Mechanical and electrical properties of Ni0.65Zn0.35CuxFe2−xO4 ferrite

A series of samples in the system Ni_0.65Zn_0.35Cu_xFe_2?xO₄ (x=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by the usual ceramic technique. X-ray analysis showed that they were cubic spinel (single phase). Young's modulus, the dielectric loss and the change in capacitance under mechanical stress were measured for the samples. Young's modulus decreased with increasing Cu content. This is due to the fact that Cu²⁺ ions entered the lattice substitutionally for Fe³⁺ ions at the octahedral sites, creating lattice vacancies gave rise to lattice strain. The minimum value of the dielectric loss corresponding tox=0.3 may be due to the formation of lattice vacancies retarding the jump frequency to be far from the frequency of the applied a.c. field. The increase in capacitance of the samples with mechanical stress may be explained via the mechanism of dielectric polarization.     

Solid state reaction synthesis of filled skutterudite compounds (Ce or Y)yFexCo4-xSb12 and the effect of filling atoms Ce or Y on lattice thermal conductivity

The synthesis of filled skutterudite compounds (Ce or Y)yFexCo₄-xSb12, through a solid state reaction using chloride of Ce or Y, high purity powder of Co, Fe, and Sb as starting materials, was investigated. (Ce or Y)yFexCo₄-xSb12 (x = 0 1.0,y = 0 0.15) compounds were obtained at 850 1 123 K. The results of Rietveld analysis demonstrate that (Ce or Y)yFexCo₄-xSb12 synthesized by a solid state reaction possesses a filled skutterudite structure. The filling fraction of Ce or Y obtained by Rietveld analysis agrees well with the composition obtained by chemical analysis. The lattice constant of CeyFexCo₄-xSb12 increases with increasing substitution of Fe at Co sites, and with an increasing Ce filling fraction in the Sb-dodecahedron voids. The lattice thermal conductivity of (Ce or Y)yFexCo₄-xSb12 decreases significantly with an increasing Ce or Y filling fraction in the voids and with substitution of Fe at Co sites.     

Observation of ordering in silicon and germanium mullites

The local order is studied in silicon and germanium mullites Al_4+2xM_2−2xO_10−x (M = Si or Ge) using high resolution transmission electron microscopy. A superstructure is observed for x = 0.5 and a structural model is proposed from image simulation. Images reconstructed after selecting the diffuse scattering located in the (010) reciprocal plane reveal short range ordered domains for compositions with a low concentration of oxygen vacancies (x < 0.40).     

Thermal Expansion in the Zr and 1-, 2-valent Complex Phosphates of NaZr2(PO4)3 (NZP) Structure

A_5–4xZr_xZr(PO₄)₃ (A=Na, K;0≤x≤1.25), Na_1-xCd_0.5xZr₂(PO₄)₃ (0≤x≤1), Na_5–xCd_0.5xZr(PO₄)₃ (0≤x≤4) compositions which belong to the NZP structural family were synthesized using the sol-gel method. The lattice thermal expansion of members of these rows were determined up to 600°C by high-temperature X-ray diffractometry. The axial thermal expansion coefficients change from -5.8·10^-6to 7.5·10^-6 °C^-1 (α_a) and from 2.6·10^–6 to 22·10^–6 °C^-1 (α_c). These results, in addition to those for other NZP compounds allow us to explain their low thermal expansion. The mechanism can be attributed to strongly bonded three-dimensional network structure, the existence of structural holes capable to damp some of the thermal vibrations and anisotropyin the thermal expansion of the lattice. This revised version was published online in August 2006 with corrections to the Cover Date.     

Physicochemical and catalytic properties of systems based on CeO<Subscript>2</Subscript>

The effect of synthesis conditions, the nature of components, and the ratio between the components on the phase composition, the texture, and the redox and catalytic properties of the Ce-Zr-O, Ce-Zr-M₁-O (M₁ = Mn, Ni, Cu, Y, La, Pr, or Nd), N/Ce-Zr-O (N = Rh, Pd, or Pt), and Pd/Ce-Zr-M₂-O/Al₂O₃ (M₂ = Mg, Ca, Sr, Ba, Y, La, Pr, Nd, or Sm) was considered. A cubic solid solution with the fluorite structure was formed on the introduction of <50 mol % zirconium into CeO₂, and the stability of this solid solution depended on preparation procedure and treatment conditions. The presence of transition or rare earth elements in certain concentrations extended the range of compositions with the retained fluorite structure. The texture of the Ce-Zr-O system mainly depended on treatment temperature. An increase in this temperature resulted in a decrease in the specific surface area of the samples. The total pore volume varied over the range of 0.2–0.3 cm³/g and depended on the Ce/Zr ratio. The presence of transition or rare earth elements either increased the specific surface area of the system or made it more stable to thermal treatment. The introduction of the isovalent cation Zr⁴⁺ into CeO₂ increased the number of lattice defects both on the surface and in the bulk to increase the mobility of oxygen and facilitate its diffusion in the Ce_{1 − x} Zr _x O₂ lattice. The catalytic properties of the Ce-Zr-M₁-O or N/Ce-Zr-M₂-O systems were due to the presence of anion vacancies and the easy transitions Ce⁴⁺ ai Ce³⁺, M₁²ⁿ⁺ ai M₁ ⁿ⁺, and N ^δ+ → N ⁰ in the case of noble metals.     

New Compounds of the ThCr2Si2-Type and the Electronic Structure of CaM2Ge2 (M: Mn-Zn)

Two new compounds were synthesized by heating mixtures of the elements at 975-1025 K and characterized by single-crystal X-ray methods. CaZn₂Si₂ (a=4.173(2) Å, c=10.576(5) Å) and EuZn₂Ge₂ (a=4.348(2) Å, c=10.589(9) Å) crystallize in the ThCr₂Si₂-type structure (space group I4/mmm; Z=2). Magnetic susceptibility measurements of EuZn₂Ge₂ show Curie-Weiss behavior with a magnetic moment of 7.85(5)μ_B/Eu and a paramagnetic Curie temperature of 10(1) K. EuZn₂Ge₂ orders antiferromagnetically at T_N=10.0(5) K and undergoes a metamagnetic transition at a low critical field of about 0.3(2) T. The saturation magnetization at 2 K and 5.5 T is 6.60(5) μ_B/Eu. ¹⁵¹Eu Mössbauer spectroscopic experiments show one signal at 78 K at an isomer shift of −11.4(1) mm/s and a line width of 2.7(1) mm/s compatible with divalent europium. At 4.2 K full magnetic hyperfine field splitting with a field of 26.4(4) T is detected. The already known compounds CaM₂Ge₂ (M: Mn-Zn) also crystallize in the ThCr₂Si₂-type structure. Their MGe₄ tetrahedra are strongly distorted with M=Ni and nearly undistorted with M=Mn or Zn. According to LMTO electronic band structure calculations, the distortion is driven by a charge transfer from M-Ge antibonding to bonding levels.     

Positron lifetime study of Pr1−xCaxMnO3 (x=0.5, 0.3) during magnetic transition

We measured the positron lifetime in perovskite manganites Pr_1−xCa_xMnO₃ (x=0.3, 0.5). Two lifetime components were observed for each compound; they were attributed to the annihilation of free positrons and positrons trapped at the A-site vacancies. The positron lifetime at the A-site vacancies changed significantly during the antiferromagnetic transition in both the compounds, whereas it was constant around the charge-ordering transition. This change indicates that the electron distribution at the vacancies changed possibly due to the change in the electron distribution of neighboring oxygen atoms. This result indicates that positron lifetime measurements can provide unique information on electronic states during a spin-related phase transition in various oxide materials.     

Tuning thermal stability and electronic properties of germanium oxide on Ge(001) surface with the incorporation of nitrogen

The application of germanium as a channel material of transistors in near future requires an improved understanding of the interface between germanium and its potential passivation layer. In this study, we study effects of nitrogen incorporation on the thermal stability and electronic properties of GeO_xN_y/Ge interface by using high‐resolution X‐ray photoemission spectroscopy. We find that with the increasing nitrogen concentration in the GeO_xN_y films, the thermal stability can be increased, while the valence band offset with the Ge(001)substrate is decreased. First‐principles calculations further suggest that the unpaired p orbitals of nitrogen atoms induce electronic states near valence band edge, contributing to the reduction of the valence band offset. Our results provide a possibility to tune electronic and thermal properties of GeO_xN_y/Ge interface by controlling nitrogen concentrations during the growth.     

Optimisation des facteurs influencant la conductivité anionique dans quelques fluorures de structure fluorine

The large influence in the M_1?xM′_xF_2+x solid solutions (M = Sr, Pb, M′ = Y, In, Sb, Bi) of the covalency of the MF₂ “starting lattice” on the electrical properties of fluorides of fluorite-type structure is clearly shown in a comparative investigation. The influence of the polarizability of the substituting trivalent ion is only significant as far as the starting lattice contains a weakly polarizable cation. Enhancement of the electrical performances of β-PbF₂ by substitution of Pb²⁺ by trivalent cations seems to be due mainly to increasing disordering within the anionic sublattice and hence the role of cationic polarizability is apparently a second-order effect.     

Doping studies of the magnetic cobaltocuprate CoSr2Y2−xCexCu2O9±δ

A structural, magnetic and electronic study of the cobaltocuprate CoSr₂Y_2−xCe_xCu₂O_9±δ (x=0.5-0.8) has been performed. All materials crystallise in the orthorhombic Cmcm symmetry space group in which chains of corner linked CoO₄ tetrahedra run parallel to the 1 1 0 direction. An antiferromagnetic transition is observed for x=0.5-0.8; T_M increases with x. A change in the dimensionality of the magnetic order occurs at x=0.8 as the interchain distance increases to a critical value. There is charge transfer between the cuprate planes and cobaltate layer as Ce doping increases, so that Co³⁺ is partially oxidised to Co⁴⁺ with a concomitant reduction in the valence of Cu. Superconductivity is not observed in any of the samples and a crossover from Mott to Efros and Shklovskii variable range hopping behaviour is evidenced as x increases from 0.5 to 0.8.     

Structural and Electronic Effects of Carbon‐Supported PtxPd1−x Nanoparticles on the Electrocatalytic Activity of the Oxygen‐Reduction Reaction and on Methanol Tolerance

We report a systematic investigation on the structural and electronic effects of carbon‐supported Pt_xPd_1?x bimetallic nanoparticles on the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) in acid electrolyte. Pt_xPd_1?x/C nanocatalysts with various Pt/Pd atomic ratios (x=0.25, 0.5, and 0.75) were synthesized by using a borohydride‐reduction method. Rotating‐disk electrode measurements revealed that the Pt₃Pd₁/C nanocatalyst has a synergistic effect on the ORR, showing 50 % enhancement, and an antagonistic effect on the MOR, showing 90 % reduction, relative to JM 20 Pt/C on a mass basis. The extent of alloying and Pt d‐band vacancies of the Pt_xPd_1?x/C nanocatalysts were explored by extended X‐ray absorption fine‐structure spectroscopy (EXAFS) and X‐ray absorption near‐edge structure spectroscopy (XANES). The structure–activity relationship indicates that ORR activity and methanol tolerance of the nanocatalysts strongly depend on their extent of alloying and d‐band vacancies. The optimal composition for enhanced ORR activity is Pt₃Pd₁/C, with high extent of alloying and low Pt d‐band vacancies, owing to favorable O? O scission and inhibited formation of oxygenated intermediates. MOR activity also shows structure dependence. For example, Pt₁Pd₃/C with Pt_rich?corePd_rich?shell structure possesses lower MOR activity than the Pt₃Pd₁/C nanocatalyst with random alloy structure. Herein, extent of alloying and d‐band vacancies reveal new insights into the synergistic and antagonistic effects of the Pt_xPd_1?x/C nanocatalysts on surface reactivity.     

Cation ordering in the fluorite-like transparent conductors In4+xSn3−2xSbxO12 and In6TeO12

The cation ordering in the fluorite-like transparent conductors In_4+xSn_3−2xSb_xO₁₂ and In₆TeO₁₂, was investigated by Time of Flight Neutron Powder Diffraction and X-ray Powder Diffraction (tellurate). The structural results including atomic positions, cation distributions, metal-oxygen distances and metal-oxygen-metal angles point to a progressive cation ordering on both sites of the Tb₇O₁₂-type structure with a strong preference of the smaller 4d¹⁰ cations (Sn⁴⁺, Sb⁵⁺, Te⁶⁺) for the octahedral sites. The corresponding increase of the overall structure-bonding anisotropy is analyzed in terms of the crystal chemical properties of the OM₄ tetrahedral network of the antistructure. The relationships between the M₇O₁₂ and the M₂O₃ bixbyite-type structures are explored. Within the whole series of compositions In_4+xM_3−xO₁₂ (M=Sn, Sb, Te) there exists an increase of the symmetry gap between the more symmetrical bixbyite structure and the M₇O₁₂ type. This is tentatively correlated with the progressive weakening of thermal stability of these compositions from Sn to Te via Sb.     

Thermally activated and non-activated charge transport processes in the oxides NaxMo2

The electrical properties of Na_xM_1+yO₂ (M=Mn,Co) were measured as a function of temperature under conditions of thermodynamic equilibrium for a quenched defect structure. The results of electrochemical studies of Na_xM_1+yO₂ are also presented. A correlation is demonstrated between the structures of the ionic and electronic defects in these materials and the potential changes of the cathode in Na/Na⁺/Na_xM_1+yO₂.The author is grateful to Prof. S. Mrowec and Dr. A. Stoklosa for helpful discussions.     

Targeting Vacancies in Nitridosilicates: Aliovalent Substitution of M2+ (M=Ca,Sr) by Sc3+ and U3+

Based on the known linking options of their fundamental building unit, that is the SiN₄ tetrahedron, nitridosilicates belong to the inorganic compound classes with the greatest structural variability. Although facilitating the discovery of novel Si–N networks, this variability represents a challenge when targeting non‐stoichometric compounds. Meeting this challenge, a strategy for targeted creation of vacancies in highly condensed nitridosilicates by exchanging divalent M²⁺ for trivalent M³⁺ using the ion exchange approach is reported. As proof of concept, the first Sc and U nitridosilicates were prepared from α‐Ca₂Si₅N₈ and Sr₂Si₅N₈. Powder X‐ray diffraction (XRD) and synchrotron single‐crystal XRD showed random vacancy distribution in Sc_0.2Ca_1.7Si₅N₈, and partial vacancy ordering in U_0.5xSr_2?0.75xSi₅N₈ with x≈1.05. The high chemical stability of U nitridosilicates makes them interesting candidates for immobilization of actinides.     

Oxygen Vacancies in Amorphous InOx Nanoribbons Enhance CO2 Adsorption and Activation for CO2 Electroreduction

Tuning surface electron transfer process by oxygen (O)‐vacancy engineering is an efficient strategy to develop enhanced catalysts for CO₂ electroreduction (CO₂ER). Herein, a series of distinct InO_x NRs with different numbers of O‐vacancies, namely, pristine (P‐InO_x), low vacancy (O‐InO_x) and high‐vacancy (H‐InO_x) NRs, have been prepared by simple thermal treatments. The H‐InO_x NRs show enhanced performance with a best formic acid (HCOOH) selectivity of up to 91.7 % as well as high HCOOH partial current density over a wide range of potentials, largely outperforming those of the P‐InO_x and O‐InO_x NRs. The H‐InO_x NRs are more durable and have a limited activity decay after continuous operating for more than 20 h. The improved performance is attributable to the abundant O‐vacancies in the amorphous H‐InO_x NRs, which optimizes CO₂ adsorption/activation and facilitates electron transfer for efficient CO₂ER.     

A Study of Different Doped Metal Cations on the Physicochemical Properties and Catalytic Activities of Ce20M1Ox (M=Zr,Cr, Mn,Fe, Co,Sn) Composite Oxides for Nitric Oxide Reduction by Carbon Monoxide

This work is mainly focused on investigating the effects of different doped metal cations on the formation of Ce₂₀M₁O_x (M=Zr, Cr, Mn, Fe, Co, Sn) composite oxides and their physicochemical and catalytic properties for NO reduction by CO as a model reaction. The obtained samples were characterized by using N₂ physisorption, X‐ray diffraction, laser Raman spectroscopy, UV/Vis diffuse reflectance spectroscopy, inductively coupled plasma atomic emission spectroscopy, X‐ray photoelectron spectroscopy, temperature‐programmed reduction by hydrogen and by oxygen (H₂‐TPR and O₂‐TPD), in situ diffuse reflectance infrared Fourier transform spectroscopy, and the NO+CO model reaction. The results imply that the introduction of M^x+ into the lattice of CeO₂ increases the specific surface area and pore volume, especially for variable valence metal cations, and enhances the catalytic performance to a great extent. In this regard, increases in the oxygen vacancies, reduction properties, and chemisorbed O₂^? (and/or O^?) species of these Ce₂₀M₁O_x composite oxides (M refers to variable valence metals) play significant roles in this reaction. Among the samples, Ce₂₀Cr₁O_x exhibited the best catalytic performance, mainly because it has the best reducibility and more chemisorbed oxygen, and significant reasons for these attributes may be closely related to favorable synergistic interactions of the vacancies and near‐surface Ce³⁺ and Cr³⁺. Finally, a possible reaction mechanism was tentatively proposed to understand the reactions.     

Preparation,Crystallographic, Spectroscopic and Magnetic Characterization of Low‐Valency Nitridometalates Li2[(Li1‐xMx)N] with M = Cu,Ni

Thermogravimetric and difference thermal analyses show that the reactions of lithium nitride with the transition metals Cu and Ni under molecular nitrogen to form phases Li₂[(Li_1‐xM^I_x)N] take place above 673 K. The maximum weight gains are reached at 926 K and 968 K for M = Cu and Ni, respectively. At higher temperatures, the ternary phases Li₂[(Li_1‐xM^I_x)N] decompose, limiting the substitutional level x. In the temperature range of 773 K — 873 K, the successful synthesis of Li₂[(Li_1‐xNi^I_x)N] (0 < x ≤ 0.85(1)) single phase products is demonstrated. Maximum substitution obtained for the Cu phases is x_max= 0.43(1). The dependence of the lattice parameters of the hexagonal unit cell on x is almost linear. The magnetic moment of M strongly depends on x. At low x the magnetic moments in phases with M = Ni are presumably enhanced by orbital effects. A decrease of μ_eff with x to μ_eff(x = 1) → 0 is explained by delocalization of the magnetic moments and by the gradual formation of a metal for the hypothetical compound Li₂[NiN] (x = 1). XAS spectroscopy at the transition metal K‐edges shows that Cu and Ni principally correspond to d¹⁰‐ and d⁹‐configurations, respectively.     

Characterization of phase transition of Ba2-xSrxIn2O5 by thermal analysis and high temperature X-ray diffraction

The phase transitions of Ba_2-xSr_xIn₂O₅ were investigated with various thermal analyses and high-temperature X-ray diffraction. It was clarified that crystal structure of Ba_2-xSr_xIn₂O₅ with x=0.0~0.4 varies from brownmillerite through distorted perovskite to another distorted perovskite with increase of temperature. The phase transition from brownmillerite to distorted perovskite was revealed to be first order, whereas transition from distorted perovskite to another one was second order. The specimen with x≥0.5 showed only one first order phase transition from brownmillerite to distorted perovskite. The phase diagram of Ba_2-xSr_xIn₂O₅ was established and existence of tricritical point at ~1100°C with x=0.4~0.5 was suggested. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electric properties of solid solutions based on strontium tantalate with perovskite-type structure. Oxygen-ionic conductivity

Total conductivity of Sr_{6 − 2x} Ta_{2 + 2x} O_{11 + 3x} (0 ≤ x ≤ 0.33) solid solutions with a cryolite structure is studied in the atmosphere with a low water vapor content under variation of the temperature (500 < T < 1000°C) and oxygen activity in the gas phase (10⁻¹⁸ < aO₂ < 0.21). Conductivity is divided into components. It is found that both the oxygen-ionic conductivity and the mobility of oxygen ions increase and the percentage of p-type electronic conductivity decreases at an increase in the concentration of oxygen vacancies. It is shown that compositions with a high strontium oxide content (0 ≤ x < 0.15) and accordingly high concentration of oxygen vacancies correspond to the maximum values of the oxygen-ionic conductivity and low activation energies. The Sr₆Ta₂O₁₁ and Sr_5.92Ta_2.08O_11.12 compositions in a wide range of aO₂ are characterized by negligibly low fractions of electronic conductivity component. For the compositions of x > 0.15, a transition occurs from the electrolytic region to mixed oxygen-hole conductivity character (at aO₂ = 0.21) at an increase in the oxygen activity.     

Targeting Vacancies in Nitridosilicates: Aliovalent Substitution of M2+ (M=Ca,Sr) by Sc3+ and U3+

Based on the known linking options of their fundamental building unit, that is the SiN₄ tetrahedron, nitridosilicates belong to the inorganic compound classes with the greatest structural variability. Although facilitating the discovery of novel Si–N networks, this variability represents a challenge when targeting non‐stoichometric compounds. Meeting this challenge, a strategy for targeted creation of vacancies in highly condensed nitridosilicates by exchanging divalent M²⁺ for trivalent M³⁺ using the ion exchange approach is reported. As proof of concept, the first Sc and U nitridosilicates were prepared from α‐Ca₂Si₅N₈ and Sr₂Si₅N₈. Powder X‐ray diffraction (XRD) and synchrotron single‐crystal XRD showed random vacancy distribution in Sc_0.2Ca_1.7Si₅N₈, and partial vacancy ordering in U_0.5xSr_2?0.75xSi₅N₈ with x≈1.05. The high chemical stability of U nitridosilicates makes them interesting candidates for immobilization of actinides.