Anisotropy of nonlinear absorption in Co<Superscript>2+</Superscript>:MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> crystal

Anisotropy of the nonlinear absorption of Co²⁺ ions in MgAl₂O₄ single crystal at the wavelengths of 1.35 and 1.54 μm has been experimentally demonstrated. The experimental data are analyzed in the framework of a phenomenological model when the Co²⁺ ions are described as three sets of linear dipoles oriented along the crystallographic axes. Ground-state and excited state absorption cross-sections at 1.35 and 1.54 μm are evaluated to be σ_gsa=(4.0±0.3)×10^-19, σ_esa=(3.6±0.4)×10^-20 cm² and σ_gsa=(5.1±0.3)×10^-19, σ_esa=(4.6±0.4)×10^-20 cm², respectively. PACS 42.55.Rz; 71.20.Be     

Atomic and Electronic Structures of Intrinsic Defects in Ta<Subscript>2</Subscript>O<Subscript>5</Subscript>: Ab Initio Simulation

The atomic and electronic structure of intrinsic point defects in orthorhombic tantalum oxide has been studied by numerical simulation within the density functional theory. It has been shown that all defects responsible for metal enrichment of Ta₂O₅ serve as electron and hole traps. Under conditions of strong oxygen depletion and at a metal–insulator interface, which are characteristic of resistive memory elements, interstitial tantalum atoms compete with an oxygen vacancy in the formation of a conducting filament. Interstitial oxygen atoms are not involved in charge transport. Tantalum substituting oxygen can be considered as a combination of the oxygen vacancy and interstitial tantalum. The analysis of the calculated thermal and optical energies of trap ionization shows that the oxygen vacancy is a key defect for charge transport in Ta₂O₅.     

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.     

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.     

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.     

Synthesis and electrocatalytic activities of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanocubes

In this article, a hydrothermal method was developed to synthesize Co₃O₄ nanocubes using hydrogen peroxide (H₂O₂) as oxidant, Co(NO₃)₂·6H₂O as a cobalt source. The products are characterized in detail by multiform techniques including X-ray diffraction (XRD), energy dispersive X-ray analysis (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results show that the obtained products are Co₃O₄ nanocubes with size ranging between 20 and 40 nm. The effects of the hydrogen peroxide concentration on the size of the products have been studied. The electrocatalytic activities of H₂O₂ reduction on Co₃O₄ nanocubes in phosphate buffer were also evaluated.     

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).     

CeO<Subscript>2</Subscript> coating to improve the performance of Li[Li<Subscript>0.2</Subscript>Mn<Subscript>0.54</Subscript>Ni<Subscript>0.13</Subscript>Co<Subscript>0.13</Subscript>]O<Subscript>2</Subscript>

The Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ coated with CeO₂ has been fabricated by an ionic interfusion method. Both the bare and the CeO₂-coated samples have a typical layered structure with R-3m and C2/m space group. The results of XRD and TEM images display that the CeO₂ coating layer on the precursor could enhance the growth of electrochemically active surface planes ((010), (110), and (100) planes) in the following ionic interfusion process. The results of galvanostatic cycling tests demonstrate that the CeO₂-coated sample has a discharge capacity of 261.81 mAh g^?1 with an increased initial Coulombic efficiency from 62.4 to 69.1% at 0.05 °C compared with that of bare sample and delivers an improved capacity retention from 71.7 to 83.4% after 100 cycles at 1 °C (1 °C?=?250 mA g^?1). The results of electrochemical performances confirm that the surface modification sample exhibits less capacity fading, lower voltage decay, and less polarization.     

Effects of Sn doping on electrochemical characterizations of Li[Ni<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>]O<Subscript>2</Subscript> cathode material

Li[Ni_1/3Co_1/3Mn_1/3]O₂ and Sn-doped Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode materials for lithium battery are synthesized by a solid-state method. The samples are characterized by X-ray diffraction, scanning electron microscope, electrochemical impedance spectroscopy (EIS), and charge–discharge test. The results show that the Sn-doped Li[Ni_1/3Co_1/3Mn_1/3]O₂ has a typical hexagonal α-NaFeO₂ structure and strawberry-like shape with uniform particle size. It has also been found that the Sn-doped Li[Ni_1/3Co_1/3Mn_1/3]O₂ reveals better electrochemical performances than that without Sn doping. The EIS results suggest that Sn presence decreases the total resistance of Li[Ni_1/3Co_1/3Mn_1/3]O₂, which should be related to the improvement on the electrochemical properties.     

Tunneling conductivity in lithiated transition metal oxide cathode Li<Subscript>0.9</Subscript>[Ni<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>]O<Subscript>1.95</Subscript>

Electrical complex ac conductivity of the compound Li_0.9[Ni_1/3Mn_1/3Co_1/3]O_1.95 has been studied in the frequency range 10 Hz–2 MHz and in the temperature range 93–373 K. It has been observed that the frequency dependence of the ac conductivity obeys a power law and the temperature dependence of the ac conductivity is quite weak. The experimental data have been analyzed in the framework of several theoretical models based on quantum mechanical tunneling and classical hopping over barriers. It has been observed that the electron tunneling is dominant in the temperature range from 93 K to 193 K. A crossover of relaxation mechanism from electron tunneling to polaron tunneling is observed at 193 K. Out of the several models discussed, the electron tunneling and the polaron tunneling models are quite consistent with the experimental data for the complex ac conductivity. The various parameters obtained from the fits of the experimental results for the real and imaginary parts of the conductivity to the predictions of these models are quite reasonable.     

Electronic structure of an oxygen vacancy in Al<Subscript>2</Subscript>O<Subscript>3</Subscript> from the results of Ab Initio quantum-chemical calculations and photoluminescence experiments

The electronic structure of an oxygen vacancy in α-Al₂O₃ and γ-Al₂O₃ is calculated. The calculation predicts an absorption peak at an energy of 6.4 and 6.3 eV in α-Al₂O₃ and γ-Al₂O₃, respectively. The luminescence and luminescence excitation spectra of amorphous Al₂O₃ are measured using synchrotron radiation. The presence of a luminescence band at 2.9 eV and a peak at 6.2 eV in the luminescence excitation spectrum indicates the presence of oxygen vacancies in amorphous Al₂O₃.     

Enhanced electrochemical properties and thermal stability of LiNi<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> by surface modification with Eu<Subscript>2</Subscript>O<Subscript>3</Subscript>

Layered LiNi_1/3Co_1/3Mn_1/3O₂ cathode material is synthesized via a sol-gel method and subsequently surface-modified with Eu₂O₃ layer by a wet chemical process. The effect of Eu₂O₃ coating on the electrochemical performances and thermal stability of LiNi_1/3Co_1/3Mn_1/3O₂@Eu₂O₃ cells is investigated systematically by the charge/discharge testing, cyclic voltammograms, AC impedance spectroscopy, and DSC measurements, respectively. In comparison, the Eu₂O₃-coated sample demonstrates better electrochemical performances and thermal stability than that of the pristine one. After 100 cycles at 1C, the Eu₂O₃-coated LiNi_1/3Co_1/3Mn_1/3O₂ cathode demonstrates stable cyclability with capacity retention of 92.9 %, which is higher than that (75.5 %) of the pristine one in voltage range 3.0–4.6 V. Analysis from the electrochemical measurements reveals that the remarkably improved performances of the surface-modified composites are mainly ascribed to the presence of Eu₂O₃-coating layer, which could efficiently suppress the undesirable side reaction and increasing impedance, and enhance the structural stability of active material.     

A study on capacity and power fading characteristics of Li(Ni<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>)O<Subscript>2</Subscript>-based lithium-ion batteries

The objective of this study is to find out the factor that accounts for the capacity fading and to predict the cycle life of lithium-ion batteries by the driving cycle test. A new method, incremental polarization resistance, is elected to analyze the gradation mechanism based on incremental capacity analysis. It is summarized that the two major factors, the loss of lithium ions and the cathode fading, make the capacity loss in different stages. In the first stage, the loss of lithium ions, caused by the solid electrolyte interface (SEI) lay reaction, is the main reason for battery degradation. In the second stage, the cathode starts to decay and make the capacity loss, because of the less intercalation in cathode. In the third stage, the cathode degradation gradually outpaces the loss of Li and becomes the limit factor for the battery recession. Finally, a cycle life model was established to predict the capacity loss with cycle numbers.     

Synthesis and electrochemical properties of Li<Subscript>1.2</Subscript>Mn<Subscript>0.54</Subscript>Ni<Subscript>0.13</Subscript>Co<Subscript>0.13</Subscript>O<Subscript>2</Subscript> cathode material for lithium-ion battery

The layered Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ lithium-rich manganese-based solid solution cathode material has been synthesized by a simple solid-state method. The as-prepared material has a typical layered structure with R-3m and C2/m space group. The synthesized Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ has an irregular shape with the size range from 200 to 500 nm, and the primary particle of Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ has regular sphere morphology with a diameter of 320 nm. Electrochemical performances also have been investigated. The results show that the cathode material Li_1.2Mn_0.54Ni_0.13Co_0.13O₂ prepared at 900 °C for 12 h has a good electrochemical performance, which can deliver a high initial discharge capacity of 233.5, 214.2, 199.3, and 168.1 mAh g^?1 at 0.1, 0.2, 0.5, and 1 C, respectively. After 50 cycles, the capacity retains 178.0, 166.3, 162.1, and 155.9 mAh g^?1 at 0.1, 0.2, 0.5, and 1 C, respectively. The results indicate that the simple method has a great potential in synthesizing manganese-based cathode materials for Li-ion batteries.     

Rectifying and photovoltage characteristics of Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>Co<Subscript>2</Subscript>O<Subscript>y</Subscript>/Nb-doped SrTiO<Subscript>3</Subscript> heterojunction

A heterojunction was fabricated by growing a layer of Bi₂Sr₂Co₂O _y thin film on the 0.7 wt% Nb-doped SrTiO₃ substrate. Such heterojunction showed good rectifying characteristics over a wide temperature range, and its transport mechanism under the forward bias can be attributed to a space charge limited conduction mechanism via defects near the interface of the heterojunction. Photovoltaic properties of the heterojunction were studied by using both continuous-wave and pulsed irradiations and the results can be well explained by the photovoltaic effect of a p–n junction.     

Magnetic properties of aerugite Co<Subscript>10</Subscript>Ge<Subscript>3</Subscript>O<Subscript>16</Subscript>

This paper reports on the first study of the temperature and field dependences of the magnetization, heat capacity, and electrical properties of synthesized polycrystalline samples of aerugite Co₁₀Ge₃O₁₆, as well as on x-ray diffraction analysis of this compound. It is shown that the cobalt ions in this compound occupy three nonequivalent positions. The results of the experimental and theoretical studies suggest that aerugite is a ferrimagnet with two uncompensated magnetic moments of the cobalt atom per formula unit.     

Photoinduced processes in thin films of MoO<Subscript>3</Subscript> and mixed V<Subscript>2</Subscript>O<Subscript>5</Subscript>: MoO<Subscript>3</Subscript> oxides

Photoinduced processes in thin films of MoO₃ and mixed V₂O₅: MoO₃ oxides prepared by polycondensation of the respective oxoacids under solvothermal conditions are studied using Raman spectroscopy, ESR, and AFM. It is shown that, under UV irradiation, the photoinduced polycondensation occurs in the oxide films, leading to the formation of the oxygen bridges, an effect that opens up the possibility of developing a new photolithographic process.     

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.     

Mott Transition in GdMnO<Subscript>3</Subscript>: an Ab Initio Study

Orthorhombic GdMnO₃ is studied using density functional theory considering the pseudo-potential plane-wave method within local-spin-density approximation, LSDA. The electronic band structure and density of states, for several hydrostatic pressures, are studied. The Mott transition was observed at 60 GPa. Calculated lattice parameters are close to the experimental measurements, and some indirect band gaps (S→Γ) were obtained within the LSDA level of calculation, between the occupied O-2p and unoccupied Gd-4f states. The variation of the gap reduces with increasing pressure, being well fitted to a quadratic function.