Influence of Al-substitution on structures,resistivity and magnetic properties of LaxSr(2−x)Fe(1+y)Mo(1−y)O6 (x=3y,y=0.05, 0.1, 0.15, 0.2)

Two series of single-phased La_xSr_(2−x)Fe_(1+y)Mo_(1−y)O₆ and La_xSr_(2−x)Fe_(1+0.5y)Al_0.5yMo_(1−y)O₆ (x=3y, y=0.05, 0.1, 0.15 and 0.2) double perovskites were prepared by solid-state reaction. The effects of Al-substitution on the structures, resistivity and magnetic properties of La_xSr_(2−x)Fe_(1+y)Mo_(1−y)O₆ were investigated. Although Al-replacement exhibits a negligible influence of on the B-site ordering degree, it results in the suppression of magnetisation caused by non-magnetic Al³⁺ ions. Reduction of grain sizes leads to increased resistivity, thus an optimised magnetoresistance (MR) behaviour is observed. The greatest MR extent improvement can be obtained when y is 0.15 and the MR% of the Al-doped ceramics reaches −10.5% (10 K, 1 T), which is 2 times greater than that of the undoped ceramics (−4.6%, 10 K, 1 T). Interestingly, the Curie temperature (Tc) of both Al-doped and undoped samples maintained relatively constant values of approximately 420 K and 405 K, respectively, which were different results from the data obtained for similar electron-doping systems in the literature.     

Sol-gel synthesis and spectral characteristics of crystal phosphors Sr2Y8(1 ? x)Eu8x Si6O26

Solid solutions of the compositions Sr₂Y_{8(1 ? x)}Eu_8x Si₆O₂₆ (x = 0.005, 0.01, 0.03, 0.05, 0.1, 0.2, 0.4) based on silicates with the oxyapatite structure have been synthesized using the sol-gel method. The samples have been characterized using X-ray diffraction and energy dispersive X-ray analyses. The morphology of the synthesized solid solutions has been examined. The spectral-luminescent characteristics of crystal phosphors have been investigated.     

Preparation and dielectric properties of (Sr1−xCax)Fe0.5Nb0.5O3; (x=0.0, 0.1, 0.2) ceramics

In the present work, strontium calcium iron niobate ((Sr_1?xCa_x)Fe_0.5Nb_0.5O₃; SCFN) (x=0, 0.1, and 0.2) powders were synthesized for the first time using a molten salt technique. The pure phase perovskite obtained at a relative low calcination temperature of 800 °C was characterized using the X-ray diffraction technique (XRD). SCFN ceramics were fabricated and their properties were investigated. The XRD data of the SCFN ceramics was consistent with an orthorhombic symmetry. However, the solubility of Ca in the SCFN ceramics had an upper limit at x=0.1. All ceramics showed a large dielectric constants. The Ca doping inhibited grain growth, but produced an improvement in dielectric–temperature stability. Furthermore, the doping reduced loss tangent, especially for the x=0.1 sample. These results suggest that the SCFN ceramics prepared from molten salt synthesis exhibit a good dielectric performances, compared to many high dielectric materials that have been prepared using the conventional method.     

Growth of whisker nanocrystals in the (1 ? x)In2O3 · xSeO2 system

PbSe and In films with surface areas of 3 × 3 mm² and thicknesses of 1?C1.5 ??m were consecutively deposited on substrates made of glass of S-29 grade using the vacuum deposition method. The films were oxidized in the dry air atmosphere at a temperature of 550°C. The possibility of forming whisker nanocrystals (1 ? x)In₂O₃ · xSeO₂ on glass substrates was demonstrated. The results of X-ray microanalysis demonstrated the presence of not only lead and indium selenide, but also (1 ? x)In₂O₃ · xSeO₂, PbSeO₃, PbO and SeO₂ on the surfaces of the films under study.     

Colossal dielectric permittivity and electrical properties of the grain boundary of Ca1−3x/2YbxCu3−yMgyTi4O12 (x=0.05, y=0.05 and 0.30)

Dielectric properties of Ca_1−3x/2Yb_xCu_3−yMg_yTi₄O₁₂ (x=0.05, y=0.05 and 0.30) prepared using a modified sol–gel method and sintered at 1070 °C for 4 h were investigated. The mean grain sizes of the CaCu₃Ti₄O₁₂ and co-doped Ca_0.925Yb_0.05Cu_3−yMg_yTi₄O₁₂ (y=0.05 and 0.30) ceramics were ≈15.86, ≈3.37, and ≈2.32 μm, respectively. Interestingly, the dielectric properties can be effectively improved by co-doping with Yb³⁺ and Mg²⁺ ions to simultaneously control the microstructure and properties of grain boundaries, respectively. These properties were improved over those of single-doped and un-doped CaCu₃Ti₄O₁₂ ceramics. A highly frequency−independent colossal dielectric permittivity (≈10⁴) in the range of 10²–10⁶ Hz with very low loss tangent values of 0.018–0.028 at 1 kHz were successfully achieved in the co-doped Ca_0.925Yb_0.05Cu_3−yMg_yTi₄O₁₂ ceramics. Furthermore, the temperature stability of the colossal dielectric response of Ca_1−3x/2Yb_xCu_3−yMg_yTi₄O₁₂ was also improved to values of less than ±15% in the temperature range from −70 to 100 °C.     

Manganese-rich SmBaCo2−x−yMnxMgyO5+δ (x = 0.5, 1, 1.5 and y = 0.05, 0.1) with stable structure and low thermal expansion coefficient as cathode materials for IT-SOFCs

SmBaCo_2−x−yMn_xMg_yO_5+δ (x = 0.5, 1, 1.5 and y = 0.05, 0.1) samples are synthesized by sol-gel method. The influence of different substitution of Mn and Mg for Co on crystal structures, thermal expansion coefficient (TEC), electrical conductivities and electrochemical performances have been investigated. The generation of the secondary phase BaMnO₃ is suppressed with Mg²⁺ increasing. Demonstrated by temperature-dependent X-ray diffraction from 25 °C to 700 °C, the structure of SmBaCo_0.4Mn_1.5Mg_0.1O_5+δ in high temperature is stable. The TEC of SmBaCo_1.45Mn_0.5Mg_0.05O_5+δ, SmBaCo_0.95MnMg_0.05O_5+δ, SmBaCo_0.45Mn_1.5Mg_0.05O_5+δ and SmBaCo_0.4Mn_1.5Mg_0.1O_5+δ are 15.77 × 10⁻⁶ K⁻¹, 16.20 × 10⁻⁶ K⁻¹, 12.19 × 10⁻⁶ K⁻¹ and 12.58 × 10⁻⁶ K⁻¹, respectively, which are much lower than those of cobalt-based layered perovskites and more compatible with the thermal expansion of SDC electrolyte. Although the electrochemical performances of SmBaCo_2−x−yMn_xMg_yO_5+δ (x = 0.5, 1, 1.5 and y = 0.05, 0.1) decrease slightly with Mn increasing, the polarization resistances of the SmBaCo_1.45Mn_0.5Mg_0.05O_5+δ and SmBaCo_0.4Mn_1.5Mg_0.1O_5+δ are 0.17 Ω cm² and 0.30 Ω cm² at 800 °C, respectively, which can meet the electrochemical performance requirements of cathode materials. Among the samples, the SmBaCo_1.45Mn_0.5Mg_0.05O_5+δ and SmBaCo_0.4Mn_1.5Mg_0.1O_5+δ show better tradeoff properties between TEC and electrochemical performance as cathode materials for IT-SOFCs.     

High entropy BaFe12-x(Ti/Mn/Ga/In)xO19 (x = 1–7) oxides: Correlation of the composition,entropy state,magnetic characteristics,and terahertz properties

The BaFe_12-x (Ti/Mn/Ga/In)_xO₁₉ (x = 1–7) high-entropy oxides (HEOs) were obtained by solid-phase synthesis. The correlation of the chemical composition (the level of the Fe replacement), structural parameters, magnetic characteristics, and terahertz (THz) properties was investigated. All studied samples were single-phase (SG: P6₃/mmc). Lattice parameters showed a monotonic increasing trend with an increase of (x). The key role in increasing the lattice parameters of HEOs may belong to the influence of In³⁺ ions, which are much larger in size than Fe³⁺ ions. As one of the possible reason for explanation of the lattice parameters behavior we hypothesized that partial charge transformation of the Mn³⁺ state to the Mn²⁺ state takes place in order to maintain electrical neutrality. Other cations (Ti/Ga/In) have stable oxidation state. The anisotropic nature of the increase in lattice parameters was demonstrated. No strong correlation between chemical composition and microstructural parameters of the investigated HEOs was observed. The magnetic characteristics of the BaFe_12-x (Ti/Mn/Ga/In)_xO₁₉ (x = 1–7) HEOs were investigated at broad intervals of magnetic fields and temperatures. The behavior of the magnetic characteristics was shown to be the result of the magnetic structure frustration. In addition, terahertz electrodynamic properties were studied by measuring spectra of complex dielectric permittivity at frequencies 0.2–1.2 THz in the temperature interval 20–300 K. The spectra indicate the presence of a higher-frequency infrared phonon resonance, whose damping decreases with cooling, as well as an excitation below 0.2 THz that freezes out at low temperatures, the origin of which is associated with the polycrystalline nature of the materials.     

Solution combustion synthesis of La1−xSrxFe1−yCuyO3±w (x=0, 0.2; y=0, 0.2) perovskite nanoparticles: Conventional vs. microwaves ignition

La_1−xSr_xFe_1−yCu_yO_3±w (x=0, 0.2; y=0, 0.2) nanoparticles have been prepared by solution combustion synthesis exploiting both conventional and microwave heating in the ignition of the self-sustaining reactions. Interaction of microwaves with the reaction mixture allowed significant reduction of the ignition time according to the dielectric properties of the precursor gels, which have been measured at room temperature in the 0.5–3 GHz frequency range. Both the ignition strategies led to the preparation of crystalline single-phase products without affecting particles morphology. The ignition technique influenced only the average particles size with those prepared by microwaves-ignition, possessing typically larger dimension, as a probable consequence of the higher temperatures reached due to microwave absorbing products. Perfectly crystallised nanoparticles were obtained after combustion syntheses and calcination at 600 °C for 3 h in the particle size range between 20 and 80 nm dependently upon the heating source and the dopant level.     

Effects of copper doping on the structure,electrical and low-field magnetoresistance properties of (1–x)La0.67Sr0.33MnO3/xCu (x=0–0.15) composite coatings

The doping effect of metallic Cu on the performances of manganite coatings has been investigated. The (1–x)La_0.67Sr_0.33MnO₃/xCu (x=0–0.15) composites were prepared by the sol-gel based screen printing method. XRD and SEM measurements revealed CuO phase to form by oxidation and to distribute mainly at grain boundaries (GBs) of the manganite matrix. Excess Cu entered the manganite lattice without changing its rhombohedral structure, resulting in a slight lattice expansion. The surface morphology of composite samples changed dramatically with different Cu contents, and average particle size refined with a low concentration of Cu. As the Cu content increased, the resistivity of the samples increased while the metal–insulator transition temperature decreased. The enhanced magnetoresistance (MR) effect close to the percolation threshold has been ascribed to the introduction of local disorder at GBs.     

The structural and magnetic features of perovskite oxides La1–xSrxMnO3+δ (x = 0.05, 0.10, 0.20) depending on the strontium doping content and heat treatment

Sr-doped (0<x < 0.2) ceramic samples of the lanthanum manganite oxides were obtained via sol-gel method to investigate the influence of doping on structural, magnetic end electronic responses, and their correlations. Synthesized samples of non-stoichiometric compositions are rhombohedral single-phase. After annealing the formation of a phase-separated system as a mixture of orthorhombic phases was found. The R-3c and PnmaI, PnmaII* and PnmaII phases have been studied using Mössbauer spectroscopy, XRD, SEM analysis and magnetic measurements. The magnetic temperature-concentration phase diagram of La_1–xSr_xMnO_3+δ (x = 0.05, 0.10, 0.20) was obtained.The Jahn-Teller effect or the orbital order breaking, as well as the competition between Mn³⁺–O^2—Mn⁴⁺ double- and Mn³⁺–O^2-–Mn³⁺ superexchange interaction was demonstrated under the effect of cation doping compound and interstitial oxygen value (δ). The relaxation character of the Mössbauer spectra and the type of magnetization dependences revealed nanosized magnetic clusters with fluctuation of their magnetic moment in all perovskite phases. Results are interpreted in terms of matrix – clusters: regions of sample with ferromagnetic type of ordering (cluster) exist in antiferromagnetically or ferromagnetically (with different exchange parameter) ordered matrix. Exchange interaction frustrations of the cluster with the matrix can lead to relaxation behavior of the magnetic moment of the cluster. The clusters size vary from about 3.9 to 5.7 nm. All samples are characterized by the presence of particles agglometates with a typical size about 0.4–0.8 μm; for annealed samples additional non-conducting regions with 80–220 nm in size were found. It is shown that the annealing time significantly affects the production of materials with determined properties and be useful in the applied field in technological processes.     

A new class of Sol–gel derived LiM1xM2yMn2?x?yO3.8F0.2 (M1?=?Cr, M2?=?V; x?=?y?=?0.2) cathodes for lithium batteries

A series of LiM_1xM_2yMn_2−x−yO_3.8F_0.2 (M₁ = Cr, M₂ = V; x = y = 0.2) cathodes, viz., LiMn₂O_3.8F_0.2, LiCr_0.2Mn_1.8O_3.8F_0.2 and LiCr_0.2V_0.2Mn_1.6O_3.8F_0.2 along with native LiMn₂O₄ have been synthesized by Citric Acid assisted Modified (CAM) sol–gel method, with a view to understand the effect of synthesis methodology and the effect of dual category dopants, viz., anion and/or cation upon spinel cathodes individually. An acceptable capacity retention (94%) observed up to 50 cycles for native LiMn₂O₄ cathodes is attributed to the significance of CAM sol–gel method. Similarly, the encouraging charge–discharge results of LiMn₂O_3.8F_0.2 (130 mAh g⁻¹) and LiCr_0.2Mn_1.8O_3.8F_0.2 (142 mAh g⁻¹) cathodes revealed a possible augmentation in the reversible capacity behavior of the spinels upon F⁻ substitution at 32e site and the simultaneous substitution of Cr³⁺ and F⁻ at 16d and 32e sites respectively.     

Crystal structure,microstructure and electrical properties of (1−x−y)Bi0.5Na0.5TiO3–xBi0.5K0.5TiO3–yBiFeO3 ceramics near MPB prepared via the combustion technique

(1?x?y)Bi_0.5Na_0.5TiO₃–xBi_0.5K_0.5TiO₃–yBiFeO₃ (BNKFT-x/y with 0.12≤x≤0.24, 0≤y≤0.07) lead-free piezoelectric ceramics have been prepared by the combustion technique. The effects of amounts of x and y on structures and electrical properties were examined. Powders and ceramics can be well calcined and sintered at 750 °C for 2 h and 1025–1050 °C, respectively. The results indicated that the crystalline structure and microstructure changed with the increase of x and y concentrations. XRD results of BNKFT-x/0.03 and BNKFT-0.18/y ceramics with 0.12≤x≤0.24 and 0≤y≤0.07 showed the rhombohedral–tetragonal morphotropic phase boundary (MPB). The addition of y caused a promoted grain growth while the addition of x suppressed the grain growth. The highest density (ρ=5.85 g/cm³), superior dielectric properties at T_c (ε_r=7846 and tan δ=0.02), remnant polarization measured at 40 kV/cm (P_r = 20.1 μC/cm²) and piezoelectric coefficient (d₃₃=213 pC/N) were obtained for x=0.18 and y=0.03.     

Scanning tunneling microscopy and tunneling spectroscopy studies of niobium-containing H6+xP2W18?xNbxO62 (x=0, 1, 2, 3)Wells-Dawson heteropolyacid catalysts to probe their redox property and oxidation catalysis

Niobium-containing H_6+x P₂W_18−x Nb _x O₆₂ (x=0, 1, 2, 3) Wells-Dawson heteropolyacids (HPAs) were investigated by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) in order to elucidate their redox properties. The HPAs formed two-dimensional well-ordered monolayer arrays on graphite surface and exhibited a distinctive current-voltage behavior called negative differential resistance (NDR) in their tunneling spectra. NDR peak voltage measured on HPA molecule was correlated with reduction potential and absorption edge energy determined by electrochemical method and UV-visible spectroscopy, respectively. NDR peak voltage of H_6+x P₂W_18−x Nb _x O₆₂ Wells-Dawson HPAs appeared at less negative voltage with increasing reduction potential and with decreasing absorption edge energy. Oxidative dehydrogenation of isobutyraldehyde was also carried out as a model reaction to probe oxidation catalysis of the HPAs. The trend of NDR peak voltage of H_6+x P₂W_18−x Nb _x O₆₂ Wells-Dawson HPAs was well consistent with the trend of yield for methacrolein.     

Diruthenium(I,I) Catalysts for the Formation of β‐ and γ‐Lactams via Carbenoid C?H Insertion of α‐Diazoacetamides

Intramolecular carbenoid C H insertion of five α‐diazoacetamides [N₂CH CONR₂, NR₂=NEt₂ ( 3a ), NBu₂ ( 3b ), N(i‐Pr)₂ ( 3c ), N(CH₂Ph)₂ ( 3d ), N(i‐Pr)(CH₂Ph) ( 3e )], was investigated using as catalysts dinuclear Ru(I,I) complexes of the type [Ru₂(μ‐L¹)₂(CO)₄L²₂], where L¹ is a bidentate bridging acetate, calix[4]arenedicarboxylate, saccharinate, pyridin‐2‐olate, or triazenide ligand, as well as [RuCl₂(p‐cymene)]₂. The Ru(I,I) complexes were found to be suitable catalysts for the carbenoid cyclization reactions, except in the case of 3a . With diazoamides 3b–e , [Ru₂(μ‐sac)₂(CO)₅]₂ (sac=saccharinate) and [Ru₂(μ‐6‐chloropyridin‐2‐olate)₂(CH₃CN)₂(CO)₄] are as effective as Rh₂(OAc)₄ under the same conditions, although some differences in the regioselectivity and chemoselectivity of the cyclization are observed. The carbenoid cyclization reactions yield γ‐lactams from diazoamides 3a and 3b , both a β‐ and a γ‐lactam from 3c , and a β‐lactam as well as a 3‐azabicyclo[5.3.0]deca‐5,7,9‐trien‐2‐one from 3d . With 3e , formation of γ‐lactam 21 and of bicyclic lactam 23 prevails.