Synthesis of new (Bi,La)<Subscript>3</Subscript>MSb<Subscript>2</Subscript>O<Subscript>11</Subscript> phases (M = Cr,Mn, Fe) with KSbO<Subscript>3</Subscript>-type structure and their magnetic and photocatalytic properties

Synthesis and structure of new (Bi, La)₃MSb₂O₁₁ phases (M = Cr, Mn, Fe) are reported in conjunction with their magnetic and photocatalytic properties. XRD refinements reflect that Bi₃CrSb₂O₁₁, Bi₂LaCrSb₂O₁₁, Bi₂LaMnSb₂O₁₁ and Bi₂LaFeSb₂O₁₁ adopt KSbO₃-type structure (space group, Pn[`3])Pn\overline{3}). The structure can be described through three interpenetrating networks where the first is the (M/Sb)O₆ octahedral network and other two are the identical networks having Bi₆O₄ composition. The magnetic measurements on Bi₂LaCrSb₂O₁₁ and Bi₂LaMnSb₂O₁₁ show paramagnetic behaviour with magnetic moments close to the expected spin only magnetic moments of Cr^+ 3 and Mn^+ 3. The UV-Visible diffuse reflectance spectra are broad and indicate that these materials possess a bandgap of ∼ 2 eV. The photocatalytic activity of these materials has been investigated by degrading Malachite Green (MG) under exposure to UV light.     

Low-field Magnetoresistance,Specific Heat and Magnetocaloric Effect in Sr Substituted Pr<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript>

We investigate the effect of ionic size variation on the electrical and thermodynamic properties in a series of Pr_0.7Ca_0.3−x Sr _x MnO₃ (PCSMO) samples. The increase in Sr content results in an increase of the unit cell volume, as a bigger Sr²⁺ ion replaces the smaller Ca²⁺ ions. Resistivity measurements show that the increase in the Sr content also results in the induction of a metal–insulator transition (T _MI), which increases with increasing Sr content. The activation energy (E _a), calculated from the resistivity data, decreases with increasing Sr content confirming the metallic character. The effect of the magnetic field on resistivity and specific heat has also been studied.     

Synthesis,transport and dielectric properties of polyaniline/Co<Subscript>3</Subscript>O<Subscript>4</Subscript> composites

Conducting polyaniline/cobaltous oxide composites have been synthesized using in situ deposition technique by placing fine graded/cobaltous oxide in polymerization mixture of aniline. The a.c. conductivity and dielectric properties are studied by sandwiching the pellets of these composites between the silver electrodes. It is observed that the values of conductivities increase up to 30 wt% of cobaltous oxide in polyaniline and decrease thereafter. Initial increment in conductivity is due to extended chain length of polyaniline where polarons possess sufficient energy to hop between favourable sites. Beyond 30 wt% of cobaltous oxide in polyaniline, blocking of charge carriers takes place reducing the conductivity values. It can be noted that the value of dielectric constant increases up to 10 wt% of cobaltous oxide. Thereafter, it decreases up to 30 wt% of cobaltous oxide and again increases up to 40 wt% of cobaltous oxide and decreases thereafter. The observed behaviour is attributed to the variation of a.c. conductivity. And it is observed that the dielectric loss increases up to 10 wt% of cobaltous oxide in polyaniline, decreases to a lower value of 20 wt% of cobaltous oxide and increases to 35 wt% and thereafter decreases. These values go in accordance with the values of dielectric constant. The results obtained for these composites are of greater scientific and technological importance.     

Electric resistance and magnetoresistance of a two-layer epitaxial heterostructure (30 nm)La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>/(30 nm)La<Subscript>0.67</Subscript>Ba<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>

Two-layer epitaxial heterostructures (30 nm)La_0.67Ca_0.33MnO₃/(30 nm)La_0.67Ba_0.33MnO₃ (LCMO/LBMO) have been grown by laser deposition on single crystal (001)LaAlO₃ (LAO) substrates. In this system, the upper (LCMO) layer occurs under the action of tensile stresses in the substrate plane, whereas the lower (LBMO) layer exhibits biaxial compression. The formation of a 30-nm-thick LCMO film on the surface of the 30-nm-thick LBMO layer leads to an increase in the level of mechanical stresses in the latter layer. The maximum electric resistivity ρ of the (30 nm)LCMO/(30 nm)LBMO/LAO structure was observed at a temperature 25–30 K below that corresponding to the maximum of the ρ(T) curve for a single (30 nm)LBMO film on the same LAO substrate.     

Synthesis of nanocomposites based on MO-Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-B<Subscript>2</Subscript>O<Subscript>3</Subscript> (M = Ca,Sr, Ba) glasses

This work examines the effect of KBF₄ additions on the crystallization behavior of glasses based on the multicomponent systems MO-Bi₂O₃-B₂O₃ with M = Ca, Sr, and Ba. The glass-ceramic composites obtained contain a δ-Bi₂O₃-based crystalline phase with a crystallite size of ≃7 nm, evenly distributed over the glass matrix. The 400°C electrical conductivity of the nanocomposites reaches 2 × 10⁻⁴ S/cm, and the activation energy is 1.1 eV, typical of anion conduction. These values are comparable to those reported for δ-Bi₂O₃ ceramics.     

Interface roughness influence on exchange bias effect in La<Subscript>2/3</Subscript>Ca<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript>/La<Subscript>1/3</Subscript>Ca<Subscript>2/3</Subscript>MnO<Subscript>3</Subscript> bilayers

A Monte Carlo simulation study of La_2/3Ca_1/3MnO₃/La_1/3Ca_2/3MnO₃ bilayers exchange bias (EB) properties by using a classical Heisenberg model and Monte Carlo method with Metropolis algorithm is addressed. Samples were built atom-by-atom in order to resemble the real roughness. In this model, several contributions included nearest neighbors exchange interactions; two different interface couplings, magnetocrystalline anisotropy and Zeeman term, were considered. Here, an influence of the relaxation steps on the interface roughness is present. Our study focuses on the influence of interface roughness on hysteresis loops, particularly EB field (H _ex) and coercive force (H _c). Results reveal that H _ex and H _c decrease as the interface roughness increases.     

Substrate effect on the properties of La<Subscript>0.775</Subscript>Sr<Subscript>0.225</Subscript>MnO<Subscript>3</Subscript> films

La_0.775Sr_0.225MnO₃ films have been produced by screen printing on various substrates (Al₂O₃, BaTi_0.85Zr_0.11Sn_0.04O₃, Ba_0.996Y_0.004TiO₃, Ba_0.996Y_0.004TiO₃ + 0.04%Mn, and Ba_0.996Y_0.004Ti_0.65Sn_0.35O₃), and their electrical properties have been studied in comparison with those of bulk materials. The structural properties of the substrates are shown to influence the electrical properties of the films.     

Thermoelectric properties of WO<Subscript>3</Subscript>-based ceramics doped with Co<Subscript>2</Subscript>O<Subscript>3</Subscript>

Tungsten trioxide (WO₃) doped with cobalt sesquioxide (Co₂O₃) was prepared by a conventional mixed oxide processing route and the thermoelectric properties were studied from 300 up to 1,000 K. The addition of Co₂O₃ to WO₃ resulted in an increase in both the grain size and porosity, indicating that Co₂O₃ promotes the grain grown of WO₃. The magnitude of the electrical conductivity (σ) and the absolute value of the Seebeck coefficient (|S|) depended strongly on the Co₂O₃ content. As for the power factor (σS ² ), the 5.0 mol% sample has the maximum value of the power factor which is 0.12 μWm⁻¹K⁻² at 873 K.     

Tunneling magnetoresistance in a Eu<Subscript>0.7</Subscript>Pb<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> (single crystal)-Fe (film) structure

We have studied the magnetoresistive properties of a structure comprising single crystal manganite Eu_0.7Pb_0.3MnO₃ covered with an epitaxial iron film. At temperatures below T _C of the manganite crystal, the structure exhibits positive magnetoresistance. The behavior of the resistance as a function of the magnetic field is characteristic of a tunneling junction with ferromagnetic electrodes separated by a thin insulating film. The observed effect is related to the formation of a transition layer at the manganite-Fe interface, which is depleted of oxygen and possesses dielectric properties. The sensitivity of the resistance with respect to the magnetic field is determined both by the negative magnetoresistance of the manganite crystal and by the tunneling contribution to the conductivity, whereby the tunneling current depends on the mutual orientation of magnetic moments of the electrodes (Eu_0.7Pb_0.3MnO₃ crystal and Fe film).     

Synthesis and structure of (NH<Subscript>4</Subscript>)<Subscript>3</Subscript>[UO<Subscript>2</Subscript>(CH<Subscript>3</Subscript>COO)<Subscript>3</Subscript>]<Subscript>2</Subscript>(NCS)

The compound (NH₄)₃[UO₂(CH₃COO)₃]₂(NCS) (I) was synthesized and examined by single crystal X-ray diffraction analysis. The compound crystallizes in the rhombic system with the unit cell parameters a = 11.5546(4), b = 18.5548(7), c = 6.7222(3) Å, V = 1441.19(10) ų, space group P2₁2₁2, Z = 2, R = 0.0345. The uranium-containing structural units of crystals of I are isolated mononuclear groups [UO₂(CH₃COO)₃]^? belonging to crystal-chemical group AB ₃ ⁰¹ (A = UO ₂ ²⁺ , B⁰¹ = CH₃COO^?) of uranyl complexes. The specific features of packing of the uranium-containing complexes in the crystal structure are considered.     

Synthesis,microstructure, and gas-sensing properties of SnO<Subscript>2</Subscript>/MoO<Subscript>3</Subscript> nanocomposites

SnO₂/MoO₃ nanocomposites are synthesized in a broad composition range through chemical precipitation from solution, and their phase composition and microstructure are investigated by x-ray diffraction. The gas sensitivity of the nanocomposites to lower alcohols (C_nH_{2n + 1} OH, n=1–4) is studied by in situ conductance measurements. The results are interpreted in terms of the acid-base properties of the nanocomposite surface.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 4, 2005, pp. 442–449.Original Russian Text Copyright © 2005 by Makeeva, Rumyantseva, Gaskov.     

Ac and dc magnetotransport properties of the phase-separated La<Subscript>0.6</Subscript>Y<Subscript>0.1</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> manganite

The physical properties of the La_0.6Y_0.1Ca_0.3MnO₃ compound have been investigated, focusing on the magnetoresistance phenomenon studied by both dc and ac electrical transport measurements. X-ray diffraction and scanning electron microscopy analysis of ceramic samples prepared by the sol–gel method revealed that specimens are single phase and have average grain size of ∼0.5 μm. Magnetization and 4-probe dc electrical resistivity ρ(T,H) experiments showed that a ferromagnetic transition at T _C ∼ 170 K is closely related to a metal-insulator (MI) transition occurring at essentially the same temperature T _MI . The magnetoresistance effect was found to be more pronounced at low applied fields (H ≤ 2.5 T) and temperatures close to the MI transition. The ac electrical transport was investigated by impedance spectroscopy Z(f,T,H) under applied magnetic field H up to 1 T. The Z(f,T,H) data exhibited two well-defined relaxation processes that exhibit different behaviors depending on the temperature and applied magnetic field. Pronounced effects were observed close to T _C and were associated with the coexistence of clusters with different electronic and magnetic properties. In addition, the appreciable decrease of the electrical permittivity ε′(T,H) is consistent with changes in the concentration of e _g mobile holes, a feature much more pronounced close to T _C .     

Synthesis of anisotropic NaNbO<Subscript>3</Subscript> seed crystals and fabrication of textured (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript>-based ceramics

High aspect ratio patelike NaNbO₃ particles with pure perovskite structure have been successfully synthesized by topochemical microcrystal conversion (TMC) from plate-like precursor particles of the layer-structured Bi_2.5Na_3.5Nb₅O₁₈. By changing the Bi_2.5Na_3.5Nb₅O₁₈/Na₂CO₃ ratio, large and thin NaNbO₃ particles with a thickness of approximately 0.5 μm and a width of approximately 20 μm were obtained. The obtained NaNbO₃ particles is quite suitable for fabricating textured (K_0.5Na_0.5)NbO₃-based ceramics. Using the fine platelike NaNbO₃ particles as templates, dense <001> -oriented (K_0.5Na_0.5)NbO₃-0.5 mol %MnO₂ ceramics with high texture quality (Lotgering factor F ₀₀₁ = 87 %) and excellent piezoelectric properties were produced by templated grain growth. Compared with randomly oriented ceramics, textured samples show greatly enhanced properties. The room-temperature strain S, the piezoelectric coefficient d ₃₃ ^* and d ₃₃ reach up to 0.093 %, 233 pm/V and 195pC/N, respectively, which are all about 1.5 times larger than those of non-textured ceramics.     

Structural and Magnetic Properties of La<Subscript>2/3</Subscript>Sr<Subscript>1/3</Subscript>MnO<Subscript>3</Subscript>/SrTiO<Subscript>3</Subscript>/CoFe<Subscript>2</Subscript> Hard-Soft Magnetic Systems

We report on the growth and magnetic properties of La_2/3Sr_1/3MnO₃/SrTiO₃/CoFe₂ hard-soft magnetic systems prepared by pulsed laser deposition on SrTiO₃(001) substrates. In situ reflection high-energy electron diffraction along the [100]SrTiO₃ substrate azimuth and atomic force microscopy measurements reveal that La_2/3Sr_1/3MnO₃ and SrTiO₃ grow both in a three dimensional mode and that the roughness of the lower and upper magnetic/non-magnetic interfaces ranges between 2 and 4 Å. Cross-section transmission electron microscopy observations show that the layers are continuous, with an homogeneous thickness, and that the interfaces are mostly sharp and correlated. The magnetization curves show a two step reversal of the magnetization, with very distinct coercive fields. A small anisotropy is observed for the CoFe₂ layer with an in plane easy magnetization axis along the [110]SrTiO₃ direction. Minor magnetization loops indicate that the coupling between the magnetic layers is negligible.     

Synthesis and characterization of M<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> (M = Ca,Sr and Ba) by a solid-state metathesis approach

A solid-state metathesis approach initiated by microwave energy has been successfully applied for the synthesis of orthovanadates, M₃V₂O₈ (M = Ca, Sr, and Ba). The structural, vibrational, thermal, optical and chemical properties of synthesized powders are determined by powder X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, magnetic property measurements and diffused reflectance spectra in the UV-VIS range. The direct bandgap of the synthesized materials was found to be 3·55 ± 0·2 eV, 3·75 ± 0·2 eV and 3·57 ± 0·2 eV for Ca₃V₂O₈, Sr₃V₂O₈ and Ba₃V₂O₈, respectively.     

Photoinduced characteristics in La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> film

A systematic investigation of photoinduced properties is carried out in La_0.67Ca_0.33MnO₃ film prepared on LaAlO₃ (100) substrate by magnetron sputtering method. At T < 270 K, the resistivity of film induced by laser increases because of the demagnetization effect of manganites. The photoinduced relaxation character of film indicates that the time constant increases with increasing temperature, which is attributed to the growing thermal fluctuation. After laser irradiation, the resistivity returns to the original value and the relaxation time seems to be independent of temperature. In insulating state, laser irradiation induces the reduction in resistivity of film due to the excitation of small polarons.     

Features of the electromechanical properties of 0–3 composites of the Pb(Zr,Ti)O<Subscript>3</Subscript>-based ferroelectric ceramics-polymer type

We have analyzed the experimental concentration dependences of the permittivity ʵ ₃₃ ^*σ (m _FC) and the piezoelectric coefficient d ₃₃ ^* (m _FC) of 0–3 connectivity composites comprising ferroelectric ceramic inclusions with a volume fraction m _FC dispersed in a piezoactive or piezopassive polymer matrix. Both concentration dependences at 0.1 < m _FC ≤ 0.6 can be interpreted within the framework of a new model of the 0–3 composite representing two systems of inclusions—spherical and cylindrical (elongated in the polarization axis direction). It is established that the elongated inclusions determine to a considerable extent the piezoelectric and dielectric properties of the 0–3 composites of the Pb(Zr,Ti)O₃-based ferroelectric ceramics-polymer type with m _FC ≥ 0.2.     

Structure and luminescent properties of Cs<Subscript>2</Subscript>Sr(VO<Subscript>3</Subscript>)<Subscript>4</Subscript>:Mn<Superscript>2+</Superscript>

We have developed a procedure for thermally stimulated synthesis of a cesium strontium metavanadate, Cs₂Sr(VO₃)₄:Mn²⁺ (0.01, 0.50, 1.00, 5.00 at % Mn²⁺), using MnO-containing starting mixtures. The EPR spectrum of the material containing 0.01 at % Mn²⁺ shows a hyperfine structure due to the incorporation of a small amount of manganese into the diamagnetic double metavanadate host. The luminescent and optical properties of Cs₂Sr(VO₃)₄:Mn²⁺ depend on manganese content. In contrast to higher doping levels, doping with 0.01 at % Mn²⁺ increases the integrated emission intensity of the vanadate by 10% and improves its chromaticity characteristics (approaching them to those of white light). We assume that this is due to the reduction in the density of vacancy-type growth defects, such as oxygen vacancies.