State of atoms and interatomic interactions in complex perovskite-like oxides: XXX. Influence of the nature of diamagnetic substituents on the dynamics of clustering in lanthanum gallate doped with strontium,chromium, and magnesium

The fractions of clusters and single chromium atoms were calculated by the diluted solution and Heisenberg-Dirac-van-Vleck models for a series of solid solutions containing chromium, strontium, and magnesium in the ratio 5:1:1, respectively. A principal genetic relationship was revealed between La_1−0.2x Sr_0.2x ·Cr _x Ga_1−x O_3−δ, La_1−0.5x Sr_0.5x Cr _x Ga_1−x O_3−δ, and La_1−0.2x Sr_0.2x Cr _x Ga_1−1.2x Mg_0.2x O_3−δ systems. Deviations from Curie-Weiss law are observed for the systems with the ratio [Cr]:[Sr]:[Mg]=5:1:1, which point to a noncompensated magnetic moment.     

Phase composition, electroconductivity, oxygen ion transport number, and microhardness of Ln1 ? xSrxGa0.5 ? y/2Al0.5 ? y/2MgyO3 ? δ (Ln = La, Pr, Nd; x, y = 0.10, 0.15)

Phase composition, electroconductivity, oxygen ion transport number, and microhardness of samples of Ln_{1 − x} Sr_xGa_{0.5 − y/2}Al_{0.5 − y/2}Mg_yO_{3 − δ} (Ln = La, Pr, Nd; x, y = 0.10, 0.15) synthesized by a ceramic methods are studied. Methods of x-ray diffraction analysis and scanning electron microscopy reveal the La-containing samples to be homogeneous and have a perovskite structure. Magnesium does not dissolve in Pr-and Nd-containing systems but forms an individual phase based on magnesium oxide. Apart from magnesium oxide, in these systems there form extrinsic phases, specifically, LnSrGa₃O₇ and an unknown phase. The electroconductivity of La_{1 − x} Sr_xGa_{1 − y} Mg_yO_{3 − δ} decreases after substituting Al for Ga. Ceramic La_{1 − x} Sr_xGa_{0.5 − y/2}Al_{0.5 − y/2}Mg_yO_3−δ is a purely ionic conductor in the temperature interval 500 to 1000°C; Nd_xSr_xGa_{0.5 − y/2}Al_{0.5 − y/2}Mg_yO_{3 − δ} has predominantly ionic conduction; and the predominant type of conduction in Pr_{1 − x} Sr_xGa_{0.5 − y/2}Al_{0.5 − y/2}Mg_yO_{3 − δ} is electronic below 700–800°C, with the contribution of ionic conduction increasing at higher temperatures. Substituting Al for Ga raises the hardness of ceramics under study. Among the compositions studied, La_0.85Sr_0.15Ga_0.45Al_0.45Mg_0.10O_{3 − δ} and La_0.85Sr_0.15Ga_0.425Al_0.425Mg_0.15O_{3 − δ} exhibit a combination of electroconductivity and hardness that is optimal for application as electrolyte at reduced temperatures (600–800°C). The Pr_{1 − x} Sr_xGa_{0.5 − y/2}Al_{0.5 − y/2}Mg_yO_{3 − δ} system possesses mixed ionic-electronic conduction and high hardness, which makes it appealing for application as oxygen-penetrable membranes. Original Russian Text ? Yu.V. Danilov, A.D. Neuimin, L.A. Dunyushkina, L.A. Kuz’mina, N.S. Zybko, Z.S. Martem’yanova, A.A. Pankratov, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 1, pp. 57–65.     

Conductivity,oxygen interfacial exchange and diffusion in oxides based on lanthanum gallate

The method of isotopic exchange was used to study the oxygen exchange kinetics in the oxides of La_0.88Sr_0.12Ga_0.82Mg_0.18O_{3 − δ} and La_0.80Sr_0.20Ga_0.85−x Mg_0.15Co _x O_{3 − δ} (x = 0.05, 0.15, 0.20, 0.25). The rates of oxygen exchange and its diffusion coefficients were determined in the temperature range of 600–900°C at the oxygen pressure of 5 torr. The fractions of the three exchange types for the oxides studied were determined at the temperature of 817°C and oxygen pressure of 5 torr. The total conductivity of the oxides of La_0.80Sr_0.20Ga_0.85−x Mg_0.15Co _x O_{3 − δ} (x = 0.05, 0.15, 0.20, 0.25) was measured in the temperature range of 550–850°C in air and at the temperature of 800°C in the range of oxygen pressures of 1–760 torr. It was shown that an increase in the electronic conductivity component due to an increase in the cobalt fraction in the gallium sublattice results in growing interfacial exchange rate, total conductivity of the studied systems, and a decrease in the effective conductivity activation energy.     

Catalytic properties of Mg-modified Ni-based hexaaluminate catalysts for CO2 reforming of methane to synthesis gas

CO₂ reforming of CH₄ over hexaaluminates LaNi _x Mg_1−x Al₁₁O_19+δ and LaNi _x Mg_0.8Al_11.2−x O_19+δ were studied. XRD analysis confirmed that modifier Ni as well as Mg was inlaid into the hexaaluminate lattice. Hexaaluminate LaNi _x Mg_1−x Al₁₁O_19+δ (1.0 ≥ x ≥ 0.4) showed better catalytic activities and higher resistance to carbon deposition, which was attributed to their pure magnetoplumbite-type structure. For this reaction, x-value between 0.6 and 0.4 was the most suitable.     

Solid electrolytes based on CeO<Subscript>2</Subscript> for medium-temperature electrochemical devices

CeO₂-based solid solutions with a fluorite structure are promising materials as electrolytes of medium-temperature electrochemical devices: electrolytic cells, oxygen sensors, and solid oxide fuel cells. In this work, studies are presented of the effect of the dopant cation radius and its concentration on the physico-chemical properties of the Ce_{1 − x} Ln _x O_{2 − δ} solid solutions (x = 0–0.20; Ln = La, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb) and also of multicomponent solid solutions of Ce_{1 − x} Ln _x/2Ln′ _x/2O_{2 − δ} (x = 0–0.20; Ln = Sm, La, Gd and Ln′ = Dy, Nd, Y) and Ce_{1 − x − y} Sm _x M _y O_{2 − δ} (M = Ca, Sr, Ba) obtained using the solid-phase synthesis technique. Electric properties of the samples were studied in the temperature range of 623–1173 K and in the oxygen partial pressure range of 0.01–10⁻²² MPa. The values of oxygen critical pressure ( p_O2 ^* )\left( {p_{O_2 }^* } \right) are presented, at which the ionic and electron conductivity values are equal. The values were calculated on the basis of experimental dependences at 1023 K at the assumption that the ionic conductivity value is determined only by the dopant concentration and its effective ionic radius and is independent of the oxygen partial pressure.     

Homogeneity regions of yttrium and ytterbium manganites in air

Homogeneous solid solutions and heterogeneous systems of the general formula R_{2 − x} Mn _x O_{3 ± δ} (0.90 ≤ x ≤] 1.10 for R = Y and 0.88 ≤ x ≤ 1.14 for R = Yb; Δx = 0.02) were produced by ceramic synthesis from oxides in air within the temperature range 900–1400°C. The solubility boundaries of simple oxides R₂O₃ (R = Y, Yb), Mn₃O₄, and binary oxide RMn₂O₅ in yttrium and ytterbium manganites RMnO_{3 ± δ} were determined X-ray powder diffraction of these solutions and systems. The results were presented as fragments of phase diagrams of the systems Y-Mn-O and Yb-Mn-O in air. The solubility of Y₂O₃ and Mn₃O₄ in YMnO_{3 ± δ} was found to increase with increasing temperature, and the solubility of Yb₂O₃ and Mn₃O₄ in YbMnO_{3 ± δ} to be insensitive to varying temperature. It was suggested that the solubility of Y₂O₃ and Mn₃O₄ in YMnO_{3 ± δ} and of Yb₂O₃ and Mn₃O₄ in YbMnO_{3 ± δ} is caused by crystal structure defects of yttrium and ytterbium manganites and their related oxygen nonstoichiometry. In dissolving RMn₂O₅ in RMnO_{3 ± δ} (R = Y, Yb) in air within a narrow (∼20°C) temperature range adjacent to the RMn₂O₅ = RMnO₃ + 1/3Mn₃O₄ + 1/3O₂ equilibrium temperature, the solubility of RMn₂O₅ in RMnO_{3 ± δ} ecreases abruptly until almost zero. Conclusion is made that structural studies are necessary necessary to determine the oxygen nonstoichiometry δ of R_{2 − x} Mn _x O_{3 ± δ} solid solutions as a function of x and synthesis temperature; together with the results of this work, these studies will allow one to construct unique crystal-chemical models of these solid solutions.     

Platinum electrocatalysts based on oxide supports for hydrogen and methanol fuel cells

Platinum electrocatalysts for fuel cells based on individual oxides Pt/SnO₂ and Pt/TiO₂ and their solid solutions Pt/Ti_1−x M _x O₂ (M = Ru, Nb) and Pt/Sn_1−x M′ _x O_2−δ(M′ = Sb, Ru) were prepared. The influence of the composition of the oxide supports on the activity of the supported platinum catalysts in electrooxidation of methanol and hydrogen in the presence of CO was studied. The prepared platinum catalysts supported on solid solutions of tin dioxide Sn_1−x M _x O_2−δ(M = Sb, Ru; x = 0.4−0.9) and Ti_1−x M _x O₂ (M = Ru, Nb; x = 0.7) exhibited higher tolerance to CO poisoning and higher activities for methanol electrooxidation than commercial Pt,Ru catalysts on carbon support. The use of the proposed oxide supported catalysts in hydrogen and direct methanol fuel cells improved their performances in comparison with that for the fuel cells with traditional Pt,Ru catalysts on carbon support.     

Oxygen ionic transport in Bi2O3-based oxides: The solid solutions Bi2O3–Nb2O5

The minimum concentration of niobium to stabilize the fluorite-type f.c.c. phase in the Bi₂O₃–Nb₂O₅ oxide system at temperatures below 996 K was ascertained to be about 10 mol%. Thermal expansion, electrical conductivity and crystal lattice parameters of the Bi(Nb)O_1.5+δ solid solutions decrease with increasing niobium content. Thermal expansion coefficients were calculated from the dilatometric data to be (10.314.5)×10⁻⁶ K⁻¹ at temperatures in the range 300–700 K and (17.526.0)×10⁻⁶ K⁻¹ at 700–1100 K. The conductivity of the Bi_{1− x} Nb _x O_1.5+δ ceramics is predominantly ionic. The p-type electronic transference numbers of the Bi(Nb)O_1.5+δ solid solutions in air were determined to be less than 0.1. Annealing at temperatures below 900 K results in a sharp decrease in conductivity of the Bi_{1− x} Nb _x O_1.5+δ ceramics. Received: 18 August 1997 / Accepted: 20 October 1997     

Hydrolytic precipitation of magnesium polyvanadates in vanadium (IV, V) solutions

The phase and chemical composition of precipitates formed in Mg(VO₃)₂-VOSO4-H₂O system at initial pH from 1 to 7 and temperature from 80 to 90°C was studied. Polyvanadates of variable composition Mg _x V _y ⁴⁺V_12-y ⁵⁺¹O_31–δ · nH₂O (0.7 ≤ x ≤ 1.3, 1.2 ≤ y ≤ 2.4, 0.7 ≤ δ = 1.4) were formed at pH from 1 to 4 and V⁴⁺/V⁵⁺ ratio from 0.43 to 9. Compounds with the general formula Mg _x V _y ⁴⁺V_6-y ⁵⁺O_16-δ · nH₂O (0.7 ≤ x ≤ 0.65, y = 1.0, 0.8 ≤ δ ≤ 0.85) were formed at pH from 6.0 to 7.0 and V⁴⁺/V⁵⁺ ratios from 0.11 to 0.25. The maximum V⁴⁺ concentration (y = 2.4) in the precipitates was achieved at the VV⁴⁺/V⁵⁺ solution ratio of 1.0 and pH = 3. The precipitates in solutions with pH 3 were formed only upon addition of VO²⁺ ions with the maximum rate at a V⁴⁺/V⁵⁺ ratio of 0.33. These processes were limited by second-order reactions on the surface of polyvanadates.     

Some features of the preparation,structure, and properties of BICUTIVOX

This work continues the search for optimal preparation methods and studies of structural and transport characteristics of BICUTIVOX solid solutions of the general composition Bi₄V_{2 − x} Cu _x/2Ti _x/2O_{11 − δ} (x = 0.0−0.35).     

Structure and magnetic and electric properties of bismuth niobates doped with d elements: IV. Magnetic behavior of chromium-containing solid solutions with the structure of bismuth niobate, Bi5Nb3O15

The magnetic susceptibility and electrophysical properties of chromium-containing Bi₅Nb_3−3x · Cr_3x O_15−δ solid solutions with the perovskite-like structure were studied. Parameters of exchange interactions in the dimeric clusters of chromium atoms and also the distribution of chromium monomers and dimers in the solid solutions were calculated. Original Russian Text ? N.A. Zhuk, I.V. Piir, A.L. Pimenov, N.V. Chezhina, 2007, published in Zhurnal Obshchei Khimii, 2007, Vol. 77, No. 6, pp. 898–904. For communication III, see [1].     

Structural and thermal stability of BIMEVOX oxygen semiconductors

The results on the structural and thermal stability of different modifications of solid solutions Bi₄V_{2 − x} Fe _x O_{11 − δ} (BIFEVOX), where x = 0.05–0.6, and Bi₄V_{2 − x} Cu _x/2Ti _x/2O_{11 − δ} (BICUTIVOX), where x = 0.025–0.50, are shown. The stability is assessed by varying the thermodynamic parameters of the medium and also the time parameters with the use of modern methods, namely, the high-temperature X-ray diffraction analysis, the differential scanning calorimetry, and the dilatometry. The γ-modification of BIFEVOX is shown to be stable in a wide range of temperatures and oxygen partial pressures.     

Development of Cocatalysts for Photocatalytic Overall Water Splitting on (Ga1? x Zn x )(N1? x O x ) Solid Solution

The development of cocatalysts promoting overall water splitting on (Ga_1−x Zn _x )(N_1−x O _x ) solid solution photocatalyst is presented. The (Ga_1−x Zn _x )(N_1−x O _x ) is a stable visible-light-driven photocatalyst for stoichiometric water splitting upon loading with a suitable nanoparticulate cocatalyst. Loading with a combination of Cr and Rh oxides, Rh_2−y Cr _y O₃, is demonstrated to raise the quantum efficiency of (Ga_1−x Zn _x )(N_1−x O _x ) for overall water splitting to 2.5% at 420–440 nm. This represents a 10-fold increase in efficiency over the highest efficiency previously obtained using nanoparticulate RuO₂ as a cocatalyst. In addition to the composition, the dispersion and size of cocatalyst nanoparticles are identified as important factors affecting the degree of enhancement for stoichiometric water splitting. Kazuhiko Maeda—Research Fellow of the Japan Society for the Promotion of Science (JSPS).     

Cation distribution and interatomic interactions in oxides with heterovalent isomorphism: XII. Gd2Sr1? x Ca x Al2O7 solid solutions

The area of existence of Gd₂Sr_1−x Ca _x Al₂O₇ solid solutions at x ≤ 0.5 was determined by the X-ray phase analysis. It was found by full-profile X-ray structural analysis that, in contrast to La₂Sr_1−x Ca _x Al₂O₇ solid solutions, the Ca²⁺ cations occupy not only AO₉ nine-vertex fragments, but also AO₁₂ oxygen cubooctahedra. Full ordering of Sr²⁺ cations in the perovskite layer is observed at the calcium content x 0.5. Original Russian Text ? I.A. Zvereva, A.G. Cherepova, Yu.E. Smirnov, 2007, published in Zhurnal Obshchei Khimii, 2007, Vol. 77, No. 4, pp. 557–563. For communication XI, see [1].     

Oxygen ionic transport in Bi2O3-based oxides: II. The Bi2O3–ZrO2–Y2O3 and Bi2O3–Nb2O5–Ho2O3 solid solutions

Electrical conductivity, fluorite-type cubic unit cell volume and thermal expansion of the (Bi_{1− x} Nb _x )_{1− y} Ho _y O_1.5+δ (x=0.05 and 0.08; y=0.10−0.15) and (Bi_{1− x} Zr _x )_{1− y} Y _y O_1.5+δ (x=0.05 and 0.07; y=0.15) solid solutions have been found to decrease regularly with increasing dopant content. Annealing at temperatures below 900 K leads to a phase decomposition and to a sharp decrease in conductivity of the ceramics. Oxygen ion transference numbers have been determined by the e.m.f. method and by Faradaic efficiency measurement to exceed 0.9. A new technique of studying Faradaic efficiency has been proposed and verified using (Bi_0.95Zr_0.05)_0.85Y_0.15O_1.5+δ and Zr_0.90Y_0.10O_1.95 ceramic samples. Received: 31 October 1997 / Accepted: 18 December 1997     

Physicochemical properties of the surface of the (Y,La0.1)Ce x Zr1? x O2?δ( x = 0.1–0.7) and Y0.1Pr0.3Zr0.6O2?δ complex oxide systems

The physicochemical properties of the surface of the Y_0.1Ce _x Zr_1−x O_2−δ, La_0.1Ce _x Zr_1−x O_2−δ (x=0.1–0.7), and Y_0.1Pr_0.3Zr_0.6O_2−δ. complex oxide systems were studied using IR and X-ray photoelectron spectroscopies. An appreciable enrichment of the surface of the solids in rare-earth-metal cations (cerium or praseodymium) during the synthesis was revealed. While cations are uniformly spread over the surface of cerium-zirconium solid solutions, the Y_0.1Pr_0.3Zr_0.6O_2−δ surface is covered by the clusters or even a phase of praseodymia. Reductive treatment in hydrogen with subsequent reoxidation results in the segregation of cerium ions on the Y_0.1Ce_0.3Zr_0.6O_2−δ surface at a temperature as low as 770 K. Original Russian Text ? A.N. Kharlanov, L.N. Ikryannikova, V.V. Lunin, A. Yu. Stakheev, 2007, published in Zhurnal Fizicheskoi Khimii, 2007, Vol. 81, No. 7, pp. 1271–1277.     

Cr3+在不同基质Y3Ga5O12或Ga2O3中的荧光特性

以Ga_2O_3、Y_2O_3、Cr(NO_3)_3·9H_2O为原料,柠檬酸为配位剂,通过溶胶-凝胶高温固相合成法制备出Ga_(2-2x)O_3∶2xCr~(3+)(Ga_2O_3∶xCr)与Y_3Ga_(5-5x)O_(12)∶5xCr~(3+)(YGG∶xCr)2种多晶粉体(x=0.01,0.03,0.05,0.07)。并采用X射线衍射(XRD)、红外光谱(IR)、扫描电镜(SEM)、荧光光谱(PL)对样品的结构、组成、形貌和荧光性能进行测试分析。XRD和IR分析结果显示在900℃煅烧后Ga_2O_3∶xCr和YGG∶xCr两种样品均成相。SEM照片显示Ga_2O_3∶xCr样品形貌为柱形多面体,YGG∶xCr为短棒状。PL结果显示Cr~(3+)在Ga_2O_3和YGG两种基质中的最强荧光发射峰分别位于742和740 nm,均属于Cr~(3+)的~2E-~4A_2跃迁,对比发现Cr~(3+)在YGG基质中的荧光发射强度更强,在远红光区的荧光性能更好,能满足温室照明中植物光合作用的需求。     

Phase formation in the V<Subscript>2</Subscript>O<Subscript>5</Subscript>-Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>-MoO<Subscript>3</Subscript> system

Solid-phase interactions in the V₂O₅-Ta₂O₅-MoO₃ system were studied. The formation of com- pounds TaVO₅ and VTa₉O₂₅ in the V₂O₅-Ta₂O₅ binary system was verified. Tetragonal VTa₉O₂₅-base solid solutions of the general formula Ta_{5 + 4x} V_{5 − 4x} O₂₅ (x = 0.25–1) and TaVO₅-base solid solutions of the general formula Ta _x Mo_{1 − x} V_{2 − x} O_{8 − 3x} (x = 0.625–1) were found to form. Subsolidus phase equilibria in the V₂O₅-Ta₂O₅-MoO₃ were determined.     

Effect of oxygen nonstoichiometry on kinetics of oxygen exchange and diffusion in lanthanum-strontium cobaltites

The method of isotopic exchange was used to study the kinetics of oxygen exchange and diffusion in complex oxides of La_{1 − x} Sr _x Co_{1 − y} Fe _y O_{3 − δ} (x = 0.0, y = 0.0; x = 0.6, y = 0.2, 0.4). The rates of oxygen interfacial exchange and its diffusion coefficient were determined for LaCoO_{3 − δ} at the pressure of 5 torr in the temperature range of 600–850°C and at the temperature of 700°C in the pressure range of 1–70 torr. The contributions of the three exchange types were calculated. The order of the dependence of the interfacial exchange rate on the oxygen pressure was 0.51 ± 0.01. In the case of La_0.4Sr_0.6Co_{1 − y} Fe _y O_{3 − δ} (y = 0.2, 0.4) in the temperature range of 600–900°C at the oxygen pressure of 10 torr, the oxygen exchange rates and diffusion coefficients were determined in the material bulk and in the subsurface region; contributions of the three types of exchange were calculated. The paper considers the mechanism of oxygen exchange and diffusion as compared to nonstoichiometry in the oxygen sublattice of the La_{1 − x} Sr _x Co_{1 − y} Fe _y O_{3 − δ} oxides.     

Phase formation and physicochemical properties of perovskite complex oxides based on bismuth ferrite

The effect of the cationic composition on the formation of perovskite solid solutions based on bismuth ferrite(Bi_{1 − x/3}□ _x/3)[Fe_{1 − x} Ti _x ]O₃ with x = 0.00–0.48, Δx = 0.03, has been studied. The homogeneity range of the perovskite phase under the conditions of the solid-phase synthesis of solid solutions has been determined, and the effect of the cationic composition on the physicochemical properties of solid solutions has been studied.