Magnetic and photoluminescence properties of Fe3O4@SiO2@YP1−xVxO4:Dy nanocomposites

In this paper, we report on the bifunctional Fe₃O₄@SiO₂@YP_0.1V_0.9O₄:Dy³⁺ nanocomposites were prepared by the solvothermal method and sol-gel method. The structure, photoluminescence (PL) and magnetic properties of the nanocomposites were characterized by means of X-ray diffraction, scanning electron microscope, transmission electron microscope, PL excitation and emission spectra and vibration sample magnetometry. It is shown that Fe₃O₄@SiO₂@YP_0.1V_0.9O₄:Dy³⁺ nanocomposites with a core-shell structure present excellent fluorescent and magnetic properties. Additionally, the effects of the magnetic field on the luminescence properties of nanocomposites were discussed.     

Dielectric investigation of MM(PO4)2 double orthophosphates (M = Ca, Sr, Ba, Pb; M = Ti, Zr, Hf, Ge, Sn)

M^IIM^IV(PO₄)₂ (M^II = Ca, Sr, Ba, Pb; M^IV = Ti, Zr, Hf, Ge, Sn) ceramics were prepared by solid state reaction method and consolidated by spark plasma sintering in order to study their electrical properties. The dielectric study performed at room temperature was aimed to determine the basic electrical properties of these compounds. Maxwell-Wagner polarization contributions, which are active at low frequencies cause a strong extrinsic increase of both real and imaginary part of permittivity as well of the dc-conductivity for BaZr(PO₄)₂, BaTi(PO₄)₂, BaHf(PO₄)₂, and SrGe(PO₄)₂ double orthophosphates. Moderate real (ε^′r=20-150) and imaginary low-frequency permittivity (ε^′r=7-300) and typical Debye relaxation with relaxation time in the range of ms is typical for: PbHf(PO₄)₂, PbZn(PO₄)₂, and CaGe(PO₄)₂. At high-frequency (f = 10⁹ Hz), the ceramics have permittivities of 2.29÷8.02 and tangent loss of 0.003÷0.153. The compounds SrGe(PO₄)₂, BaGe(PO₄)₂, BaZr(PO₄)₂ and BaSn(PO₄)₂ have excellent high-frequency dielectric characteristics, with losses of 3÷6% and permittivity slightly above 2 and are possible candidate as microwave ceramics.     

One-step synthesis of MFe2O4 (M = Fe, Co) hollow spheres by template-free solvothermal method

Monodispersed magnetic MFe₂O₄ (M = Fe, Co) hollow spheres were synthesized by simple template free solvothermal method in ethylene glycol (EG) solution. The hollow spheres were in the same size with an average diameter of about 360 nm and the shells of these spheres were about 80 nm, consisted of closely packed nanocrystallines due to Ostwald ripening. EG plays the key role in the synthesis of hollow spheres in contrast with octahedral crystals synthesized in aqueous solution. The products synthesized in aqueous solution were calcined at 800 °C and 1000 °C. The amount of spinel ferrite products increased monotonically with the increase of temperature and appeared as a single phase at 1000 °C. The saturation magnetization (M_s), remanent magnetization (M_r) and coercivity (H_c) for Fe₃O₄ hollow spheres was 74.47 emu/g, 2.59 emu/g and 32.503 Oe respectively whereas the reading of the same indicators for CoFe₂O₄ hollow spheres was 69.07 emu/g, 14.46 emu/g and 242.79 Oe, respectively. The magnetic variation between Fe₃O₄ and CoFe₂O₄ hollow spheres was caused by the radius difference of Fe²⁺ (3d⁶) and Co²⁺ (3d⁷) ions and it was also relevant with nanocrystal sizes of the spin disorder of crystal surface.     

Fabrication of V3O7·H2O@C core-shell nanostructured composites and the effect of V3O7·H2O and V3O7·H2O@C on decomposition of ammonium perchlorate

V₃O₇·H₂O@C core-shell materials have been synthesized using V₃O₇·H₂O nanobelts as the cores and glucose as the source of carbon via an environmental hydrothermal method. The as-obtained V₃O₇·H₂O@C core-shell materials were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), elemental analysis (EA), Fourier transform infrared spectroscopy (FT-IR) and Raman spectrum. The influences of the reaction temperature, concentration of glucose and reaction time on the morphologies of the samples were respectively discussed in detail. The possible formation mechanism of V₃O₇·H₂O@C was proposed according to our experimental results. Furthermore, the effect of V₃O₇·H₂O and V₃O₇·H₂O@C on the thermal decomposition of ammonium perchlorate (AP) were investigated by thermal gravimetric analyzer (TG) and differential thermal analysis (DTA). The thermal decomposition temperatures of AP in the presence of V₃O₇·H₂O and V₃O₇·H₂O@C were reduced by 70 and 89 °C, respectively, which indicates that V₃O₇·H₂O@C core-shell composites have higher activity than V₃O₇·H₂O.     

Dielectric relaxation and conduction mechanism in LaNi3/4M1/4O3 (M = Mo, W) at low temperature

In order to investigate the electrical transport in LaNi_3/4Mo_1/4O₃ and LaNi_3/4W_1/4O₃, the dc conductivity and dielectric properties in these polycrystalline materials are investigated in the temperature range from 163 K to 383 K and frequency range from 50 Hz to 1 MHz. The X-ray diffraction patterns of the samples show monoclinic phase at room temperature. The homogeneity of the samples is determined by energy dispersive analysis of X-ray (EDAX) attached with a scanning electron microscope. The temperature dependence of dc conductivity shows the semiconducting nature of the materials. The complex impedance plane plots show that the relaxation (conduction) mechanism in these materials is purely a bulk effect arising from the semiconductive grains. The frequency-dependent electrical data are also analyzed in the framework of ac conductivity formalism. The ac conductivity spectra follow the universal power law. The activation energies required for bulk conduction is 0.143 and 0.165 eV for LNM and LNW respectively. The scaling behaviour of loss tangent suggests that the relaxation describes the same mechanism at various temperatures.     

Red-emitting Ln2−xEuxTe2O6:RE (Ln = La, Y; RE = Sm, Gd) phosphors prepared by the Pechini sol-gel method

La_1.90Eu_0.10TeO₆:RE³⁺ (RE = Gd, Sm) and Y₂TeO₆:Eu³⁺nanophosphors were prepared by the Pechini sol-gel process, using lanthanide sesquioxides and telluric acid as precursors. X-ray diffraction (XRD), scanning electron microscopy (SEM), photoluminescence spectra (PL) and fluorescence lifetime were used to characterize the resulting phosphors. The results of XRD indicate that all samples crystallized completely at 1073 K and are isostructural with orthorhombic Ln₂TeO₆. The SEM study reveals that the samples have a strong tendency to form agglomerates with an average size ranging from 40 to 65 nm. The luminescence decay curves suggest for all samples a monoexponential behavior. The photoluminescence intensity and chromaticity were improved for excitation at 395 nm when the co-doping concentration reaches the 1% mol. The optimized phosphorsLa_1.88Eu_0.10Gd_0.02TeO₆and La_1.88Eu_0.10Sm_0.02TeO₆, could be considered an efficient red-emitting phosphor for solid-state lighting devices based on InGaN LEDs.     

Optical properties of CeNi5 and CeNi4M (M = Al, Cu) compounds

The experimental study of the optical properties of CeNi₅, CeNi₄Cu and CeNi₄Al compounds was carried out in the 0.083-5.64 eV energy range using the ellipsometry method. The optical constants, dielectric functions and electronic parameters (plasma and relaxation frequencies) were determined. The energy dependencies of the optical interband conductivities are discussed by using the available information on the electronic band structure of these compounds. In the ternary alloys the optical spectra show the presence of peculiarities related to effect of Cu or Al substitution at Ni sites.     

The effects of thermal treatment on the physical properties of Sr2FeMo1−xMxO6 perovskite with M = W, Ta and x ≤ 0.3

The Sr₂FeMo_1−xM_xO₆ double perovskites with M = W and Ta and x ≤ 0.3, were obtained by sintering at 1300 °C, during 4 and 8 h, respectively. The perovskites crystallize in a tetragonal structure having I4/mmm space group. The grains are more homogeneous when sintering time is increased. The samples sintered longer time show higher values of saturation magnetizations, resistivities and magnetoresistivities than the samples sintered during 4 h. The intergrain tunnelling magnetoresistance as well as the intragrain contributions, respectively were analysed as function of temperature and external field. The changes in the physical properties, when the sintering time is increased, have been correlated with the number of antisite defects as well as the nature of grain boundaries.     

Two-dimensional organic metals θ-(BETS)4MBr4(PhBr), M = Cd,Hg with differently oriented conducting layers

New bis(ethylenedithio)tetraselenafulvalene (BETS) based radical cation salts with tetrahedral dianions [CdBr₄]²⁻ and [HgBr₄]²⁻ of the (BETS)₄MBr₄(PhBr) composition were prepared by electrochemical crystallization. Room-temperature crystal structure of (BETS)₄CdBr₄(PhBr) determined by single crystal X-ray diffraction involves BETS radical cation layers of the θ-type packing and insulating layers consisting of [CdBr₄]²⁻ anions and PhBr molecules. In the neighboring conducting layers, the stacks are arranged perpendicular to each other. A metal-to-metal transition within 225–230 K range was found in both (BETS)₄CdBr₄(PhBr) and (BETS)₄HgBr₄(PhBr). The behavior of electrical resistivity of these salts differs substantially along and across conducting layers. The study of magnetoresistance of (BETS)₄HgBr₄(PhBr) revealed weak Shubnikov-de Haas oscillations in fields higher than 6 T.     

Synthesis and ageing effect in FeO nanoparticles: Transformation to core-shell FeO/Fe3O4 and their magnetic characterization

This paper reports the magnetic properties of partially oxidized FeO nanoparticles (NPs) prepared using thermal decomposition of iron acetylacetonate at high temperature. X-ray diffraction (XRD) analysis confirmed that the resulting NPs comprise a mixture of wüstite and magnetite phases, which are subsequently confirmed using high resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) pattern. Magnetic properties were investigated using vibrating sample magnetometer (VSM), which exhibit superparamagnetic (SPM) behavior at room temperature. Alternatively, below 200 K, a large exchange bias field has been observed in field cooled mode whose magnitude increases with the decrease in measuring temperature attaining a maximum value of ∼2.3 kOe at 2 K accompanied by coercivity enhancement (∼3.4 kOe) and high field of irreversibility (>50 kOe). The results are discussed taking into account the role of interface exchange coupling on the macroscopic magnetic properties of the nanoparticles.     

Strong correlation between structural, magnetic and transport properties of non-stoichiometric Sr2FexMo2−xO6 (0.8 ≤ x ≤ 1.5) double perovskites

Sr₂Fe_xMo_2−xO₆ (x = 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4 and 1.5 wt.%) (SFMO) double perovskite oxides of different compositions have been prepared by sol-gel method. These materials were subjected to X-ray diffraction and found that crystal structure changes from tetragonal to cubic around x = 1.2 wt.%. Lattice parameters and unit cell volume have been calculated using X-ray diffraction data. Magnetization studies have been carried out using Vibrating Sample Magnetometer ranging from −15 kOe to +15 kOe and saturation magnetization (M_s) has been determined. Electrical resistivity and magnetoresistance studies have been carried out in the magnetic field range of −40 kOe to +40 kOe keeping the temperature constant at 5, 150 and 300 K using standard four-probe method. Resistivity studies have also been carried out in the temperature ranging from 5 to 300 K keeping the magnetic field constant at 0, 10, 20 and 40 kOe. Maximum degree of Fe/Mo ordering (η_max) of SFMO has been calculated and compared with magnetic and transport properties. It has been found that there is a strong correlation between 3 parameters η_max, M_s and MR (%), i.e. all of them show a maximum at x = 1.0 wt.% and decreases as x deviates from 1.0 in SFMO. It has been also found that there is a different resistivity behavior between x ≤ 1.2 wt.% and x > 1.2 wt.% samples of SFMO. Semiconductor metal transition temperature was found to be maximum at x = 1.0 wt.%.     

Synthesis,characterization, morphology control of poly (p-phenylenediamine)-Fe3O4 magnetic micro-composite and their application for the removal of Cr2O7 from water

In this paper, poly (p-phenylenediamine)-Fe₃O₄ (PpPD-Fe₃O₄) magnetic micro-composite was synthesized via chemical oxidation polymerization of p-phenylenediamine (pPD) monomers with (NH₄)₂S₂O₈ (APS) and Fe(NO₃)₃ at room temperature. The results demonstrated that the morphologies of this magnetic micro-composite were varied by tuning the consistence of Fe(NO₃)₃ in solution. The structures, morphologies and properties of the composites were characterized with IR, Raman spectroscopy, XRD, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and flame atomic absorption spectrometry (FAAS). This magnetic composite showed excellent ability on absorption of Cr₂O₇²⁻. This magnetic composite is low cost, easy to operate and separate from solution. The mechanism research shows that the chemical adsorption and physical adsorption exist at the same time in adsorption process.     

Electronic structure of antiferromagnetic PbCrO3 (0 0 1) surfaces

Surfaces of cubic perovksite PbCrO₃ in (0 0 1) plane are investigated through density functional theory. The plane wave pseudopotential method is applied with generalized gradient approximation scheme. Hubbard U correction (GGA + U) is included in all calculations in order to simulate on-site Coulomb interactions between Cr-d states. Two types of terminations, namely, PbO- and CrO₂-terminations are considered in construction of the surfaces. Surfaces of both terminations show convergence at 9-layer slab geometry. The density of states calculations on the converged slab geometry yield a metallic behavior for both PbO- and CrO₂-terminations. Both metal atoms, Pb and Cr, in the uppermost layer of the respective terminations, have inward atomic relaxations much larger in magnitude than the oxygen atoms of the respective layer. However, Cr atoms which are labeled as up and down according to their spin orientation show different relaxations. The interlayer distance between the uppermost layer and the first one next to it decreases in both PbO- and CrO₂-terminated surface geometries. The calculations of the relative movement of the oxygen atom with respect to the Pb or Cr atom in each terminations give a positive rumpling in the uppermost layer.     

Preparation and characteristics of Fe3O4@YVO4:Eu bifunctional magnetic-luminescent nanocomposites

A facile direct precipitation method has been developed for the synthesis of bifunctional magnetic-luminescent nanocomposites with Fe₃O₄ nanoparticles as the core and YVO₄:Eu³⁺ as the shell. Transmission electron microscopy (TEM) images revealed that the obtained bifunctional nanocomposites had a core-shell structure and a spherical morphology. The average size was ∼150 nm, and the thickness of the shell was ∼15 nm. The X-ray diffraction (XRD) patterns showed that a cubic spinel structure of Fe₃O₄ core and a tetragonal phase of YVO₄ shell were obtained. Fourier transform infrared (FT-IR) spectra confirmed that the YVO₄:Eu³⁺ had been successfully deposited on the surface of Fe₃O₄ nanoparticles. Photoluminescence (PL) spectra indicated that the nanocomposites displayed a strong red characteristic emission of Eu³⁺. Magnetic measurements showed that the obtained bifunctional nanocomposites exhibited superparamagnetic behavior at room temperature. Therefore, the bifunctional nanocomposites are expected to develop many potential applications in biomedical fields.     

Substitution effect on metal-insulator transition in Cu(Ir1−xMx)2S4 (M = Sn, Hf)

A spinel CuIr₂S₄ exhibits a temperature-induced metal-insulator (M-I) transition at around 226 K. Non-magnetic substitution effect on the M-I transition, T_M-I, in Cu(Ir_1−xM_x)₂S₄ (M = Sn, Hf) has been studied on the focus of the rather low composition region of x. Magnetic property of Cu(Ir_1−xM_x)₂S₄ (M = Sn, Hf) has been examined experimentally. The T_M-I decreases with increasing x and the temperature hysteresis becomes unclear within the experimental errors. The step anomaly in the magnetic susceptibility smears out and the T_M-I becomes ill defined around x = 0.20 in Cu(Ir_1−xSn_x)₂S₄, and x = 0.10 in Cu(Ir_1−xHf_x)₂S₄, respectively. These substitutions play an important role in decoupling the spin-dimerization of Ir⁴⁺-Ir⁴⁺ in CuIr₂S₄, and lead the destruction of the metal-insulator transition.     

Synthesis, structure, and properties of Li2Pb2CuB4O10 with isolated [CuB4O10] units

The copper borate Li₂Pb₂CuB₄O₁₀ has been synthesized in air by the standard solid-state reaction at temperature in the range 550-650 °C and the structure of Li₂Pb₂CuB₄O₁₀ was determined by single-crystal X-ray diffraction. Li₂Pb₂CuB₄O₁₀ crystallizes in the monoclinic space group C2/c (no. 15) with a = 16.8419(12), b = 4.7895(4), c = 13.8976(10) Å, and β = 125.3620(10)°, V = 914.22(12) Å³, and Z = 4, as determined by single-crystal X-ray diffraction. The Li₂Pb₂CuB₄O₁₀ structure exhibits isolated units of stoichiometry [CuB₄O₁₀]⁶⁻ that are built from CuO₄ distorted square planes and triangular BO₃ groups. The IR spectroscopy and thermal analysis investigations of Li₂Pb₂CuB₄O₁₀ are also presented.     

Mechanical synthesis and rapid consolidation of a nanocrystalline 3.3Fe0.6Cr0.3Al0.1-Al2O3 composite by high frequency induction heating

Nanopowders of 3.3Fe_0.6Cr_0.3Al_0.1 and Al₂O₃ were synthesized from Fe₂O₃, Cr, and Al powders by high-energy ball milling. A high density nanocrystalline 3.3Fe_0.6Cr_0.3Al_0.1-Al₂O₃ composite was consolidated by a high frequency induction heated sintering (HFIHS) method within 3 min from mechanically synthesized powders of Al₂O₃ and 3.3Fe_0.6Cr_0.3Al_0.1. The average grain sizes of Al₂O₃ and 3.3Fe_0.6Cr_0.3Al_0.1 were 84 and 32 nm, respectively.     

Magnetoelectric CoFe2O4/Pb(Zr0.53Ti0.47)O3 composite thin films of 2-2 type structure derived by a sol-gel process

CoFe₂O₄/Pb(Zr_0.53Ti_0.47)O₃ (CFO/PZT) magnetoelectric composite thin films of 2-2 type structure have been prepared onto Pt/Ti/SiO₂/Si substrate by a sol-gel process and spin coating technique. The optimal annealing process of composite thin films was determined by using X-ray diffraction (XRD) and differential scanning calorimetry (DSC). It is found that the amount of the citric acid and concentration of CFO starting precursor solution have great impact on morphologies of composite thin films. Subsequent scanning electron microscopy (SEM) investigations show that the prepared thin films exhibit good morphologies and compact structure, and cross-sectional micrographs clearly display a multilayered nanostructure of multilayered thin films. The purpose of this work is to determine the optimal annealing processes of composite thin films and to prepare magnetoelectric composite thin films with good microstructure. It is shown that the films exhibit both good magnetic and ferroelectric properties, as well as a magnetoelectric effect.     

Phase relations in the SrO–Ga2O3–B2O3 system

The subsolidus phase relations in the SrO–Ga₂O₃–B₂O₃ system were investigated. The system contains 10 binary compounds and two ternary compounds, and can be divided into 15 three-phase regions. The new ternary compound SrGaBO₄ has two modifications (- and β-phases), both of which crystallize in the orthorhombic system but with different space groups.     

Synthesis, structural and microwave dielectric properties of Al2W3−xMoxO12 (x = 0-3) ceramics

Low dielectric ceramics in the Al₂W_3−xMo_xO₁₂ (x = 0-3) system have been prepared through solid state ceramic route. The phase purity of the ceramic compositions has been studied using powder X-ray diffraction (XRD) studies. The microstructure of the sintered ceramics was evaluated by Scanning Electron Microscopy (SEM). The crystal structure of the ceramic compositions as a result of Mo substitution has been studied using Laser Raman spectroscopy. The microwave dielectric properties of the ceramics were studied by Hakki and Coleman post resonator and cavity perturbation techniques. Al₂Mo_xW_3−xO₁₂ (x = 0-3) ceramics exhibited low dielectric constant and relatively high unloaded quality factor. The temperature coefficient of resonant frequency of the compositions is found to be in the range −41 to −72 ppm/°C.