Wide range magnetoresistance and high temperature coefficient of resistance in La<Subscript>0.7</Subscript>Sr<Subscript>0.3−x</Subscript>Ag<Subscript>x</Subscript>MnO<Subscript>3</Subscript> system

We report observation of composition and process parameter dependant wide range magnetoresistance and high temperature coefficient of resistance in La_0.7Sr_0.3−xAg_xMnO₃ (0 ≤ x ≤ 0.3) system. The polycrystalline samples synthesized through solid state reaction method exhibited rhombohedral structure with systematic change in lattice parameters. The samples with x ≤ 0.2 showed metallic behavior below room temperature. A broad metal-insulator transition is observed for x = 0.3, which becomes sharp for the samples sintered at high temperature. The broad transition resulted in magnetoresistance (MR) value around 35% over a wide temperature range whereas the sharp transition results in MR value as high as 85%. The temperature coefficient of resistance value of 11% is seen as a consequence of sharp transition. The magnetic transition was sharp for the samples sintered at high temperature.     

Low temperature synthesis of nanosized Mn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> ferrites and their characterizations

Nanosized Mn_1−x Zn _x Fe₂O₄ (x = 0, 0·1, 0·3, 0·5, 0·6, 0·7, 0·9) mixed ferrite samples of particle size < 12 nm were prepared using the co-precipitation technique by doping the Zn²⁺ ion impurities. Autoclave was employed to maintain constant temperature of 80°C and a constant pressure. The X-ray analysis and the IR spectrum analysis were carried out to confirm the spinel phase formation as well as to ascertain the cation distribution in the ferrite samples. This clearly points to the fact that the Zn²⁺ ion’s presence is not restricted to A-site alone for some of the Mn-Zn ferrite series. The real part of a.c. susceptibility measurements clearly indicated the superparamagnetic behaviour of the ferrite samples. There is a systematic decrease in the particle size, Curie temperature and magnetization with the increase in the Zn²⁺ ion doping, measured using magneto thermal gravimetric analysis (MTGA) and vibrating sample magnetometer (VSM), respectively. The lattice constant is found to be constantly decreasing till x = 0·6 and beyond this an unusual slight increase in the lattice constant is found.     

Novel temperature stable Ba<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>V<Subscript>2</Subscript>O<Subscript>6</Subscript> microwave dielectric ceramics with ultra-low sintering temperature

New temperature stability Ba_1?xSr_xV₂O₆ (0.35?≤?x?≤?0.55) microwave dielectric ceramics prepared by the conventional solid-state route were investigated. X-ray diffraction confirmed that all the specimens formed a solid solution single phase with orthorhombic structure. The microwave dielectric properties strongly depended on the compositions, densification and microstructure of the specimens. Furthermore, partial Sr ions substitution for Ba ions in Ba_1?xSr_xV₂O₆ lattices not only successfully improved the temperature stability of BaV₂O₆-based ceramic but also promoted the sinterability of SrV₂O₆-based one. Out of these compositions, Ba_0.5Sr_0.5V₂O₆ sintered at 625?°C exhibited a near-zero τ_f together with a low permittivity ε_r?~?11.5 and a quality factor Q?×?f?~?14 100 GHz, which also showed good chemical compatibility with Al electrodes.     

La<Subscript>1−x</Subscript>Ag<Subscript>x</Subscript>FeO<Subscript>3</Subscript>/halloysites nanocomposite with enhanced visible light photocatalytic performance

La_1?xAg_xFeO₃/halloysites nanotubes (HNTs) nanocomposite was synthesized by sol–gel method. It was characterized by X-ray diffraction, transmission electron microscope, Fourier transform infrared spectroscopy and UV–visible diffused reflectance spectroscopy measurements. The photo-activity of the La_1?xAg_xFeO₃/HNTs nanocomposite was evaluated via degradation of methylene blue (MB) under visible-light irradiation. The results showed that the HNTs with unique pore structure favored the adsorption of organic molecules. Adequate Ag⁺ doping improved the absorption ability for visible light. The La_0.95Ag_0.05FeO₃/HNTs demonstrated the best photocatalytic performance, which achieved as high as 99 % for MB degradation exposed 2 h irradiation. However,further increasing of Ag⁺ doping gradually reduced the photocatalytic activity. The nanocomposite catalyst showed outstanding recyclability after eight cycles which still remained up to 90 %.     

Low temperature synthesis and magneto- electrical studies in Nd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Pb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>MnO<Subscript>3</Subscript> system

The system Nd_{1 − x}Pb_xMnO₃ with x ≤ 0.50 has been synthesized by citrate-gel route. All these samples have shown single phase structure with cubic symmetry. Magnetic and electrical behavior has been studied in the temperature range 4–300 K. The temperature dependent resistivity data shows that metal-semiconductor transitions occurs at temperature T_P∼ 166, 174 and 182 K for x = 0.30, 0.40 and 0.50 respectively. Under the field of 1 T, this transition shifts to higher temperature. These samples have shown maximum MR upto 40% under the field of 1 T at temperature lower than T_P. These results are supported by magnetic measurements over the observed temperature range. Resistivity data above T_P has been found to fit well with Tⁿ where n = −1/4 for x = 0.30 and n = −1/2 for x = 0.50 which is attributed to the variable range hopping of small polarons.     

Microscopic and macroscopic magnetic properties of MgAl<Subscript>x</Subscript>Cr<Subscript>x</Subscript>Fe<Subscript>2 − 2x</Subscript>O<Subscript>4</Subscript> spinel ferrite system

In order to study the effect of co-substitution of Al^{3 +} and Cr^{3 +} for Fe³⁺ in MgFe₂O₄ on its structural and magnetic properties, the spinel system MgAl _x Cr_xFe_{2 ? 2x}O₄ (x = 0.0–0.8) has been characterized by X-ray diffraction, high field magnetization, low field a.c. susceptibility and ⁵⁷Fe Mössbauer spectroscopy measurements. Contrary to the earlier reports, about 50% of Al^{3 +} is found to occupy the tetrahedral sites. The system exhibits canted spin structure and a central paramagnetic doublet was found superimposed on the magnetic sextet in the Mössbauer spectra (0.5 > x > 0.2). Thermal variation of a.c. susceptibility exhibits normal ferrimagnetic behaviour.     

Raman and photoluminescence spectra of crystalline and glassy GeS<Subscript>2<Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>2−2<Emphasis Type="Italic">x</Emphasis></Subscript> solid solutions

We have studied the effects of compositional and structural disorder on the Raman and photoluminescence (PL) spectra of crystalline and glassy GeS_2x Se_2?2x solid solutions. The influence of structural disorder (variations in bond lengths and bond angles at the vertices of [GeX₄] (X = S, Se) tetrahedra) is shown to be comparable to that of local inhomogeneities in the solid solutions. The PL spectra of the crystalline phases are, on the whole, similar to those of the glasses: they show one, broad emission band, which shifts to lower photon energies in going from the crystals to glasses over the entire solid-solution series. The luminescence excitation spectra of the crystals differ markedly from those of the glassy solid solutions. In contrast to crystalline GeS₂ and GeSe₂, the solid solutions exhibit PL fatigue. Our results indicate that radiative processes in the crystalline and glassy GeS_2x Se_2?2x solid solutions are determined by native point defects and depend little on the degree of long-range ordering, whereas nonradiative processes are rather sensitive to topological disorder in the structure of the solid solutions.     

Stability region of Eu<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript>x</Subscript>O<Subscript>3+δ</Subscript> solid solutions in air

The homogeneity range of EuMnO₃ has been determined using x-ray diffraction analysis of single-and mixed-phase Eu_2?x Mn_xO_3+δ samples (0.90≤x≤1.20, Δx = 0.02) prepared from oxide mixtures by solid-state reactions in air between 900 and 1400°C. The results have been used to construct a partial phase diagram of the Eu-Mn-O system in air. The dependences of unit-cell parameters on x and synthesis temperature are presented for the samples synthesized at 1100 and 1400°C and for EuMnO₃, respectively. The solubility of europium oxide in EuMnO₃ is tentatively attributed to structural defects, and that of managanese oxides is interpreted in terms of structural defects, oxygen nonstoichiometry of europium manganite, the disproportionation reaction 2Mn³⁺ = Mn²⁺ + Mn⁴⁺, and partial substitution of the resulting Mn²⁺ for Eu³⁺ on the cuboctahedral site of the perovskite-like structure. To check these assumptions, systematic studies of the oxygen nonstoichiometry and structure of Eu_2?x Mn_xO_3+δ solid solutions synthesized at different temperatures are needed.     

Photocatalysis of several organic dyes by a hierarchical Ag<Subscript>2</Subscript>V<Subscript>4</Subscript>O<Subscript>11</Subscript> micro–nanostructures

Hierarchical Ag₂V₄O₁₁ nano-protuberance-assembled submicrofibers have been successfully synthesized through a facile and surfactant-free hydrothermal strategy. Morphology and microstructures of as-fabricated products are carefully examined using TEM, SA-ED, HRTEM, FE-SEM, XPS and X-ray diffraction. A possible growth mechanism of the as-obtained Ag₂V₄O₁₁ hierarchical submicrofibers is proposed by a time dependent experiments and the dosage of Ag. The as-prepared Ag₂V₄O₁₁ products are further regarded as a kind of photocatalyst for the catalytic degradation of three toxic organic dyes, methyl blue (MB), methyl orange and rhodamine blue. Radical trapping experiments have been applied to confirm the photoreaction mechanism. Compared with N-doped TiO₂ and P25, the catalyst Ag₂V₄O₁₁ prepared at 24 h reaction time exhibited much higher photocatalytic activity for MB degradation under visible-light illumination. It was ascribed to the formation of superoxide radicals species, and hierarchical structure. The use of ESI-MS analyses showed that the MB dye molecules were broken up by the action of the Ag₂V₄O₁₁ photocatalyst, indicating the occurrence of a mineralization process. Additionally, kinetic results showed that the degradation process follows Langmuir–Hinshelwood (L–H) first-order kinetic reaction.     

Synthesis of Zn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Co<Subscript>3−<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>4</Subscript> spinels at low temperature and atmospheric pressure

In order to synthesize cobalt-based spinel oxides, air was bubbled through aqueous hydroxides suspensions of cobalt and zinc with atomic ratios of Co:Zn = 100:0, 85:15, 67:33 (precisely 2:1), 50:50, 30:70, 15:85, and 0:100 at 70 °C and atmospheric pressure. When cobalt was absent in the suspensions, hexagonal ZnO nanocrystals with ca. 82.2 nm size were observed, whereas cubic Co₃O₄ ones with ca. 52.7 nm were seen when zinc was not present. Zinc-contained cobalt spinel oxides, i.e., Zn _x Co_3?x O₄ (x = 0–1), were obtained when both hydroxides were present, e.g., spherical nanoparticles with ca. 107.6 and 85.0 nm diameters were observed for Co:Zn = 50:50 and 67:33, respectively. The lattice constant, a, for the cubic spinel increased with the increase in zinc atomic %, suggesting the increase in zinc concentration in the spinel. The Zn _x Co_3?x O₄ synthesized was normal spinel with a cubic crystal structure, whereas it was also suggested that a very small portion of zinc ion was incorporated into the octahedrally coordinated sites (inversion of spinel). BET surface area of the synthesized catalyst increased with increasing the cobalt atomic % except for Co:Zn = 67:33, for which local minimal surface area was obtained. Oxygen storage capacity of the catalyst was the largest for Co:Zn = 85:15 at 150 and 200 °C, whereas it was for 50:50 at 250 °C. After reducing the synthesized catalysts with hydrogen, metallic cobalt was formed on zinc oxide. CO chemisorption number on the cobalt was the largest for Co:Zn = 67:33, for which the smallest metallic cobalt diameter was also obtained. On the other hand, catalytic CO₂ hydrogenation activity and methane selectivity were the highest for Co:Zn = 50:50, suggesting that zinc oxide as a basic support played an important role in the hydrogenation of acidic CO₂. It was also shown that the catalytic hydrogenation activity and the methane selectivity were higher for the catalysts prepared by the present liquid-phase approach than for those prepared by conventional coprecipitation and impregnation methods, which were ascribed to larger surface area and number of active sites for the former preparation technique than for the latter two.     

1.06 μm laser characteristics of Nd:KY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

This paper reported the pulse laser characteristics of Nd³⁺:KY(WO₄)₂ crystal grown in our laboratory with an impurity of 4.57%. The maximum laser output was 61.6 mJ at 1.06 m, pumped by a single Xenon flash-lamp. The laser energy threshold value E_th was 0.32 J. The overall and slope efficiencies were 0.43% and 0.45%.     

Influence of bismuth and calcium on the luminescence intensity of NaY<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Eu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> phosphors

We have studied the effect of Bi³⁺ and Ca²⁺ cation additions and partial Si, P, and V substitutions for Mo on the photoluminescence of NaY_1−x Eu _x (MoO₄)₂ scheelite-like red phosphors. The results demonstrate that the photoluminescence intensity in NaY_{1 − x} Eux(MoO₄)₂ doped with Bi³⁺ and Ca²⁺ increases with europium concentration, up to x = 0.5, without concentration quenching. When the atomic fraction of Bi³⁺ is 0.2, it has a sensitizing effect on Eu³⁺. Ca²⁺ doping to an atomic fraction of 0.45 also increases the photoluminescence intensity. Partial Si, P, and V substitutions for Mo have little effect on the photoluminescence intensity.     

Thermoelectric properties of TlIn<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Eu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te<Subscript>2</Subscript> crystals

The thermoelectric properties of TlIn_{1 ? x} Eu _x Te₂ (0 < x ≤ 0.09) solid solutions have been studied at temperatures from 300 to 500 K. The results indicate that the thermoelectric figure of merit of the alloys increases with temperature and europium content.     

Preparation and tunable photoluminescence of alloyed CdS<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> nanorods

Ternary alloys of CdS _x Se_1−x nanorods have been synthesized by the thermal treatment of Cd²⁺ dispersed polyethylene glycol 2000 gel (PEG2000) with ethylenediamine solution of sulfur and selenium in a sealed system at 180 °C for 24 h, during which the proportion between S and Se in the nanorods was controlled by the ratios of every starting material to each other. The alloyed ternary CdS _x Se_1−x nanorods are highly crystalline without any other phase. The optical property these nanorods could be manipulated by modulating the composition of S and Se.     

Coexistence of Superconductivity and Ferromagnetism in Ni-Doped Bi<Subscript>4 −x</Subscript>Ni<Subscript>x</Subscript>O<Subscript>4</Subscript>S<Subscript>3</Subscript> (0.075 ≤<Emphasis Type="Italic">x</Emphasis>≤ 0.150)

The effects of Ni doping on the physical properties of the newly discovered layered superconductor Bi₄O₄S₃ are studied. X-ray diffraction data indicates that the lattice constants a and c decrease with the increasing Ni doping. From resistivity-temperature curves, the superconducting transition temperature (\(T_{c}^{\text {onset}})\) is suppressed by only 0.5 K with the increase of Ni doping from 0.075 to 0.15; the \(T_{c}^{\text {zero}}\) is almost the same constant at different Ni ions’ doping level. The magnetic susceptibility results suggest the coexistence of superconductivity and ferromagnetism in this system. A possible superconductivity transition is observed around ?14 K from M-T (FC) curves in x = 0.125, 0.15 samples, which may result from the doped magnetic Ni ions.     

Characterization and electrical conductivity of La<Subscript>1−x</Subscript>Sr<Subscript>x</Subscript>CrO<Subscript>3</Subscript> NTC ceramics

The La_1?xSr_xCrO₃ (x?=?0–0.1) negative temperature coefficient (NTC) ceramics have been prepared by the traditional solid state reaction method. X-ray diffraction (XRD) analysis has revealed that the as-sintered ceramics crystallize in a single perovskite structure. Scanning Electron Microscope (SEM) images show that the doped Sr²⁺ contributes to in the decrease in porosity. X-ray photoelectron spectroscopy (XPS) analysis indicates the existence of Cr³⁺ and Cr⁶⁺ ions on lattice sites, which result in hopping conduction. The presence of the Cr⁶⁺ is one of the key factors that affect the electrical conductivity of La_1?xSr_xCrO₃. Resistance–temperature characteristics were studied in the range of ?80 to 10?°C for the ceramic samples, the electrical characterizations show that the electrical resistivity and material constant B decrease with the increase of the strontium content.     

Photoluminescence,Judd–Ofelt analysis and photometric characterization of Eu<Superscript>3+</Superscript> activated Ca<Subscript>0.5</Subscript>Gd(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> red phosphor

The red emitting Ca_0.5Gd(WO₄)₂:Eu³⁺ was synthesized by a solid-state reaction method. X-ray diffraction patterns were used to characterize crystal structure as well as phase purity. The results suggest that the as synthesized powder phosphor possess scheelite crystal structure with tetragonal symmetry along with the space group of I4₁/a. SEM studies reveal that the as synthesized sample show polyhedral morphology with particle size of 5.5 µm. Photoluminescence excitation spectrum depicts that a broad band (from 200 to 300 nm) centered at 242 nm is attributed to the ligand to metal charge transfer transition of WO₄ ^2? and three intense with sharp absorption bands (observed at 394, 464 and 535 nm) are designated as f–f electronic transitions of Eu³⁺. Photoluminescence emission studies indicate that, under 394 nm UV excitation, a hypersensitive red emission was observed at 617 nm due to the transition from upper ⁵D₀ level to the ⁷F₂ lower level of Eu³⁺ ion. The spectroscopic behaviour of the as synthesized phosphor Ca_0.5Gd(WO₄)₂:Eu³⁺ was determined using Judd–Ofelt theory. The CIE color coordinates, colour correlated temperature and luminous efficacies of radiation were estimated. The as obtained results indicating that the Ca_0.5Gd(WO₄)₂:Eu³⁺ red phosphor is most suitable for solid state lighting applications.     

Electrical and oxygen sensing properties of Nd<Subscript>1−x</Subscript>Ba<Subscript>x</Subscript>CoO<Subscript>3</Subscript> ceramics

Nd_1?xBa_xCoO₃ (0?≤?x?≤?0.2) ceramics was synthesized by solid state reaction. All the samples have an orthorhombic perovskite structure (Space group P n m a). The electrical transport property indicates that Ba doped NdCoO₃ ceramics goes through semiconductor–metal phase transition. The electrical resistivity of Nd_1?xBa_xCoO₃ (0?≤?x?≤?0.15) ceramics decreases, while the electrical resistivity of Nd_0.8Ba_0.2CoO₃ ceramics increases with the increase of temperature. The chemical-sensing property shows that Nd_1?xBa_xCoO₃ ceramics is very sensitive to oxygen. Also, increasing Ba²⁺ doping concentration can reduce the oxygen desorption rate and increase the sensitivity of resistivity. These results indicate that Ba²⁺ doped NdCoO₃ ceramics is not only the good candidate of the cathode materials of solid fuel cells but also the good materials of gas sensor devices.     

Microstructure and properties of the Ba<Subscript>0.75−x</Subscript>Ca<Subscript>x</Subscript>La<Subscript>0.25</Subscript>Fe<Subscript>11.6</Subscript>Co<Subscript>0.25</Subscript>Ti<Subscript>0.15</Subscript>O<Subscript>19</Subscript> hexaferrites

In the case of Ti⁴⁺ remain unchanged, the Ca²⁺ substituted Ba_0.75?xCa_xLa_0.25Fe_11.6Co_0.25Ti_0.15O₁₉ (0?≤?x?≤?0.05) were prepared by conventional solid-state reaction method at temperature of 1280 °C. A ball-to-power weight ratio of 10:1. Their crystal structure and magnetic properties were mainly investigated. The results show that the single magnetoplumbite phase structure transformed into the multiphase structure. Meanwhile, the small amount of α-Fe₂O₃ phase existed in M-type phase. The micrographs were observed by a field emission scanning electron microscopy (SEM). Vibrating sample magnetometer (VSM) was used to analyze the magnetic properties. The saturation magnetization (M _s ) first increases then decreases when x from 0 to 0.03. But, when x from 0.03 to 0.05, the saturation magnetization (M _s ) first increases then decreases too. The maximum value is at x?=?0.04 (M _s ?=?70.73 emu/g). The value of coercivity (H _c ) first increases then decreases when x from 0 to 0.04. But, the value increased when x from 0.04 to 0.05. The maximum value is at x?=?0.02 (H _c ?=?1691 Oe).     

Negative temperature coefficient thermistor based on BaFe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Sn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>3−ε</Subscript> solid solutions

A negative temperature coefficient (NTC) thermistors based on BaFe _x Sn_1−x O_3−ε were fabricated by conventional solid-state reaction method. The microstructure and electrical properties of the NTC thermistors were characterized by X-ray diffraction (XRD), electric (R(T)), and impedance measurements. The XRD analysis shows that the BaFe _x Sn_1−x O_3−ε still remains cubic perovskite structure and the crystal growth orientation changes at higher Fe content. Similarly, the electric measurements indicates that all the samples show typical NTCR behavior; with increasing Fe content, the room temperature resistivity, activation energy, and thermistor constant decrease, are in the range of 2.52–217 KΩ cm, 0.343–0.43 eV, and 3900–4896 K, respectively. The impedance spectroscopy confirms that the observed ac resistance, consisting of the grain bulks, grain boundaries, and electrode–ceramic interface, mainly attributes to the grain boundaries and grain bulks; with the rise in temperature the grain boundary resistance shows more rapid change than the grain resistance; moreover, the Fe concentration can greatly affect the grain and grain boundary resistance.