Features of the electrochemical lithium insertion into monophosphate tungsten bronzes (PO2)4(WO3)2m (m=9 and 10)

Electrochemical lithium insertion into (PO₂)₄(WO₃)_2m, where m=9 and 10, has allowed the determination of several phases Li_x(PO₂)₄(WO₃)_2m between 3.4 and 0.01 V vs Li⁺/Li⁰. After the first cycle the electrochemical system was unable to maintain the high specific capacity of the cells (540 Ah/kg) due to irreversible processes. Nevertheless at high voltage values, above 1.4 V vs Li⁺/Li⁰, the lithium insertion proceeded through a reversible mechanism. By means of X-ray diffraction experiments we have detected the nature of different phases Li_x(PO₂)₄(WO₃)_2m formed and we have established a correlation with the reversible/irreversible processes detected during the electrochemical insertion.     

Photoemission study of Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0)

An angle-resolved photoemission study for Ag nanofilm grown on pseudomorphic metastable-fcc-phase Fe(1 0 0) has been done in order to investigate in detail the quantized electronic structures. From the low-energy electron-diffraction and angle-resolved photoemission spectra, it is found that the present Ag nanofilms were grown in the direction of [1 1 1] on pseudomorphic fcc Fe(1 0 0) substrates. The angle-resolved photoemission spectra of Ag nanofilms grown on pseudomorphic fcc Fe(1 0 0) exhibit the features derived from Shockley-type surface state and additional fine-structures derived from the quantized state of Ag sp valence electron. The experimental nanofilm-thickness dependence of binding energies of these quantized states is compared with the theoretical calculation based on the phase accumulation model, taking into account the phase shifts of electron reflection at both interfaces of the Ag nanofilm. From these results, we discuss the quantized electronic structure in Ag nanofilm grown on pseudomorphic fcc Fe(1 0 0).     

Multiferroic studies on (BiFeO3)m(BaTiO3)n superlattice structures

BiFeO₃ has been studied extensively due to its room temperature multiferroic features and has been proven as a promising candidate for device applications. But BiFeO₃ possesses some drawbacks like high leakage current and complicated magnetic ordering, giving rise to a canted antiferromagnetic behavior. Hence, a superlattice approach of BiFeO₃ and BaTiO₃ with a good lattice matching was fabricated and the room temperature ferroelectric and ferromagnetic properties were studied. The macroscopic and local probe studies reveal a ferroelectric nature at room temperature, and most importantly a weak ferromagnetic like behavior was observed. The ferromagnetic behavior is expected to arise due to the variation introduced in the spin modulation of single BiFeO₃ layer due to the superstructure formation.     

Electrical conductivity of (Er,U)O2+x and (Ce,U)O2+x

The electrical conductivity (σ) of (Er_yU_1−y)O_2+x (y=0.06, 0.20) and (Ce_yU_1−y)O_2+x (y=0.05, 0.15, 0.25) has been measured as a function of oxygen partial pressure in the temperature range of 1100≤T/°C≤1300 by a d.c. 4-probe method. Both of the oxides exhibited Po₂-regions where the electrical conductivity is independent of oxygen partial pressure, which indicates that doped Er and Ce exist as trivalent cations on uranium sites and fix the hole concentration by acting as electron acceptors, i.e. [h]=[Er′_U] and [h]=[Ce′_U], respectively. It is considered that strong oxidization tendency of uranium and reduction tendency of cerium simultaneously render the cerium ions exist exclusively as Ce³⁺ in the uranium dioxide. The electron-hole mobility of (Er_yU_1−y)O_2+x and (Ce_yU_1−y)O_2+x in the Po₂ region where σ is constant has been calculated by the combination of the electrical conductivity and charge carrier concentration; the activation energy (E_H) of each oxide has been obtained from the temperature dependence of the mobility. Small polaron hopping conduction mechanism was confirmed by small magnitude of the mobility (0.018-0.052 cm² V⁻¹ s⁻¹) and the activation energy (0.12-0.22 eV).     

Preparation of REFeAsO1−xFx (RE=Sm and Gd) superconductors at a relatively low temperature

A series of the SmFeAsO_1−xF_x and GdFeAsO_1−xF_x (x=0.05, 0.1, 0.15, 0.2, 0.25) samples have been prepared using nano-scaled ReF₃ as the fluorine resource at a relatively low temperature. The samples have been sintered at 1100 and 1120 °C for SmFeAsO_1−xF_x and GdFeAsO_1−xF_x, respectively. These temperatures are at least 50-60° lower than other previous reports. All of the so-prepared samples possess a tetragonal ZrCuSiAs-type structure. Dramatically supression of the lattice parameters and increase in T_c proved that this low temperature process was more effective to introduce fluorine into R_EFeAsO. Superconducting transition appeared at 39.5 K for SmFeAsO_1−xF_x with x=0.05 and at 22 K for GdFeAsO_1−xF_x with x=0.1. The highest T_c was detected to be 54 K in SmFeAsO_0.8F_0.2 and 40.2 K in GdFeAsO_0.75F_0.25. The use of the nano-scaled ReF₃ compounds has improved the efficiency of the present low temperature method in synthesizing the fluorine-doped iron-based superconductors.     

Magnetic, electric, and optical functionalities of (PLZT)x(BiFeO3)1−x ferroelectric-ferromagnetic thin films

We have prepared a series of (PLZT)_x(BiFeO₃)_1−x transparent thin films with thickness of 300 nm by a thermal pyrolysis method. Only films with x≦0.10 formed a single phase of perovskite structure. The film where x=0.10 exhibited both ferromagnetic and ferroelectric properties at room temperature with spontaneous magnetization and coercive magnetic fields of 0.0027μ_B and 5500 G, respectively. The remanent electric polarization and coercive electric field for the film where x=0.10 were 3.0 μC/cm² and 24 kV/cm, respectively. Additionally, films with 0.02≦x≦0.10 showed both magneto-optical effects and the second harmonic generation of transmitted light.     

Magnetic coupling in Co1−xPtx (x>0.5) nanowires electrodeposited on an AAO template

Thirty nanometer diameter Co-Pt nanowires of different composition were fabricated by electrodepositing the Co and Pt atoms to nanoporous anodized aluminium oxide (AAO) templates. The structure and magnetic properties are studied by transmission electron microscopy (TEM), induction-coupled plasma spectrometer (ICP), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). The as deposited nanowires with Pt content about 50 at.% present a single ferromagnetic phase of fcc CoPt. When the Pt content of the nanowires varies from about 55 to about 75 at.%, the nanowires include a soft phase of fcc CoPt₃ and a relatively hard phase of fcc CoPt and the two phases are separate as seen from the hysteresis loops. After annealing to 600 °C, the two phases coupled completely and the coupled phase has the same coercivity as the original hard one.     

Influence of lithium phosphate concentration and temperature on the electrical conduction of (76V2O5-24P2O5)1−X (Li3PO4)X glasses

Characterization of the (76V₂O₅-24P₂O₅)_1−X (Li₃PO₅)_X, where X=0.0,0.01,0.02,0.10 and 0.15, glass has been done using X-ray diffraction and differential thermal analysis (DTA). The dc conductivity of the glass samples was studied over a temperature range from 300 to 593 K. The temperature dependence of dc conductivity shows two regions. One at relatively high temperature range, above θ_D/2, and the other at relatively low temperature range, below θ_D/2. The I-V characteristics of the glasses have been studied as a function of both temperature and Li₃PO₄ content. The I-V characteristics exhibits threshold switching with differential negative resistance. It's found that both the threshold voltage (V_th) and threshold current (I_th) are dependent on the temperature and lithium phosphate concentration.     

Increase of Tc in the 1:2:3 superconductors YBCO and (CaxLa1−x)(LauBa1−u)2Cu3Oy after slow cooling or low temperature annealing

The well known phenomenon of the increase of T_c of YBCO after slow cooling or low temperature annealing without change of the oxygen content, was found also for the YBCO like tetragonal superconductors of (Ca_xLa_1−x)(La_uBa_1−u)₂Cu₃O_y (this compound has been previously denoted as CLBLCO, CLBCO or CaLaBaCuO). It has been observed at 150 and 100 °C for oxygen underdoped, optimally- and overdoped ceramics. The products retain their tetragonal unit cells. The possible reasons of this phenomenon are discussed.     

Magnetic and transport properties of SmCoAsO1−xFx

A series of SmCoAsO_1−xF_x (with x=0, 0.05, 0.1, and 0.2) samples have been prepared by solid state reactions. X-ray powder diffraction proved that all samples can be indexed as a tetragonal ZrCuSiAs-type structure. A clear shrinkage of the lattice constants a and c with increasing F content indicated that F has been doped into the lattice. The magnetic and transport properties of the samples have been investigated. Parent SmCoAsO compound exhibited complicated magnetism including antiferromagnetism, ferromagnetism, and ferrimagnetism. For the fluorine doped samples, the antiferromagnetic Néel temperatures were almost independent of the F content and metamagnetic transitions were observed below antiferromagnetic Néel temperatures. With increasing F content, high temperature (below 142 K) ferrimagnetic state gradually changed to ferromagnetic state. In the resistivity result, metallic conduction in the region of 2-300 K and Fermi liquid behavior at low temperatures were shown in all samples. Transport properties at applied magnetic fields showed anomalies at low temperatures.     

Kondo behaviour of the solid solution (Ce1−xLax)PtGa

The solid solution (Ce_1−xLa_x)PtGa has been studied through X-ray diffraction, magnetization (σ(B)), magnetic susceptibility (χ(T)), electrical resistivity (ρ(T)), magnetoresistivity (MR) and heat capacity (C_P(T)) measurements. The Néel temperature (T_N=3.3 K) for CePtGa is lowered upon La substitution as observed from χ(T) and ρ(T) measurements. The Kondo temperature T_K as calculated from MR measurements is comparable to T_N and also decreases with La substitution. The volume dependence of T_K is in accordance with the compressible Kondo lattice model and a Doniach diagram of the results is presented. C_P(T) measurements are presented for CePtGa, Ce_0.2La_0.8PtGa and LaPtGa and the results are discussed in terms of the electronic and magnetic properties. Other features of interest are anomalies in ρ(T) and C_P(T) due to crystalline electric field effects and metamagnetism as observed in σ(B) studies for samples with 0≤x≤ 0.3.     

Diffusion of hydrogen isotopes in the monohydride phase of Ti1−zVzHxDy studied by H and H NMR spin-lattice relaxation times

The ¹H and ²H spin-lattice relaxation times in the monohydride phase of the V-H-D and Ti_0.1V_0.9-H-D systems have been re-analyzed by Bloembergen-Purcell-Pound (BPP) equations with a log-normal distribution of correlation times, and the obtained parameters have been compared with those of the Ti_0.33V_0.67-H-D system. The mean activation energy and the magnitude of the distribution of H and D diffusions are discussed in relation to the metal compositions, the hydrogen site, the crystal structure and the [H]/[D] ratios.     

Synthesis of high tap density Li3V2(PO4)3 cathode materials using mixed lithium precursors

High tap density Li₃V₂(PO₄)₃ cathode materials were synthesized using mixed LiF and LiNO₃ as lithium precursors, LiNO₃ was used as the sintering agent. Rietveld refinement results show that no impurities phases are detected in products. Particle size distribution and tap density measurement results show that particle size and tap density of products can be increased by the addition of LiNO₃. Electrochemical characterization results show that electrochemical performance of products is declined with the increase in contents of LiNO₃ in the lithium precursors. Only a small amount of LiNO₃ added in the lithium precursors (mole ratio of LiNO₃ to LiF is 1:9) can increase the tap density and also retain the good performance of products. Scanning electron microscopy (SEM) images indicate that the samples prepared by mixed lithium precursors present particles agglomerate, and the particle size increased with increase in contents of LiNO₃. Large amount of LiNO₃ added in the lithium precursors induces the particles to become spheric and smooth, which worsens the performance. The particles obtained with the mole ratio of LiNO₃ to LiF in 1:9 show a flake-like shape with a high specific surface area, which leads to good electrochemical performance.     

Anomalous thermoelastic behaviour of orientationally disordered (NH4I)x(KI)1−x mixed crystals

We have incorporated the translational rotational (TR) coupling effects in the framework of three body force shell model (TSM) to develop an extended TSM (ETSM). This ETSM has been applied to reveal the second order elastic constants (C₁₁, C₁₂ and C₄₄) in the dilute regimes 0≤x≤0.50. Our theoretical results have reproduced well the observed variations of C₁₁ (x=0.14, 0.43) and C₁₂ (x=0.14, 0.43) with temperature. The anomalous elastic behaviour observed in C₄₄ below 150 K for x=0.43 has been predicted well by ETSM results in the orientationally disordered (NH₄I)_x(KI)_1−x mixed crystals. Our results for C₁₂ are in good agreement with the experimental data available for x=0.14 and 0.43. Besides, the third and fourth order elastic constants and the results on cohesive and thermophysical properties are also discussed.     

X-ray diffraction and magnetic susceptibility of sodium fullerides NaxC60

The X-ray diffraction (XRD), magnetic susceptibility and electron spin resonance (ESR) measurements have been carried out for Na_xC₆₀. The XRD profiles with x<4 can be assigned to a face-centered cubic (fcc) lattice, while those with 4≦x to a hexagonal one. The temperature dependence of magnetic susceptibility χ for Na_xC₆₀ using SQUID was fitted to the Curie law, and estimated temperature-independent component χ₀. The composition x dependence of the χ₀ for Na_xC₆₀ shows two maxima at around x=3 and x=10, and minimum at x=6. The absence of Pauli contribution at x=6 was confirmed using ESR. A trace of superconducting transition at 14 K has been found for some Na_xC₆₀ specimens with 8<x<9.     

The influence of microscopic parameters on the ionic conductivity of SrBi2(Nb1−xVx)2O9−δ(0≤x≤0.3) ceramics

Layered SrBi₂(Nb_1−xV_x)₂O_9−δ (SBVN) ceramics with x lying in the range 0-0.3 (30 mol%) were fabricated by the conventional sintering technique. The microstructural studies confirmed the truncating effect of V₂O₅ on the abnormal platy growth of SBN grains. The electrical conductivity studies were centred in the 573-823 K as the Curie temperature lies in this range. The concentration of mobile charge carriers (n), the diffusion constant (D₀) and the mean free path (a) were calculated by using Rice and Roth formalism. The conductivity parameters such as ion-hopping rate (ω_p) and the charge carrier concentration (K′) term have been calculated using Almond and West formalism. The aforementioned microscopic parameters were found to be V₂O₅ content dependent on SrBi₂(Nb_1−xV_x)₂O_9−δ ceramics.     

Urbach rule and disordering processes in Cu6P(S1−xSex)5Br1−yIy superionic conductors

Compositional behavior of Urbach absorption edge is studied as well as the effect of compositional disordering on the parameters of exciton-phonon interaction, phase transition temperatures and electric conductivity in Cu₆P(S_1−xSe_x)₅Br_1−yI_y superionic solid solutions. The effect of different types of disordering on the optical absorption processes and specific features of compositional changes in the absorption edge spectra under S→Se and Br→I anion substitution in the mixed crystals are investigated. (x, T) phase diagrams for Cu₆P(S_1−xSe_x)₅X (X=I, Br) solid solutions are studied.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

EPR investigations of oxidation effects in (V1−xMox)2−δO3

Electron paramagnetic resonance (EPR) investigations has been carried out on the new family of molybdenum doped vanadium sesquioxides (V_1−xMo_x)_2−δO₃. The oxidation effects were monitored from the rate of paramagnetic V⁴⁺ created when the sample is exposed to the air. The effects of the oxidation time, sample temperature, and annealing at 1000 °C under a diluted hydrogen atmosphere on the EPR signal features are analyzed. The V⁴⁺ concentration in the oxidized samples is determined and the relaxation effects driven by the conduction electrons are pointed out from the thermal behaviour of the EPR line features. EPR spectra of all the oxidized samples also reveal a small ferromagnetic contribution strongly correlated with the V⁴⁺ content.     

Metal-insulator transition in the spinel-type Cu(Ir1−xCrx)2S4 system

A normal thiospinel CuIr₂S₄ exhibits a temperature-induced metal-insulator (M-I) transition around 230 K with structural transformation, showing hysteresis on heating and cooling. On the other hand, CuCr₂S₄ has the same normal spinel structure without the structural transformation. CuCr₂S₄ has been found to be metallic and ferromagnetic with the Curie temperature T_c~377 K. In order to see the effect of substituting Cr for Ir on the M-I transition, we have carried out a systematic experimental study of electrical and magnetic properties of Cu(Ir_1−xCr_x)₂S₄. The M-I transition temperature shifts to lower temperature with increasing Cr-concentration x and this transition is not detected above x~0.05. The ferromagnetic transition temperature decreases as x is decreased and the transition does not occur below x~0.20.