Synthesis,Crystal Structure,and Magnetic Properties of the YbFeTi<Subscript>2</Subscript>O<Subscript>7</Subscript> Compound

We report on the synthesis conductions and results of experimental investigations of the crystal structure and magnetic properties of a new magnetic compound YbFeTi₂O₇. According to the X-ray diffractometry data, the crystal structure of the investigated compound is described by the rhombic space group Pcnb with unit cell parameters of a = 9.8115(1) Å, b = 13.5106(2) Å, and c = 7.31302(9) Å and atomic disordering in the distribution of iron ions Fe³⁺ over five structural sites. The magnetic measurements in the lowtemperature region revealed a kink in the temperature dependence of the magnetic moment and its dependence on the sample magnetic prehistory. The experimental results obtained suggest that with a decrease in temperature the sample passes from the paramagnetic state to the spin-glass-like magnetic state characterized by a freezing temperature of T _f = 4.5 K at the preferred antiferromagnetic exchange coupling in the sample spin system. The chemical pressure variation upon replacement of rare-earth ion R by Yb in the RFeTi₂O₇ system does not change the crystal lattice symmetry and magnetic state.     

Synthesis,crystal structure,Hirshfeld surface analysis,and vibrational and DFT investigation of [C<Subscript>6</Subscript>H<Subscript>10</Subscript>(NH<Subscript>3</Subscript>)<Subscript>2</Subscript>]<Subscript>3</Subscript>CuBr<Subscript>4</Subscript>.3Br

Single crystal of a new organic–inorganic hybrid material [C₆H₁₀(NH₃)₂]₃CuBr₄.3Br was synthesized by the slow evaporation method at room temperature and characterized by X-ray diffraction, FTIR, Raman spectroscopy, UV–Vis, dielectric measurements, and Hirschfield surface analysis. The title compound crystallizes in trigonal system \( P\overline{3} \).The crystal packing is governed by the N-H…Br and non-classical C-H…Br hydrogen-bonding interactions between the 1, 2-diamoniumcyclohexane cations, the tetrahedral [CuBr₄]^3? anions, and the isolated ion Br^?. Theoretical calculations were performed using density functional theory (DFT) for studying the molecular structure, vibrational spectra, and optical properties of the investigated molecule in the ground state. The optimized geometrical parameters obtained by DFT calculations are in good agreement with single crystal XRD data. The optical properties were investigated by optical absorption and show two bands at 260 and 305 nm.     

Synthesis,sintering, and conductivity of Mn<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>

Studies on the sintering of manganese pyrovanadate depending on the temperature and the crystallite size show that we are prevented from obtaining a bulk ceramic sample by the anisotropic growth of grains. Investigation of the electrical properties of Mn₂V₂O₇ in the temperature range of 250–800°C reveals the activation energy at which bulk conductivity is 0.62 eV.     

Magnetic Core/Shell Nanocomposites MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/SiO<Subscript>2</Subscript> for Biomedical Application: Synthesis and Properties

Magnetic core/shell (CS) nanocomposites (MNCs) are synthesized using a simple method, in which a magnesium ferrite nanoparticle (MgFe₂O₄) is a core, and an amorphous silicon dioxide (silica SiO₂) layer is a shell. The composition, morphology, and structure of synthesized particles are studied using X-ray diffraction, field emission electron microscopy, transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), scattering electrophoretic photometer, thermogravimetric analysis (TGA), and Mössbauer spectroscopy. It is found that the MgFe₂O₄/SiO₂ MNC has the core/shell structure formed by the Fe?O–Si chemical bond. After coating with silica, the MgFe₂O₄/SiO₂ MNC saturation magnetization significantly decreases in comparison with MgFe₂O₄ particles without a SiO₂ shell. Spherical particles agglomerated from MgFe₂O₄ nanocrystallites ～9.6 and ～11.5 nm in size function as cores coated with SiO₂ shells ～30 and ～50 nm thick, respectively. The total size of obtained CS MNCs is ～200 and 300 nm, respectively. Synthesized CS MgFe₂O₄/SiO₂ MNCs are very promising for biomedical applications, due to the biological compatibility of silicon dioxide, its sizes, and the fact that the Curie temperature is in the region required for hyperthermal therapy, 320 K.     

The Role of the Eu<Superscript>3+</Superscript> Concentration on the SrMoO<Subscript>4</Subscript>:Eu Phosphor Properties: Synthesis,Characterization and Photophysical Studies

SrMoO₄ doped with rare earth are still scarce nowadays and have attracted great attention due to their applications as scintillating materials in electro-optical like solid-state lasers and optical fibers, for instance. In this work Sr_1−xEu_xMoO₄ powders, where x = 0.01; 0.03 and 0.05, were synthesized by Complex Polymerization (CP) Method. The structural and optical properties of the SrMoO₄:Eu³⁺ were analyzed by powder X-ray diffraction patterns, Fourier Transform Infra-Red (FTIR), Raman Spectroscopy, and through Photoluminescent Measurements (PL). Only a crystalline scheelite-type phase was obtained when the powders were heat-treated at 800 °C for 2 h, 2θ = 27.8° (100% peak). The excitation spectra of the SrMoO₄:Eu³⁺ (λ_Em. = 614 nm) presented the characteristic band of the Eu^3 + 5L₆ transition at 394 nm and a broad band at around 288 nm ascribed to the charge-transfer from the O (2p) state to the Mo (4d) one in the SrMoO₄ matrix. The emission spectra of the SrMoO₄:Eu³⁺ powders (λ_Exc. = 394 and 288 nm) show the group of sharp emission bands among 523–554 nm and 578–699 nm, assigned to the ⁵D₁→⁷F_{0,1and 2} and ⁵D₀→⁷F_{0,1,2,3 and 4}, respectively. The band related to the ⁵D₀→⁷F₀ transition indicates the presence of Eu³⁺ site without inversion center. This hypothesis is strengthened by the fact that the band referent to the ⁵D₀→⁷F₂ transition is the most intense in the emission spectra.     

Dielectric Properties of Nanoporous Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Films Filled with Ferroelectric SC(NH<Subscript>2</Subscript>)<Subscript>2</Subscript>

Temperature dependences of permittivity ε′ and third harmonic amplitude γ_3ω of nanocomposites obtained by embedding ferroelectric SC(NH₂)₂ in porous alumina films with pore sizes of 60 and 100 nm are studied. A substantial increase in the temperatures of ferroelectric phase transition T_c1 and T_c2 and that of phase transition T_i from incommensurate phase to paraphase are also observed. The temperatures of all phase transitions are found to rise as pore diameters shrink.     

Synthesis and Luminescent Properties of M<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>: Eu (M=Sr,Ba) Nanophosphors

A solution combustion route for the synthesis of Eu³⁺-activated M₂V₂O₇ (M = Sr, Ba) and their luminescent properties have been investigated. Structure and luminescent characteristics of Sr₂V₂O₇:Eu³⁺ and Ba₂V₂O₇:Eu³⁺ nanophosphors have been studied by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, fluorescence spectrometry and Fourier transform infra-red spectroscopy. The incorporation of Eu³⁺ activator in these nanoparticles has been checked by luminescence characteristics. These nanoparticles have displayed red color under a UV source which is due to characteristics transition of Eu³⁺ from ⁵D₀ → ⁷F₂ at 613 nm in both Sr₂V₂O₇:Eu³⁺ and Ba₂V₂O₇:Eu³⁺ nanophosphors. In addition, the optimal Eu³⁺ - doped contents of Sr_2(1-x)Eu_2xV₂O₇ and Ba_2(1-x)Eu_2xV₂O₇ nanophosphors for both were 4 mol%.     

Synthesis,characterization, and thermal stability of SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript>/CR-Ag multilayered nanostructures

The controllable synthesis and characterization of novel thermally stable silver-based particles are described. The experimental approach involves the design of thermally stable nanostructures by the deposition of an interfacial thick, active titania layer between the primary substrate (SiO₂ particles) and the metal nanoparticles (Ag NPs), as well as the doping of Ag nanoparticles with an organic molecule (Congo Red, CR). The nanostructured particles were composed of a 330-nm silica core capped by a granular titania layer (10 to 13 nm in thickness), along with monodisperse 5 to 30 nm CR-Ag NPs deposited on top. The titania-coated support (SiO₂/TiO₂ particles) was shown to be chemically and thermally stable and promoted the nucleation and anchoring of CR-Ag NPs, which prevented the sintering of CR-Ag NPs when the structure was exposed to high temperatures. The thermal stability of the silver composites was examined by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Larger than 10 nm CR-Ag NPs were thermally stable up to 300 °C. Such temperature was high enough to destabilize the CR-Ag NPs due to the melting point of the CR. On the other hand, smaller than 10 nm Ag NPs were stable at temperatures up to 500 °C because of the strong metal-metal oxide binding energy. Energy dispersion X-ray spectroscopy (EDS) was carried out to qualitatively analyze the chemical stability of the structure at different temperatures which confirmed the stability of the structure and the existence of silver NPs at temperatures up to 500 °C.     

Synthesis and properties of barium ferrigermanate Ba<Subscript>2</Subscript>Fe<Subscript>2</Subscript>GeO<Subscript>7</Subscript>

The magnetic susceptibility and specific heat of single crystals of the Ba₂Fe₂GeO₇ barium ferrigermanate are investigated. It is revealed that the temperature dependence of the magnetic susceptibility exhibits a kink at a temperature T = 8.5 K. The number of nonequivalent positions of Fe³⁺ ions and their occupancies are determined using Mössbauer spectroscopy. It is shown that the Fe³⁺ ions located in tetrahedral positions T2 are ordered incompletely, which is inconsistent with the results obtained previously. An assumption is made regarding the possible ground magnetic state of the Ba₂Fe₂GeO₇ compound.     

DFT study of BaTiO<Subscript>3</Subscript> (001) surface with O and O<Subscript>2</Subscript> adsorption

Progress of scanning tunneling microscopy (STM) allowed to handle various molecules adsorbed on a given surface. New concepts emerged with molecules on surfaces considered as nano machines by themselves. In this context, a thorough knowledge of surfaces and adsorbed molecules at an atomic scale is thus particularly invaluable. In this work, within the framework of density functional theory (DFT), we present an electronic and structural ab initio study of a BaTiO₃ (001) surface (perovskite structure) in its paraelectric phase. As far as we know the atomic and molecular adsorption of oxygen at surface is then analyzed for the first time in the literature. Relaxation is taken into account for several layers. Its analysis for a depth of at least four layers enables us to conclude that a reasonable approximation for a BaTiO₃ (001) surface is provided with a slab made up of nine plans. The relative stability of two possible terminations is considered. By using a kinetic energy cut off of 400 eV, we found that a surface with BaO termination is more stable than with TiO₂ termination. Consequently, a surface with BaO termination was chosen to adsorb either O atom or O₂ molecule and the corresponding calculations were performed with a coverage 1 on a (1×1) cell. A series of cases with O₂ molecule adsorbed in various geometrical configurations are also analyzed. For O₂, the most favorable adsorption is obtained when the molecule is placed horizontally, with its axis, directed along the Ba-Ba axis and with its centre of gravity located above a Ba atom. The corresponding value of the adsorption energy is -9.70 eV per molecule (-4.85 eV per O atom). The molecule is then rather extended since the O–O distance measures 1.829 ?. By comparison, the adsorption energy of an O atom directly located above a Ba atom is only -3.50 eV. Therefore we are allowed to conclude that the O–O interaction stabilizes atomic adsorption. Also the local densities of states (LDOS) corresponding to various situations are discussed in the present paper. Up to now, we are not aware of experimental data to be compared to our calculated results.     

Synthesis and structural,magnetic, and resonance properties of the LiCuFe<Subscript>2</Subscript>(VO<Subscript>4</Subscript>)<Subscript>3</Subscript> compound

Complex studies have been performed for the structural, static magnetic, and resonance properties of a new magnet LiCuFe₂(VO₄)₃ prepared by solid-phase synthesis. The temperature dependence of the susceptibility has an anomaly at temperature T_max = 9.6 K. At high temperatures, the LiCuFe₂(VO₄)₃ sample is in the paramagnetic state described by the Curie–Weiss law at T > 50 K and mainly determined by iron ions with effective magnetic moment μ_eff(exp) = 8.6μ_B per formula unit. At low temperatures, a long-range magnetic order is observed in the magnetic subsystem of the sample; the order is predominantly characterized by the antiferromagnetic exchange interaction and high frustration level. The exchange interaction parameters are estimated in a six-sublattice representation of the LiCuFe₂(VO₄)₃ magnet. It is shown that the LiCuFe₂(VO₄)₃ compound is an antiferromagnet with strong intrachain and frustrating interchain exchange interactions.     

Spectroscopic Properties and Energy Transfer Analysis of Tm<Superscript>3+</Superscript>-Doped BaF<Subscript>2</Subscript>-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>-GeO<Subscript>2</Subscript>-La<Subscript>2</Subscript>O<Subscript>3</Subscript> Glass

This paper reports on the spectroscopic properties and energy transfer analysis of Tm³⁺-doped BaF₂-Ga₂O₃-GeO₂-La₂O₃ glasses with different Tm₂O₃ doping concentrations (0.2, 0.5, 2.0, 2.5, 3.0, 3.5, 3.5, 4.0 wt%). Mid-IR fluorescence intensities in the range of 1,300 nm−2,200 nm have been measured when excited under an 808 nm LD for all the samples with the same pump power. Energy level structure and Judd-Ofelt parameters have been calculated based on the absorption spectra of Tm³⁺, cross-relaxation rates and multi-phonon relaxation rates have been estimated with different Tm₂O₃ doping concentrations. The maximum fluorescence intensity at around 1.8 μm has been obtained in Tm₂O₃-3 wt% sample and the maximum value of calculated stimulated emission cross-section of Tm³⁺ in this sample is about 0.48 × 10⁻²⁰ cm² at 1,793 nm, and there is not any crystallization peak in the DSC curve of this sample, which indicate the potential utility of Tm³⁺-doped BaF₂-Ga₂O₃-GeO₂- La₂O₃ glass for 2.0-μm optical fiber laser.     

Synthesis and electrochemical properties of P2-Na<Subscript>2/3</Subscript>Ni<Subscript>1/3</Subscript>Mn<Subscript>2/3</Subscript>O<Subscript>2</Subscript>

The pure phase P2-Na_2/3Ni_1/3Mn_2/3O₂ was synthesized by a solid reaction process. The optimum calcination temperature was 850 °C. The as-prepared product delivered a capacity of 158 mAh g^?1 in the voltage range of 2–4.5 V, and there was a phase transition from P2 to O2 at about 4.2 V in the charge process. The P2 phase exhibited excellent intercalation behavior of Na ions. The reversible capacity is about 88.5 mAh g^?1 at 0.1 C in the voltage range of 2–4 V at room temperature. At an elevated temperature of 55 °C, it could remain as an excellent capacity retention at low current rates. The P2-Na_2/3Ni_1/3Mn_2/3O₂ is a potential cathode material for sodium-ion batteries.     

Synthesis,thermal and electrical behavior of the superprotonic conductor phase in Rb<Subscript>3</Subscript>(HSeO<Subscript>4</Subscript>)<Subscript>2.5</Subscript>(H<Subscript>2</Subscript>PO<Subscript>4</Subscript>)<Subscript>0.5</Subscript> compound

The Rb₃(HSeO₄)_2.5(H₂PO₄)_0.5 compound was prepared and its thermal behavior and electric properties were investigated. The thermogravimetry (TGA) analysis and the differential scanning calorimetric (DSC) show the presence of a structural phase transition of the title compounds at 374 K which is confirmed by the variation of fp and σ_dc as a function of temperature. The complex impedance of the Rb₃(HSeO₄)_2.5(H₂PO₄)_0.5 compound has been investigated in the temperature range of 295–453 K and in the frequency range 209 Hz–1 MHz. The impedance plots show semicircle arcs at different temperatures, and an electrical equivalent circuit has been proposed to explain the impedance results. The circuits consist of the parallel combination of bulk resistance Rp and constant phase elements CPE₁ in series with fractal capacity CPE₂. The frequency dependence of the conductivity is interpreted in terms of Jonscher’s law. The conductivity dc follows the Arrhenius relation. The near value of activation energies obtained from the analysis of modulus, conductivity data, and circuit equivalent confirm that the transport is through the ion hopping mechanism, dominated by the motion of the H⁺ proton in the structure of the investigated materials.     

Synthesis of ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript>/α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> complex substrates and growth of GaN films

With the solid phase reaction between pulsed-laser-deposited (PLD) ZnO film and α-Al₂O₃ substrate, ZnAl₂O₄/α-Al₂O₃ complex substrates were synthesized. X-ray diffraction (XRD) spectra show that as the reaction proceeds, ZnAl₂O₄ changes from the initial (111)-oriented single crystal to poly-crystal, and then to inadequate (111) orientation. Corresponding scanning electron microscope (SEM) images indicate that the surface morphology of ZnAl₂O₄ transforms from uniform islands to stick structures, and then to bulgy-line structures. In addition, XRD spectra present that ZnAl₂O₄ prepared at low temperature is unstable at the environment of higher temperature. On the as-obtained ZnAl₂O₄/α-Al₂O₃ substrates, GaN films were grown without any nitride buffer using light-radiation heating low-pressure MOCVD (LRH-LP-MOCVD). XRD spectra indicate that GaN film on this kind of complex substrate changes fromc-axis single crystal to poly-crystal as ZnAl₂O₄ layer is thickened. For the single crystal GaN, its full width at half maximum (FWHM) of X-ray rocking curve is 0.4°. Results indicate that islands on thin ZnAl₂O₄ layer can promote nucleation at initial stage of GaN growth, which leads to the (0001)-oriented GaN film.     

Synthesis and magnetic properties of FeNi<Subscript>3</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> core-shell nanocomposites

In this study, FeNi₃/Al₂O₃ core-shell nanocomposites, where individual FeNi₃ nanoparticles were coated with a thin layer of alumina, were fabricated by a modified sol-gel method. Several physical characterizations were performed on the samples of FeNi₃/Al₂O₃ nanocomposites with different thickness of Al₂O₃ shell. The encapsulation of FeNi₃ nanoparticles with alumina stops FeNi₃ agglomeration during heat treatment, and prevents interaction among the closely spaced magnetic FeNi₃ nanoparticles. The Al₂O₃ insulating shell improves the soft magnetic properties of FeNi₃. The study of the complex permeability of the samples shows that the real part μ’ of the permeability of the sample with Al molar content of 20% (Al/(Fe+Ni)) is as high as 12, and independent of frequency up to at least 1 GHz. The tunneling magnetoresistance arising from the presence of the Al₂O₃ shell have also been studied.