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%.     

Electronic structure of A15-type compounds: V<Subscript>3</Subscript>Co,V<Subscript>3</Subscript>Rh,V<Subscript>3</Subscript>Ir and V<Subscript>3</Subscript>Os

Band structure and Fermi surfaces of the A₃B compounds V₃Co, V₃Rh, V₃Ir and V₃Os are calculated in FP-LAPW calculations. From V₃Co to the V₃Os compound one observes a decrease of the overlap for d-states from both V and B atoms; the center of gravity of the d-band for V moves upwards, while for the B-atom it moves toward lower energies. Hence, despite the band widening, a weakening of interactions takes place throughout this series, which leads to a lattice expansion as experimentally observed. The bonding mechanism in these compounds is found to be dominated by the lower energy d-states. Comparatively, the DOS at E_F does not change appreciably between these compounds, except for V₃Os, where a sharp peak is observed at E_F. This feature leads to the highest electronic heat-capacity coefficient γ (2.31 ) in this compound, which otherwise possesses the smallest Bulk modulus (209.05 GPa). In V₃Os, for whom no report has been found, a stronger admixture between p-states from both metals near E_F is observed, and the Os d-states form a common d-band with the V d-states between 0.4–0.6 Ry. A stronger s–s hybridization is observed for V₃Co, which is in the source of the fact that this compound has an anomalously large quadrupole interaction and exhibits a positive Knight shift.     

Anisotropy of the Complex Permittivity of the Kagome-Staircase Compounds Co<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript> and Ni<Subscript>3</Subscript>V<Subscript>2</Subscript>O<Subscript>8</Subscript>: Experiment and Ab Initio Calculations

The anisotropy of the components of the complex permittivity of vanadate Co₃V₂O₈ and Co₃V₂O₈ single crystals in the paramagnetic phase are studied by optical ellipsometry in the spectral region 0.5–5.0 eV. Our experimental results support the weak anisotropy of the optical response detected earlier for axes a and c. The optical properties are also investigated along axis b. The properties of both compounds are compared. The optical spectra of both compounds along axis b are shifted toward low energies as compared to axes a and c. The maximum of the main interband absorption band of Co₃V₂O₈ is shifted toward low energies by 0.25–0.3 eV as compared to Co₃V₂O₈. The electronic structure parameters of both compounds are determined. Optical function spectra are analyzed using the results of ab initio band calculations.     

Synthesis and application of nanostructured MCo<Subscript>2</Subscript>O<Subscript>4</Subscript>(M=Co,Ni) for hybrid Li-air batteries

We report the synthesis of MCo₂O₄ (M=Co, Ni) on Ni-mesh by a simple metal acetate decomposition method. Stability tests of the samples in aqueous acidified LiCl, LiOH and LiTFSI in H₂O/DME showed that Co₃O₄/Ni and Co₃O₄-PVP/Ni are relatively stable in alkaline and neutral environments, with Co₃O₄/Ni being relatively more stable. For NiCo₂O₄/Ni and NiCo₂O₄-PVP/Ni, the low weight percentage change of cobalt in LiTFSI in H₂O/DME suggests that they are mostly stable in this electrolyte. The electrochemical performance of the Li-air cell was evaluated using Li anode and a LAGP ceramic separator with above mentioned electrolytes. Co₃O₄ showed slightly higher catalytic activity for oxygen reduction reaction (ORR) than for oxygen evolution reaction (OER) for the first three cycles. The cell with LiTFSI in H₂O/DME as aqueous catholyte showed that NiCo₂O₄ is a better catalyst for the OER than for the ORR, while the reverse was observed when LiOH was used as the electrolyte.     

NMR study of the paramagnetic state of low-dimensional magnets LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> and NaCu<Subscript>2</Subscript>O<Subscript>2</Subscript>

A comprehensive NMR study of the magnetic properties of single crystal LiCu₂O₂ (LCO) and NaCu₂O₂ (NCO) is carried out in the paramagnetic region of the compounds for various orientations of single crystals in an external magnetic field. The values of the electric-field gradient (EFG) tensor, as well as the dipole and transferred hyperfine magnetic fields for ^63,65Cu, ⁷Li, and ²³Na nuclei are determined. The results are compared with the data obtained in previous NMR studies of the magnetically ordered state of LCO/NCO cuprates.     

Enhancement of antiferromagnetic interaction and transition temperature in M<Subscript>3</Subscript>TeO<Subscript>6</Subscript> systems (M = Mn,Co, Ni,Cu)

Several M₃TeO₆ (M = Mn, Co, Ni, Cu) oxides order antiferromagnetically at low temperatures (?60 K), while displaying interesting dielectric properties at high temperatures (ferroelectricity below 1000 K in M = Ni case). We have investigated and analyzed the structural and magnetic properties of Mn-doped Co₃TeO₆ and Ni₃TeO₆, which order antiferromagnetically at temperatures higher than their undoped counterparts.     

Magnetic susceptibility in quasi one-dimensional Ba<Subscript>a2</Subscript>V<Subscript>3</Subscript>O<Subscript>9</Subscript>: chain segmentation versus the staggered field effect

A pronounced Curie-like upturn of the magnetic susceptibility χ( T ) of the quasi one-dimensional spin chain compound Ba₂V₃O₉ has been found recently [#!kaul:02!#]. Frequently this is taken as a signature for a staggered field mechanism due to the presence of g-factor anisotropy and Dzyaloshinskii-Moriya interaction. We calculate this contribution within a realistic structure of vanadium 3 d- and oxygen 2 p-orbitals and conclude that this mechanism is far too small to explain experimental results. We propose that the Curie term is rather due to a segmentation of spin chains caused by broken magnetic bonds which leads to uncompensated S = ? spins of segments with odd numbers of spins. Using the finite-temperature Lanczos method we calculate their effective moment and show that ∼ 1% of broken magnetic bonds is sufficient to reproduce the anomalous low-T behavior of χ( T ) in Ba₂V₃O₉. Received 19 December 2002 / Received in final form 29 January 2003 Published online 14 March 2003     

Structure,lattice dynamics,and exchange interaction in Lu<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>, Y<Subscript>2</Subscript>V<Subscript>2</Subscript>O<Subscript>7</Subscript>: an ab initio approach

Ab initio calculations of the crystal structure and fundamental vibrations of vanadium pyrochlores Lu₂V₂O₇ and Y₂V₂O₇ are performed. The calculations are performed in the framework of the density functional theory (DFT) with the use of hybrid functionals. The ions involved in the vibrations are determined by the isotope substitution method. Values of the isotropic exchange interaction constant were calculated. Theoretical results for the crystal structure parameters, the vibrational frequencies, and the isotropic exchange interaction parameter are compared with the experimental data.     

Photoaccumulating film systems based on TiO<Subscript>2</Subscript>/MoO<Subscript>3</Subscript> and TiO<Subscript>2</Subscript>/MoO<Subscript>3</Subscript>:V<Subscript>2</Subscript>O<Subscript>5</Subscript> nanoheterostructures

The thin-film photocatalysts TiO₂/MoO₃ and TiO₂/MoO₃:V₂O₅ obtained by a combination of sol–gel and sintering techniques were studied using the photooxidation of probing dyes, EPR spectroscopy, X-ray diffraction analysis, and electron microscopy. It was shown that due to charge accumulation caused by UV irradiation, these photocatalysts retain their oxidative activity and ability for self-sterilization in the dark for a long time after irradiation was terminated (up to 5 h for TiO₂/MoO₃:V₂O₅).     

Surface modification of polycrystalline oxides (V<Subscript>2</Subscript>O<Subscript>5</Subscript>, MoO<Subscript>3</Subscript>, and WO<Subscript>3</Subscript>) irradiated with high-power ion beams

The influence of a high-power ion beam on polycrystalline oxides (V₂O₅, MoO₃, and WO₃) is investigated. Oxide irradiation with ion beams with current densities of greater than ～30 A/cm² is established to initiate changes in the color of irradiated layers and lead to surface-layer particle melting. It is demonstrated that a distinctive feature of the interaction between a high-power ion beam and V₂O₅ is the formation of surface nanosheets and nanowires whose characteristic cross-sectional size and thickness are ～1 μm and up to ～40 nm, respectively. The nanosheets are generated near emerging surface cracks if the beam current density is ～100 A/cm². Possible mechanisms of surface nanostructures formation under the action of pulsed ion beams are discussed.     

Low-temperature synthesis of Cu-doped Li<Subscript>1.2</Subscript>V<Subscript>3</Subscript>O<Subscript>8</Subscript> as cathode for reversible lithium storage

Li_{1 .2}V₃O₈ and Cu-doped Li_1.2V₃O₈ were prepared at a temperature as low as 300 °C by a sol-gel method. The structure, morphology, and electrochemical performance of the as-prepared samples were characterized by means of X-ray diffraction, scanning electron microscopy, electrochemical impedance spectroscopy, and the galvanostatic discharge–charge techniques. It is found that the Cu-doped Li_1.2V₃O₈ sample exhibits less capacity loss during repeated cycling than the undoped one. The Cu-doped Li_1.2V₃O₈ sample demonstrates the first discharge capacity of 275.9 mAh/g in the range of 3.8–1.7 V at a current rate of 30 mA/g and remains at a stable discharge capacity of 264 mAh/g within 30 cycles. Furthermore, the possible role that copper plays in enhancing the cycleability of Li_1.2V₃O₈ has also been elucidated.     

Synthesis and microstructure of Co/Ni: MgGa<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles

This report discusses the preparation and microstructure of Co/Ni co-doped MgGa₂O₄ nanoparticles. The nanoparticles with the size of 20–55 nm were synthesized by sol-gel method. The phase and crystallinity were confirmed by X-ray powder diffraction (XRD) pattern. The particle size was estimated according to XRD data and transmission electron microscopy. The electronic structure was studied using X-ray photoelectron spectroscopy (XPS). The XPS studies showed that Ga³⁺ ions possess tetrahedral and octahedral sites of spinel structure and the inverse degree (two times of the fraction of tetrahedral Ga³⁺ ions) has increased with the increase of the doping concentration of Co²⁺ and Ni²⁺ ions. For Co/Ni co-doped MgGa₂O₄, two broad absorption bands of 350~500 and 550~700 nm were observed in the absorption spectra. The broad band at 350~500 nm was assigned to the combination of the absorption of octahedral Co²⁺ and Ni²⁺ ions, whereas the absorption band at 550~700 nm is mainly due to tetrahedrally coordinated Co²⁺ ions and octahedrally coordinated Ni²⁺ ions.     

On the magnetism of Ln<Subscript>2/3</Subscript>Cu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> (Ln = lanthanide)

The magnetic and thermodynamic properties of the complete Ln_2/3Cu₃Ti₄O₁₂ series were investigated. Here Ln stands for the lanthanides La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb. All the samples investigated crystallize in the space group Im[`3]Im\bar{3} with lattice constants that follow the lanthanide contraction. The lattice constant of the Ce compound reveals the presence of Ce⁴⁺ leading to the composition Ce_1/2Cu₃Ti₄O₁₂. From magnetic susceptibility and electron-spin resonance experiments it can be concluded that the copper ions always carry a spin S = 1/2 and order antiferromagnetically close to 25 K. The Curie-Weiss temperatures can approximately be calculated assuming a two-sublattice model corresponding to the copper and lanthanide ions, respectively. It seems that the magnetic moments of the heavy rare earths are weakly coupled to the copper spins, while for the light lanthanides no such coupling was found. The 4f moments remain paramagnetic down to the lowest temperatures, with the exception of the Tm compound, which indicates enhanced Van-Vleck magnetism due to a non-magnetic singlet ground state of the crystal-field split 4f manifold. From specific-heat measurements we accurately determined the antiferromagnetic ordering temperature and obtained information on the crystal-field states of the rare-earth ions.     

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.     

Synthesis and characterization of mesoporous and hollow-mesoporous M<Subscript>x</Subscript>Fe<Subscript>3-x</Subscript>O<Subscript>4</Subscript> (M=Mg,Mn, Fe,Co, Ni,Cu, Zn) microspheres for microwave-triggered controllable drug delivery

Spinel ferrites can be used in magnetic targeting and microwave heating and can therefore be used for targeted and controllable drug delivery. We used the cetyltrimethylammonium bromide-assisted solvothermal method to synthesize a series of spinel ferrites (M_xFe_3-xO₄, M=Mg, Mn, Fe, Co, Ni, Cu, Zn) with a mesoporous or hollow-mesoporous structure suitable for direct drug loading and the particle diameters ranging from 200 to 350 nm. We investigated the effects of M²⁺ cation on the morphology and properties of these products by analyzing their transmission electron microscopy images, mesoporous properties, magnetic properties, and microwave responses. We chose hollow-mesoporous M_xFe_3-xO₄ (M=Fe, Co, Zn) nanoparticles, which had better overall properties, for the drug VP16 (etoposide) loading and microwave-controlled release. The Co_xFe_3-xO₄ and Fe₃O₄ particles trapped 61.5 and 64.8%, respectively, of the VP16, which were higher than that (60.4%) of Zn_xFe_3-xO₄. Controllable drug release by these simple magnetic nanocarriers can be achieved by microwave irradiation, and VP16-loaded Co_xFe_3-xO₄ released the most VP16 molecules (more than 50% after 1 h and 69.1% after 6 h) under microwave irradiation. Our results confirm the favorable drug loading and microwave-controlled delivery by these ferrites, and lay a theoretical foundation to promote clinical application of the targeted controllable drug delivery system.

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Graphical abstract In the present study, we prepared mesoporous or hollow-mesoporous spinel ferrites (M_xFe_3-xO₄, M=Mg, Mn, Fe, Co, Ni, Cu, Zn) by CTAB-assisted solvothermal method and solved the problem of Cu and Ni impurities in Cu_xFe_3-xO₄ and Ni_xFe_3-xO₄ products by means of magnetic separation and additional redox reactions, respectively. We investigated the effects of the M²⁺ cation on the morphology, mesoporous properties, magnetic properties, and microwave responses of these ferrites. Then, the drug loading and microwave-controlled drug release of hollow-mesoporous M_xFe_3-xO₄ (M?=?Fe, Co, Zn) nanoparticles with better overall properties were also studied. Co_xFe_3-xO₄ has the best overall performances for microwave-controlled drug release.

     

Specific features of electrical conductivity of V<Subscript>3</Subscript>O<Subscript>5</Subscript> single crystals

The electrical conductivity of V₃O₅ single crystals has been investigated over a wide temperature range, including the region of existence of the metallic phase and the region of the transition from the metallic phase to the insulating phase. It has been shown that the low electrical conductivity of metallic V₃O₅ is caused, on the one hand, by a lower concentration of electrons and, on the other hand, by a strong electronelectron correlation whose role with decreasing temperature increases as the phase transition temperature is approached. The temperature dependence of the electrical conductivity of the insulating phase of V₃O₅ has been explained in the framework of the theory of hopping conduction, which takes into account the effect of thermal vibrations of atoms on the resonance integral.     

Magnetic properties of the Gd<Subscript>2</Subscript>V<Subscript>0.67</Subscript>Mo<Subscript>1.33</Subscript>O<Subscript>7</Subscript> and Y<Subscript>2</Subscript>VMoO<Subscript>7</Subscript> pyrochlore compounds

It is demonstrated that 50% substitution of vanadium for molybdenum in the pyrochlore lattice of the complex oxide Y₂(V _x Mo_{1 ? x} )₂O₇ results in a transition from the spin-glass ground state (at x = 0) to the ferromagnetic state in Y₂VMoO₇ (a = 10.1645(2) Å, T _C = 55 K). The Gd₂V_0.67Mo_1.33O₇ compound (a = 10.2862(3) Å) is a ferromagnet with T _C (84 K) exceeding that of undoped Gd₂MnO₂O₇.     

Incommensurate helical magnetic order in LiCu<Subscript>2</Subscript>O<Subscript>2</Subscript> and NaCu<Subscript>2</Subscript>O<Subscript>2</Subscript> quasi-one-dimensional compounds

The results of the measurements of the ^{6, 7}Li and ²³Na nuclear magnetic resonance (NMR) and ^{63, 65}Cu nuclear quadrupole resonance in LiCu₂O₂ and NaCu₂O₂ quasi-one-dimensional compounds with a spin chains in the paramagnetic and magnetically ordered states are presented. The shape of the NMR line below T _c = 24 and 13 K for LiCu₂O₂ and NaCu₂O₂, respectively, is characteristic of the incommensurate static modulation of the local magnetic field matching with the incommensurate spiral modulation of the magnetic moments. The differences in the shape of the NMR spectra of ²³Na and ⁷Li are discussed in terms of the features of the crystal structure of LiCu₂O₂ and NaCu₂O₂.     

Mössbauer studies of magnetic Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> nanocomposites

A large variety of glass and glass ceramics may be obtained by sol-gel process from hydrolysis of tetraethoxysilane. The transformation involves hydrolysis and polycondensation reactions leading to the growth of clusters that eventually collide together to form a gel. The structure and properties of the final product have been found to be strongly dependent on the initial conditions of preparation. Silica nanocomposites based on Fe₂O₃/SiO₂ were prepared with the help of ultrasonic activation and subsequent annealing in nitrogen atmosphere or air with concentrations of iron oxide of about 20 to 30wt.%.