for L=Y. The composition of solid solutions depends on the preparation conditions. There are some signs for an inhomogeneous distribution of B-cations in the cubic perovskites.     

Study of the formation of the apatite-type phases La9.33+x(SiO4)6O2+3x/2 synthesized from a lanthanum oxycarbonate La2O2CO3

Lanthanum silicated apatites with nominal composition La_9.33+x(SiO₄)₆O_2+3x/2 (−0.2 < x < 0.27) have been successfully synthesized by solid state reaction using a new reagent La₂O₂CO₃ and amorphous SiO₂ precursors. The formation mechanism of La₂O₂CO₃ reagent, which cannot be purchased, has been followed by in-situ temperature depend XRD of La₂O₃ under CO₂ atmosphere. The stability of this reagent during the synthesis step allowed to limit the formation of secondary phase La₂Si₂O₇ and made the weighting of the reagent easier. High purity powders could be synthesized at the temperature of 1400 °C. Dense pellets (more than 98.5%) were obtained by isostatic pressing of powders calcined at 1200 °C and then sintered at 1550 °C. Traces of La₂SiO₅ secondary phase present in synthesized powder disappeared after densification and pure oxyapatite materials were obtained for all the compositions. Electrical measurements confirmed that conductivity behaviors of the sintered pellets were dependent to the oxygen over-stoichiometry. Indeed, a relatively high conductivity of 1 × 10⁻² S cm⁻¹ was exhibited at 800 °C for the nominal composition La_9.60(SiO₄)₆O_2.405 with low activation energy around 0.79 eV. The ionic conductivity properties were comparable with that of the earlier obtained materials.     

Phase equilibria and solid solution relationships in the La2O3-TiO2-ZrO2 system

In the ternary La₂O₃-TiO₂-ZrO₂ system the subsolidus phase relations at 1350 °C were determined using X-ray diffraction, scanning electron microscopy end energy dispersive X-ray analysis. The collected results are presented in the form of a phase diagram. In the equilibrium state there are 7 ternary and 5 binary compatible subsystems. In the system TiO₂ss, ZrO₂ss, ZrTiO₄ss, La₂Zr₂O₇ss and La₂O₃ss solid solutions were confirmed and La₄Ti₉O₂ss and La₂Ti₂O₇ss solid solutions were identified. The addition of ZrO₂ does not stabilize the La_2/3TiO₃ perovskite compound, nor the addition of TiO₂ a highly temperature stable compound La_2/3ZrO₃.     

Synthesis,Characterization and Oxide Ionic Conductivity of Binary β‐(Bi2O3)1‐x(Lu2O3)x System

In this research, the effects of doping Lu₂O₃ to α‐Bi₂O₃ in the range of 0.01 < x < 0.10 in a series of different mole fractions (1% < n < 10% mole ratios) was studied. Beside, heat treatment was performed by applying a cascade temperature rise in the range of 700‐800 °C for 72 hours and new phases were obtained in the (Bi₂O₃)_1‐x(Lu₂O₃)_x system. After heat treatment for 72 hours at 800 °C; mixtures, containing 2‐8% mole Lu₂O₃, formed a tetragonal phase. As a result of subjecting mixtures, containing 9% and 10% mole Lu₂O₃, to a quenching process at 825 °C, tetragonal phases were obtained. With the help of XRD, the crystal systems and lattice parameters of the solid solutions were obtained and their characterization was carried out. Thermal measurements were made by using a simultaneous DTA/TG system. The total conductivity (σ_T) in the β‐Bi₂O₃ doped with Lu₂O₃ was measured using the four‐probe DC method.     

Polymorphie von SrTa2O6

Polymorphism of SrTa₂O₆ Orthorhombic SrTa₂O₆ is a new low temperature modification related to orthorhombic CaTa₂O₆. SrTa₂O₆(orh.) was obtained when the wellknown modification SrTa₂O₆(TTB) which is related to the tetragonal tungsten bronzes was heated in the presence of a transporting agent (chlorine) or a mineralizer (melt of B₂O₃) at temperatures below 1150°C. It could be prepared by the reaction of a 1:1 mixture of Sr(NO₃)₂ or SrCO₃ with Ta₂O₅ in a sealed quartz glass tube as well. SrTa₂O₆(orh.) also occurred as an intermediate phase of the reaction of the corresponding 1:2 mixture at temperatures below 900°C (e. g. 840°C). Indexing of Guinier powder patterns led to the following unit cell: a = 11.006 Å, b = 7.638 Å, c = 5.622 Å. At temperatures above 1220°C SrTa₂O₆(orh.) changes (in air) to SrTa₂O₆(TTB). A reversal of this transition could not be achieved without the presence of a mineralizer or a transporting agent. Ca_xSr_1?xTa₂O₆ solid solutions of the low temperature form could not definitely be established. However, at 1300°C solid TTB solutions of Ca_xSr_1?xTa₂O₆ were formed. For x > 0.05 the TTB unit cells are orthorhombically distorted. For x ≥ 0.85 the x-ray powder patterns of the solid solutions looked like the one of CaTa₂O₆(orh.) and no TTB-structure was observed at 1300°C.     

Thermal Investigations of M[La(C2O4)3]⋅xH2O (M=Cr(III) and Co(III))

The complexes M[La(C₂O₄)₃]⋅xH₂O (x=10 for M=Cr(III) and x=7 forM=Co(III)) have been synthesized and their thermal stability was investigated. The complexes were characterized by elemental analysis, IR, reflectance and powder X-ray diffraction (XRD) studies. Thermal investigations using TG, DTG and DTA techniques in air of chromium(III)tris(oxalato)lanthanum(III)decahydrate, Cr[La(C₂O₄)₃]⋅10H₂O showed the complex decomposition pattern in air. The compound released all the ten molecules of water within ∼170°C, followed by decomposition to a mixture of oxides and carbides of chromium and lanthanum, i.e. CrO₂, Cr₂O₃, Cr₃O₄, Cr₃C₂, La₂O₃, La₂C₃, LaCO, LaCrO_x (2<x<3) and C at ∼1000°C through the intermediate formation of several compounds of chromium and lanthanum at ∼374, ∼430 and ∼550°C. Thecobalt(III)tris(oxalato)lanthanum(III)heptahydrate, Co[La(C₂O₄)₃]⋅7H₂O becomes anhydrous around 225°C, followed by decomposition to Co₃O₄, La₂(CO₃)₃ and C at ∼340°C and several other mixture species of cobalt and lanthanum at∼485°C. The end products were identified to be LaCoO₃, Co₃O₄, La₂O₃, La₂C₃, Co₃C, LaCO and C at ∼ 2>1000°C. DSC studies in nitrogen of both the compounds showed several distinct steps of decomposition along with ΔH and ΔSvalues. IR and powder XRD studies have identified some of the intermediate species. The tentative mechanisms for the decomposition in air are proposed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Reactions between YBa2Cu3O7−δ and La2O3 and SrCO3

Solid state reactions at 925°C between the high-T _c ceramic superconductor YBa₂Cu₃O_7?δ and La₂O₃ and SrCO₃, respectively, mixed in various molar ratiosr=MeO_n/YBa₂Cu₃O_7?δ, were studied using X-ray powder diffraction and scanning electron microscopy. The reaction between YBa₂Cu₃O_7?δ and La₂O₃ yielded (La_1?xBa_x)₂CuO_4?δ, withx≈0.075?0.10. La_2?xBa_1+xCu₂O_6?δ, withx≈0.2?0.25 and La-doped (Y_1?xLa_x)₂BaCuO₅, withx≈0.10?0.15. Forr=3.0, Y-doped La₂BaCuO₅ resulted also. The reaction between YBa₂Cu₃O_7?δ and SrCO₃ yielded (Sr_1?zBa_z)₂CuO₃, withz≈0.1, Y₂(Ba_1?zSr_z)CuO₅, withz=0.1?0.15, and a nonsuperconducting compound with an approximate composition of Y(Ba_0.5Sr_0.5)₅Cu_3.5O_10±δ. At values ofr≤2.0, unsubstituted YBa₂Cu₃O_7?delta was found in the reaction products.     

Layered manganese oxide with O2 structure,Li(2/3)+x(Ni1/3Mn2/3)O2 as cathode for Li-ion batteries

Using LiI as the reducing agent, the compound O2-Li_(2/3)+x(Ni_1/3Mn_2/3)O₂, x∼1/3 (O2(Li+x)) has been prepared from the O2-Li_2/3(Ni_1/3Mn_2/3)O₂ (O2(Li)). Cyclic voltammetry and voltage-capacity profiles of the O2(Li+x) phase are qualitatively different from that of O2(Li) phase. The first extraction capacity of O2(Li+x) at C/10 rate is 190 mAh/g corresponding to the removal of 2/3 mole of Li from the compound. At C/5 rate it delivers a reversible capacity of 158 mAh/g at 25 °C and 184 mAh/g at 50 °C (vs Li metal; voltage window 2.5–4.6 V). In Li-ion cells, with MCMB anode and O2(Li+x) as cathode, a discharge capacity of 140 mAh/g was obtained at C/5 rate in the voltage window 2.5–4.5 V (25 °C). The charge–discharge cycling performance and the cyclic voltammograms reveal that O2(Li) and O2(Li+x) do not convert to the spinel structure.     

Mechanochemical synthesis and thermal stability of phases in the Y2O3–Yb2O3 system

Substitutional, continuous solid solution of the general formula Y_2–xYb_xO₃ was obtained from the mixture of Y₂O₃ and Yb₂O₃ oxides, for the first time by the mechanochemical method in a high-energy ball milling. The monophasic samples of nanocrystalline solid solution for x?>?0.00 and x?<?2.00 were examined by the methods: XRD, DTA, SEM, IR and UV–Vis–DR. As follows from the results, the solid solution crystallizes in cubic system and is isostructural with Y₂O₃ and Yb₂O₃. The solution is stable in the air atmosphere up to at least 900°C, and its decomposition temperature decreases with the increase in x, that is, with decreasing number of Yb³⁺ ions replacing Y³⁺ ions in the crystal lattice of Y₂O₃. The energy band gap estimated for the solid solution varies from?~?5.30 eV for x?=?0.50 to?~?4.90 eV for x?=?1.50, which means that it is an insulator.

     

Synthesis of homogeneous La<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ca<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> solid solutions by the Pechini method and their activity in methane oxidation

Homogeneous La_{1 − x} Ca _x MnO₃ solid solutions have been synthesized by the Pechini method (using polymer-solid compositions). Their microstructure, stability at high temperatures, and catalytic activity in methane oxidation are reported. A continuous series of solid solutions stable in air up to 1100°C forms in the system, and the particle surface is enriched with calcium. A distinctive microstructural feature of the particles is their microporosity. The catalytic activity of all calcium-containing samples (except for x = 0.7) below 700°C is lower than that of lanthanum manganite and decreases under the action of the reaction medium, which can be due to the decrease in the amount of weakly bound oxygen on the surface because of the enrichment of the surface with calcium and the formation of strongly bound surface carbonates. The higher activity and stability of the La_0.3Ca_0.7MnO₃ sample (calcined at 1100°C) above 500°C can be due to the formation of nanosized areas with an Mn₃O₄ structures on the perovskite particle surface in the reaction medium.     

The effects of La2O3 on the early stages of crystallization for MgO-Al2O3-SiO2-TiO2-La2O3 glass-ceramics

The early stages of crystallization for MgO-Al₂O₃-SiO₂-TiO₂-La₂O₃ glasses with different La₂O₃ concentrations were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The glass transition temperature (Tg) of the glass decreases at first and then increases again with increasing La₂O₃ concentration. This indicates that the structure of the glass becomes weaker at first and then stronger again. Lanthanum acts in glasses as network modifier and will usually decrease the network connectivity of the glass structure. Nevertheless, if the La₂O₃ concentration is high enough, the oxygen and other ions start to agglomerate around La, resulting in a more closely packed structure. Heat-treatment of the sample with x = 0.1 at 770–810 °C results in the precipitation of a droplet phase with higher mean atomic weight embedded in a matrix with lower mean atomic weight. The initial crystalline phase magnesium aluminum titanate (MAT) precipitates from the droplet phase. Nevertheless, for the sample with x = 0.4, dendrite-like structure could be observed after heat-treatment of the glass at 810 °C. Furthermore, the crystalline phase first precipitated is the lanthanum containing perrierite, which could be attributed to the rearrangement of the glass structure as an effect of La³⁺ incorporation.     

Synthesis of Na2WO4-MnxOy supported on SiO2 or La2O3 as fiber catalysts by electrospinning for oxidative coupling of methane

The oxidative coupling of methane (OCM) is an attractive route to convert natural gas directly into value-added chemical products (C₂₊). This work comparatively investigated SiO₂- or La₂O₃-supported Na₂WO₄-Mn_xO_y (denoted as NWM) catalysts in powder and fiber forms. The powder catalysts were prepared using a co-impregnation method and the fiber catalysts were prepared successfully using an electrospinning technique. The NWM/La₂O₃ fiber catalysts were activated at low temperature (500 °C) and had a 4.7% C₂₊ yield, with the maximum C₂₊ yield of 9.6% at 650 °C, while the NWM/SiO₂ fiber catalyst was activated at 650 °C and had a maximum C₂₊ yield of 20.4% at 700 °C. The XPS results in the O 1s region indicated that NWM/La₂O₃ had a lower binding energy than NWM/SiO₂, suggesting that the lattice oxygen species is easily released from the catalyst surface and creates vacancy sites that enhance performance. The stability test of the catalysts indicated that the La₂O₃-containing catalysts had excellent activity and high thermal stability, while the SiO₂-containing catalysts had a higher C₂₊ yield when the prepared catalysts were compared at 700 °C. Considering the same component catalysts, the fiber catalysts achieved higher performance because their heat and mass transfer properties were enhanced.     

Electroconductivity and transport numbers of solid electrolytes La10−x CaxA6O27−δ and La9.33+δA6−x AlxO26 (A = Si, Ge) with apatite-like structure

The ion-oxygen conductivity of apatite-like compounds based on lanthanum silicates and germanates La₁₀A₆O₂₇ (A = Si, Ge), La_10?x Ca_xSi₆O_27?δ (x = 0.25, 0.5, 1.0), La_9.75Ca_0.25Ge₆O_27?δ and La_9.33+δSi_6?x Al_xO₂₆(x=0.4, 0.8, 1.5) is studied in the interval of partial oxygen pressures pO₂ extending from 10^?16 to 10⁵ Pa, at temperatures of 500–1000°C. The electroconductivity of undoped compounds La₁₀A₆O₂₇ (A = Si, Ge) exceeds that of yttria-stabilized zirconia. The electroconductivity of lanthanum germanate (1.7 × 10^?2 and 8.5 × 10^?2S cm^?1 at 700 and 900°C, respectively) is substantially higher than that of lanthanum silicate (9.8 × 10^?3 and 3.5 × 10^?2 S cm^?1 at 700 and 900°C). Doping lanthanum germanate with calcium raises its electroconductivity (2.7 × 10^?2 and 1.3 × 10^?1 S cm^?1 for La_9.75Ca_0.25Ge₆O_27?δ at 700 and 900°C). Conversely, doping lanthanum silicate with ions of calcium or aluminum reduces the conductivity. In the pO₂ interval studied, the above compounds are ionic conductors and represent a class of solid electrolytes of promise for various electrochemical devices.     

Phase equilibria in the BaO-Bi2O3-B2O3 system

Phase equilibria in the BaO-Bi₂O₃-B₂O₃ system have been investigated by X-ray powder diffraction analysis and DTA. Quasi-binary sections have been determined, and an isothermal section of the system in the subsolidus region has been constructed. The BaO-Bi₂O₃-B₂O₃ ternary system has been divided into 22 triangles of coexisting phases. It has been found that four bismuth barium borates exist, namely, Ba₃BiB₃O₉, BaBi₂B₄O₁₀, BaBiB₁₁O₁₉, and BaBiBO₄. Ba₃BiB₃O₉ undergoes a phase transition at 850°C and exists up to 885°C, where it decomposes in the solid state. BaBiB₁₁O₁₉ and BaBi₂B₄O₁₀ melt congruently at 807 and 730°C, respectively. BaBiBO₄ melts incongruently at 780°C. X-ray powder diffraction data for the low-temperature polymorph of Ba₃BiB₃O₉ are presented.     

Thermal Characterization and Physico-Chemical Properties of Fe2O3−Mn2O3/Al2O3 Systems

The effect of ferric and manganese oxides dopants on thermal and physicochemical properties of Mn-oxide/Al₂O₃ and Fe₂O₃/Al₂O₃ systems has been studied separately. The pure and doped mixed solids were thermally treated at 400–1000°C. Pyrolysis of pure and doped mixed solids was investigated via thermal analysis (TG-DTG) techniques. The thermal products were characterized using XRD-analysis. The results revealed that pure ferric nitrate decomposes into Fe₂O₃ at 350°C and shows thermal stability up to1000°C. Crystalline Fe₃O₄ and Mn₃O₄phases were detected for some doped solids precalcined at 1000°C. Crystalline γ-Al₂O₃ phase was detected for all solids preheated up to 800°C. Ferric and manganese oxides enhanced the formation of α-Al₂O₃ phase at1000°C. Crystalline MnAl₂O₄ and MnFe₂O₄ phases were formed at 1000°C as a result of solid–solid interaction processes. The catalytic behavior of the thermal products was tested using the decomposition of H₂O₂ reaction. This revised version was published online in August 2006 with corrections to the Cover Date.     

Structure and electrochemical performances of co-substituted LiSm x La0.2-x Mn1.80O4 cathode materials for rechargeable lithium-ion batteries

Spinel LiMn₂O₄ and Sm, La co-substituted LiSm _x La_0.2-x Mn_1.80O₄ (x?=?0.05, 0.10 and 0.15) cathode materials were synthesized by sol–gel method using aqueous solutions of metal nitrates and tartaric acid as chelating agent at 600 °C for 10 h. The structure and electrochemical properties of the synthesized materials were characterized by using thermogravimetric/differential thermal analysis, X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry, charge/discharge and electrochemical impedance spectroscopy studies. XRD analysis indicated that all the prepared samples were mainly belong to cubic crystal form with Fd3m space group. LiSm_0.10La_0.10Mn_1.80O₄ exhibits capacity retention of 90 % and 82 % after 100 cycles at room temperature (30 °C) and at elevated temperature (50 °C) at a rate of 0.5-C, respectively, much higher than those of the pristine LiMn₂O₄ (74 % and 60 %). Among all the compositions, LiSm_0.10La_0.10Mn_1.80O₄ cathode has improved the structural stability, high-capacity retention, better elevated temperature performance and excellent electrochemical performances of the rechargeable lithium-ion batteries.     

Synthesis,Characterization and Oxide Ionic Conductivity of Binary δ‐(Bi2o3)1‐x(Lu2o3)x System

In this study, after doping Lu₂O₃ to α‐Bi₂O₃ in the range of 11% ≤ n ≤ 20% in a series of different mole ratios, heat treatment was performed by applying a cascade temperature rise in the range of 700‐800 °C for 72 hours and new phases were obtained in the (Bi₂o₃)_1‐x(Lu₂o₃)_x system. After 72 hours of heat treatment at 800 °C, mixtures containing 14‐16% Lu₂O₃ formed a face‐centered cubic phase. Mixtures containing 11– 13%, 17%, 18% mole Lu₂O₃ were subjected to a quenching process at 825 °C and face‐centered cubic phases were obtained. With the help of XRD, the crystal systems and lattice parameters of the solid solutions were obtained and their characterization was carried out. Thermal measurements were made by using a simultaneous DTA/TG system. The total conductivity (σ_T) in the δ‐Bi₂O₃ doped with Lu₂O₃ system was measured using the four‐probe DC method. Keywords: Bismuth oxide; lutesium oxide; oxygen ionic conductivity; X‐ray techniques; thermal analysis.     

Electrophysical characteristics and stability of solid apatite-like electrolytes La x Si6O12 + 1.5x and La x Ge6O12 + 1.5x

Oxygen-ion conduction in apatite-like compounds based on silicates and germanates of lanthanum La _x A₆O_{12 + 1.5x} (A = Si, Ge; x = 9.11–10.22) is studied. The compounds are shown to be purely ionic conductors at 600–900°C and partial oxygen pressures 10^?16 to 10⁵ Pa. The electroconductivity of the best conducting specimens of La _x A₆O_{12 + 1.5x} (A = Si, Ge; x = 9.77–10) exceeds that of electrolyte YSZ at moderate temperatures. The electroconductivity of lanthanum germanate is substantially greater than that of lanthanum silicate, specifically, 7.85 × 10^?2 and 2.35 × 10^?2 S cm^?1, respectively, at 800°C. An inflection is discovered at ～750°C in the temperature dependences of electroconductivity of La _x Ge₆O_{12 + 1.5x} (x = 9.77–10.22). A dilatometric examination points to a second-kind phase transition that may be due to the oxygen sublattice disordering. The behavior of apatite-like electrolytes La _x A₆O_{12 + 1.5x} (A = Si, Ge) during long exploitation periods in the interval of working temperatures of electrochemical devices is studied for the first time ever. The electrolytes’ aging at 800°C in air for 1000 h was investigated by the electroconductivity method. The electroconductivity of lanthanum germanates decayed with time by 5% and that of lanthanum silicates, by 9.5%. The steady-state values of electroconductivity of all compounds studied is reached after 600–700 h. The compounds studied form a class of materials that hold some promise as solid electrolytes for medium-temperature fuel cells and other electrochemical devices.     

A study of the preparation of solid solutions in the system α-Fe2O3Al2O3

In order to elucidate the formation of precipitated iron catalysts for ammonia synthesis, the formation of solid solutions between α-Fe₂O₃ and Al₂O₃ was studied in the temperature range 500–950°C. The Al₂O₃ content in the solid solutions was found to be below 15 mole%. At temperatures of 800–950°C, solid solutions are formed at an appropriate rate. Specimens with relatively large specific surface areas are obtained at 800°C.     

Synthesis and structure of bilayered manganites based on Ca3Mn2O7 with the simultaneous substitution of calcium by lanthanum and strontium

A tetragonal (space group I4/mmm) solid solution La_2-2x (Ca_1-y Sr_y)_{1 + 2x} Mn₂O₇ based on the Raddlesden-Popper phase (n = 2), which is formed by the simultaneous substitution of calcium in Ca₃Mn₂O₇ by strontium and lanthanum, is synthesized by high-temperature annealing of La₂O₃, Mn₂O₃, CaCO₃, and SrCO₃ mixtures (1500°C, air). The concentration area of the solid solution in the scheme is a pentagon, whose corners correspond to the manganites Ca₃Mn₂O₇, Ca_0.75Sr_2.25Mn₂O₇, La_0.2Sr_2.8Mn₂O₇, La_1.6Sr_1.4Mn₂O₇, and LaCa₂Mn₂O₇.