Lithium ion conductive glass–ceramics with Li3Fe2(PO4)3 and YAG laser-induced local crystallization in lithium iron phosphate glasses

The glasses with the composition of 37.5Li₂O–(25 − x)Fe₂O₃–xNb₂O₅–37.5P₂O₅ (mol%) (x = 5,10,15) are prepared, and it is found that the addition of Nb₂O₅ is effective for the glass formation in the lithium iron phosphate system. The glass–ceramics consisting of Nasicon-type Li₃Fe₂(PO₄)₃ crystals with an orthorhombic structure are developed through conventional crystallization in an electric furnace, showing electrical conductivities of 3 × 10^− 6 Scm^− 1 at room temperature and the activation energies of 0.48 eV (x = 5) and 0.51 eV (x = 10) for Li⁺ ion conduction in the temperature range of 30–200 °C. A continuous wave Nd:YAG laser (wavelength: 1064 nm) with powers of 0.14–0.30 W and a scanning speed of 10 μm/s is irradiated onto the surface of the glasses, and the formation of Li₃Fe₂(PO₄)₃ crystals is confirmed from XRD analyses and micro-Raman scattering spectra. The crystallization of the precursor glasses is considered as new route for the fabrication of Li₃Fe₂(PO₄)₃ crystals being candidates for use as electrolyte materials in lithium ion secondary batteries.     

Structural, thermal and conductive properties of Bi4−xMxV2O11 (M = La, Gd; 0 x 0.4) compounds

Bi_4−xM_xV₂O₁₁ (M = La, Gd) was prepared by solid state reactions. The amount of La and Gd in the (Bi_4−xM_xV₂O₁₁) was varied in the range of (0 x 0.4). The addition of La and Gd to Bi₄V₂O₁₁ electrolyte was found to stabilize the β crystalline phase for x 0.3. In addition, the phase transition corresponding β- to γ-phases are evident in the ionic conductivity plots as well as in XRD, DSC profiles of x 0.3 samples. The highest ionic conductivity was observed in Bi_3.9La_0.1V₂O₁₁ and Bi_3.8Gd_0.2V₂O₁₁ samples in the range of 10⁻³–10⁻⁴ S/cm for 700–500 °C. These results were supported by impedance spectroscopy, X-ray diffraction (XRD) and differential scanning calorimetry (DSC).     

Influence of octahedral tilting and composition on electrical properties of the Li0.2 − xNaxLa0.6TiO3 (0 ≤ x ≤ 0.2) series

The Rietveld analysis of ND patterns of polycrystalline Li_0.2 − xNa_xLa_0.6TiO₃ (0 ≤ x < 0.2) samples, recorded between 300 and 1075 K, shows an orthorhombic–tetragonal transformation, in which the octahedral tilting along the b axis is eliminated at ~ 773 K, but the vacancy ordering along the c axis remains. In Li rich samples, conductivity (10^− 3 Ω^− 1 cm^− 1 at 300 K) departs from the Arrhenius behaviour, decreasing activation energies from 0.37 to 0.14 eV when octahedral tilting is eliminated. Successive Maxwell–Wagner blocking processes, detected in the real part of dielectric constant plots, have been ascribed to the Li blocking at interior domains, grain-boundary and electrode–electrolyte interfaces. The substitution of Li⁺ by Na⁺ decreases the amount of vacant A-sites, decreasing several orders of magnitude the conductivity when the amount of vacancies approaches the vacancy percolation threshold (n_p = 0.27). Below the percolation threshold, Li ions only display local mobility, remaining confined into small domains of perovskites.     

Water incorporation into the Ba2(In1 − xMx)2O5‪ (M = Sc 0 ≤ x < 0.5 and M = Y 0 ≤ x < 0.35) system and protonic conduction

Ba₂(In_1 − xM_x)₂O_{5 − y / 2}(OH)_y‪□_{1 − y / 2} (y ≤ 2; M = Sc³⁺ 0 ≤ x < 0.5 and M = Y³⁺ 0 ≤ x < 0.35) compounds were prepared by reacting Ba₂(In_1 − xM_x)₂O₅‪ phases with water vapor. This reaction is reversible. Analyses of the hydration process by TG and XRD studies show that the thermal stability of hydrated phases increases when x increases and that the incorporation of water is not a single-phase reaction inducing either a crystal system or space group modification. Fully hydrated (y = 2) and dehydrated (y = 0) samples have been stabilized at room temperature and characterized for all compositions. In wet air, all phases show a proton contribution to the total conductivity at temperatures between 350 and 600 °C. At a given temperature, proton conductivity increases with the substitution ratio and reaches at 350 °C, 5.4 10^− 3 S cm^− 1 for Ba₂(In_0.65Sc_0.35)₂O_4.2□_0.2(OH)_1.6.     

Conductance studies of NaCl, KCl, NaBr, NaI, NaBPh4, Bu4NI and NaClO4 in water + 2-methoxyethanol mixtures at 298.15 K

The electrical conductances of the solutions of sodium chloride (NaCl), potassium chloride (KCl), sodium bromide (NaBr), sodium iodide (NaI), sodium tetraphenylborate (NaBPh₄), tetrabutylammonium iodide (Bu₄NI) and sodium perchlorate (NaClO₄) in water (1) + 2-methoxyethanol (2) mixtures containing 0.01, 0.025, 0.05, 0.075, 0.10, 0.15 and 0.20 mol fractions of 2-methoxyethanol have been reported at 298.15 K. The conductance data have been analyzed by the Fuoss–Justice equation. The individual limiting ionic conductivities of Na⁺, K⁺, Bu₄N⁺, BPh₄⁻, I⁻, Cl⁻, and Br⁻ ions have been determined using the Fuoss–Hirsch assumption. The dependencies of the limiting molar conductances, Λ_o, and Walden products, Λ_oη, versus mixed solvent composition have been discussed.     

Oxygen nonstoichiometry of Sr(Co,Fe)O3−δ-based perovskites: I. Coulometric titration of SrCo0.85Fe0.10Cr0.05O3−δ by the two-electrode technique

The p(O₂)–T–δ diagram of perovskite-type SrCo_0.85Fe_0.10Cr_0.05O_3−δ was determined by the coulometric titration technique in the temperature range 770–1250 K at oxygen partial pressures from 8 10⁻¹⁰ to 0.5 atm. Stability of the cubic perovskite phase of SrCo_0.85Fe_0.10Cr_0.05O_3−δ, existing down to the oxygen pressures of 10⁻³–10⁻⁵ atm, was found to be slightly higher than that of SrCo_0.80Fe_0.20O_3−δ, probably due to stabilization of oxygen octahedra neighboring Cr⁴⁺ cations. When the oxygen nonstoichiometry of the Cr-containing perovskite decreases from 0.47 to 0.38, the partial molar enthalpy and entropy for overall oxygen incorporation reaction vary in the ranges −165 to −60 kJ mol⁻¹ and 90 to 150 J mol⁻¹ K⁻¹, respectively. Within the stability limits of the single perovskite phase, the p(O₂)–T–δ diagram can be adequately described by equilibrium processes of oxygen incorporation, cobalt disproportionation and interaction of cobalt and iron cations, with the thermodynamic functions independent of defect concentrations. Increasing grain size in SrCo_0.85Fe_0.10Cr_0.05O_3−δ ceramics from submicron size to 100–200 μm has no effect on the oxygen thermodynamics. The two-electrode coulometric titration technique, based on the alternate use of electrodes for oxygen pumping and e.m.f. measurements, is described and verified by studying oxygen nonstoichiometry of La_0.3Sr_0.7CoO_3−δ and PrO_x.     

Li mobility in Li0.5 − xNaxLa0.5TiO3 perovskites (0 ≤ x ≤ 0.5): Influence of structural and compositional parameters

The dependence of Li mobility on structure and composition of Li_0.5 − xNa_xLa_0.5TiO₃ perovskites (0 ≤ x ≤  0.5) has been investigated by means of neutron diffraction, nuclear magnetic resonance and impedance spectroscopy. At 300 K, all samples display a rhombohedral superstructure (R-3c S.G.), where octahedra are out of phase tilted along [111] direction of the ideal cubic cell. The elimination of the octahedral tilting is responsible for the rhombohedral–cubic transformation, detected near 1000 K. In these perovskites, La and Na cations are randomly distributed in A sites, but Li ions are fourfold coordinated at unit cell faces of the cubic perovskite. Lithium conductivity, σ_300 K, decreases with the sodium content, decreasing from values typical of fast ionic conductors, 10^− 3 S/cm, to those of good insulators, 10^− 10 S/cm, when the interconnectivity between vacant A sites is lost (x > 0.3). In samples with x < 0.3, dc conductivity displays a non-Arrhenius behaviour, decreasing activation energy from ~ 0.37 to 0.25 eV when the sample is heated between 77 and 500 K. The temperature dependence of B_Li factors shows the existence of two regimes for Li motion. Below 373 K, Li ions remain partially located near square oxygen windows that connect contiguous A sites, but above 400 K, extended Li motions become dominant. The additional decrease of activation energy from 0.25 to 0.16 eV (low-temperature ⁷Li NMR value), should require the full elimination of octahedral tilting which is only produced above 1000 °C.     

GaN/AlN quantum dot photodetectors at 1.3–1.5 μm

We have demonstrated GaN/AlN quantum dots (QD) photodetectors, relying on intraband absorption and in-plane carrier transport in the wetting layer. The devices operate at room temperature in the wavelength range 1.3–1.5 μm. Samples with 20 periods of Si-doped GaN QD layers, separated by 3 nm-thick AlN barriers, have been grown by plasma-assisted molecular-beam epitaxy on an AlN buffer on a c-sapphire substrate. Self-organized dots are formed by the deposition of 5 monolayers of GaN under nitrogen-rich conditions. The dot height is 1.2±0.6 to 1.3±0.6 nm and the dot density is in the range 10¹¹–10¹² cm⁻². Two ohmic contacts were deposited on the sample surface and annealed in order to contact the buried QD layers. The dots exhibit TM polarized absorption linked to the s–p_z transition. The photocurrent at 300 K is slightly blue-shifted with respect to the s–p_z intraband absorption. The responsivity increases exponentially with temperature and reaches a record value of 10 mA/W at 300 K for detectors with interdigitated contacts.     

Kerr property of cubic K0.95Li0.05Ta0.60Nb0.40O3 single crystal

The Kerr (quadratic electro-optic) property of K_0.95Li_0.05Ta_0.60Nb_0.40O₃ was measured by using an automated scanning Mach–Zehnder interferometer. K_0.95Li_0.05Ta_0.60Nb_0.40O₃ has large Kerr effect with R₁₁ = 7.2 × 10⁻¹⁶ m²/V² and R₁₂ = −1.2 × 10⁻¹⁶ m²/V² at 632.8 nm near its phase-transition temperature. The dielectric and Kerr properties as function of temperature were also investigated, thus the quadratic polarization-optic coefficients were calculated. The values of g₁₁ and g₁₂ are 0.083 m⁴/C² and −0.014 m⁴/C², respectively, and do not depend on temperature within the measurement accuracy of 5%.     

Structural properties of 10 μm thick InN grown on sapphire (0001)

The structural properties of a 10 μm thick In-face InN film, grown on Al₂O₃ (0001) by radio-frequency plasma-assisted molecular beam epitaxy, were investigated by transmission electron microscopy and high resolution x-ray diffraction. Electron microscopy revealed the presence of threading dislocations of edge, screw and mixed type, and the absence of planar defects. The dislocation density near the InN/sapphire interface was 1.55×10¹⁰ cm⁻², 4.82×10⁸ cm⁻² and 1.69×10⁹ cm⁻² for the edge, screw and mixed dislocation types, respectively. Towards the free surface of InN, the density of edge and mixed type dislocations decreased to 4.35×10⁹ cm⁻² and 1.20×10⁹ cm⁻², respectively, while the density of screw dislocations remained constant. Using x-ray diffraction, dislocations with screw component were found to be 1.2×10⁹ cm⁻², in good agreement with the electron microscopy results. Comparing electron microscopy results with x-ray diffraction ones, it is suggested that pure edge dislocations are neither completely randomly distributed nor completely piled up in grain boundaries within the InN film.     

Oxide ion conductivity and relaxation in CaREZrNbO7 (RE = La, Nd, Sm, Gd, and Y) system

CaREZrNbO₇ (RE = La, Nd, Sm, Gd and Y) system changed from fluorite (F)-type to pyrochlore (P)-type structure when the ionic radius ratios, r(Ca²⁺–RE³⁺)_av/r(Zr⁴⁺–Nb⁵⁺)_av were larger than 1.34. Thus, the La, Nd, and Sm compounds have a cubic P-type structure and the Gd and Y ones have a defect F-type structure. The electrical conductivity was measured using complex-plane impedance analysis over a wide temperature (300–750 °C) and frequency (1 Hz–1 MHz) ranges. The conductivity relaxation phenomenon was observed in these compounds and the relaxation frequencies were found to show Arrhenius-type behavior and activation energies were in good agreement with those obtained from high temperature conductivity plots. These results support the idea that the relaxation process and the conductivity have the same origin. The ionic conductivity of CaREZrNbO₇ (RE = La, Nd, Sm, Gd and Y) system showed the maximum at the phase boundary between the F-type and P-type phases. On the other hand, the activation energy for the conduction decreased in the F-type phase and increased in the P-type phase with increasing ionic radius ratio. Among the prepared compounds, CaGdZrNbO₇ showed the highest ionic conductivity of 9.47 × 10^− 3 S/cm at 750 °C which was about twice as high as that observed in Gd₂Zr₂O₇ (4.2 × 10^− 3 S/cm at 800 °C). The grain morphology observation by scanning electron microscope (SEM) showed well-sintered grains. AC impedance measurements in various atmospheres further indicated that they are predominantly oxide ion conductors at elevated temperatures (> 700 °C).     

Superprotonic solid solutions between CsHSO4 and CsH2PO4

Solid solutions of (CsHSO₄)_1 − x(CsH₂PO₄)_x (x = 0.25-0.75) were synthesized by mechanical milling method over a wide range of compositions. Superprotonic cubic phase was confirmed for all these samples between 293 and 420 K depending on its composition. These superprotonic phases have primitive cubic structure similar to that of CsH₂PO₄. The kinetic stability of the supercooled cubic phase depends both on the composition x and the humidity of surrounding atmosphere. The most stable composition of the cubic phase was found around x = 0.67 and could be maintained for several days even under ambient atmosphere. The ionic conductivities of these superprotonic phases reached 10^− 2–10^− 3 S∙cm^− 1 at 450 K. With increasing x the ionic conductivity at the superprotonic phase decreased continuously associated with the increase of the activation energy. These findings suggest that the average number of the hydrogen bonds between XO₄ (X = S, P) units plays an important role on the stability of the cubic phase and also on the conductivity.     

Ab initio study of the chemical states of water on Cr2O3(0 0 0 1): From the isolated molecule to saturation coverage

The reactivity of the (0 0 0 1)-Cr–Cr₂O₃ surface towards water was studied by means of periodic DFT + U. Several water coverages were studied, from 1.2H₂O/nm² to 14.1H₂O/nm², corresponding to ¼, 1, 2 and 3 water/Cr at the (0 0 0 1)-Cr₂O₃ surface, respectively. With increasing coverage, water gradually completes the coordination sphere of the surface Cr atoms from 3 (dry surface) to 4 (1.2 and 4.7H₂O/nm²), 5 (9.4H₂O/nm²) and 6 (14.1H₂O/nm²). For all studied coverages, water replaces an O atom from the missing above plane. At coverages 1.2 and 4.7H₂O/nm², the Cr–O_s (surface oxygen) acid–base character and bond directionality govern the water adsorption. The adsorption is molecular at the lowest coverage. At 4.7H₂O/nm², molecular and dissociative states are isoenergetic. The activation energy barrier between the two states being as low as 12 kJ/mol, allowing protons exchanges between the OH groups, as evidenced by ab inito molecular dynamics at room temperature. At coverages of 9.4 and 14.1H₂O/nm², 1D- (respectively, 2D-) water networks are formed. The resulting surface terminations are –Cr(OH)₂ and –Cr(OH)₃– like, respectively. The increased stability of those terminations as compared to the previous ones are due to the stabilization of the adsorbed phase through a H-bond network and to the increase in the Cr coordination number, stabilizing the Cr (t_2g) orbitals in the valence band. An atomistic thermodynamic approach allows us to specify the temperature and water pressure domains of prevalence for each surface termination. It is found that the –Cr(OH)₃-like, –Cr(OH)₂ and anhydrous surfaces may be stabilized depending on (T, P) conditions. Calculated energies of adsorption and OH frequencies are in good agreement with published experimental data and support the full hydroxylation model, where the Cr achieves a 6-fold coordination, at saturation.     

Effect of Y2O3 addition on the conductivity and elastic modulus of (CeO2)1 − x(YO1.5)x

The conductivity and elastic modulus of (CeO₂)_1 − x(YO_1.5)_x for x values of 0.10, 0.15, 0.20, 0.30, and 0.40 were investigated by experiments and molecular dynamics simulations. The calculated conductivity exhibited a maximum value at approximately 15 mol% Y₂O₃; this trend agreed with that of the experimental results. In order to clarify the reason for the occurrence of the maximum conductivity, the paths for the transfer of oxygen vacancies were counted. The numerical result revealed that as the content of Y₂O₃ dopant increases, the number of paths for the transfer of oxygen vacancies decreases, whereas the number of oxygen vacancies for conductivity increases. Thus, the trade-off between the increase in the number of vacancy sites and the decrease in the vacancy transfer was considered to be the reason for the maximum conductivity occurring at the Y₂O₃ dopant content of approximately 15 mol%. The calculated elastic modulus also exhibited a minimum value at approximately 20 mol% Y₂O₃, which also agreed with the experimental results. It was shown that the Y–O–Y bonding energy increased with the increasing content of Y₂O₃ dopant. Thus, the trade-off between the increase in the number of vacancy sites and that in the Y–O–Y bonding energy was considered to be the reason for the minimum elastic modulus occurring at the Y₂O₃ dopant content of approximately 20 mol%.     

Study of the interaction of Na9[SbW9O33]·19.5H2O with bovine serum albumin: Spectroscopic and voltammetric methods

The interaction of Na₉[SbW₉O₃₃]·19.5H₂O (SbW) with bovine serum albumin (BSA) is studied by spectroscopic and voltammetric methods. Absorption spectroscopy of BSA and the linear sweep voltammetry of SbW proved the formation of ground-state SbW–BSA complex. Fluorescence quenching of serum albumin by SbW is also found to be a static quenching process. The binding constant K_a is 4.13×10⁴ L mol⁻¹ for SbW–BSA at pH 7.40 Tris–HCl buffer at 295 K. The number of binding sites and the apparent binding constants at different temperatures are obtained from the analysis of the fluorescence quenching data. The thermodynamic parameters determined by the Van’t Hoff analysis of the binding constants (ΔH=−80.01 kJ mol⁻¹ and ΔS=−182.85 J mol⁻¹ K⁻¹) clearly show that the binding is absolutely entropy driven. Hydrogen bonding and van der Waals interaction force play major role in stabilizing the complex. The effect of SbW on the conformation of BSA is analyzed using synchronous fluorescence spectroscopy.     

Li4Ti2.5Cr2.5O12 as anode material for lithium battery

Chromium-substituted Li₄Ti₅O₁₂ has been investigated as a negative electrode for future lithium batteries. It has been synthesized by a solid-state method followed by quenching leading to a micron-sized material. The minimum formation temperature of Li₄Ti_2.5Cr_2.5O₁₂ was found to be around 600 °C using thermogravimetric and differential thermal analysis. X-ray diffraction, scanning electron microscopy, cyclic voltammetry (CV), impedance spectroscopy, and charge–discharge cycling were used to evaluate the synthesized Li₄Ti_2.5Cr_2.5O₁₂. The particle size of the powder was around 2–4 μm. CV studies reveal a shift in the deintercalation potential by about 40 mV, i.e., from 1.54 V for Li₄Ti₅O₁₂ to 1.5 V for Li₄Ti_2.5Cr_2.5O₁₂. High-rate cyclability was exhibited by Li₄Ti_2.5Cr_2.5O₁₂ (up to 5  C) compared to the parent compound. The conduction mechanism of the compound was examined in terms of the dielectric constant and dissipation factor. The relaxation time has been evaluated and was found to be 0.07 ms. The mobility was found to be 5.133 × 10⁻⁶ cm² V⁻¹ s⁻¹.     

Advanced electrochemical performance of LiMn1.4Cr0.2Ni0.4O4 as 5 V cathode material by citric-acid-assisted method

Spinel LiMn₂O₄ and LiMn_1.4Cr_0.2Ni_0.4O₄ cathode materials were successfully synthesized by the citric-acid-assisted sol-gel method with ultrasonic irradiation stirring. The structure and electrochemical performance of the as-prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectrometer, cyclic voltamogram (CV) and the galvanostatic charge-discharge test in detail. XRD shows that all the samples have high phase purity, and the powders are well crystallized. SEM exhibits that LiMn_1.4Cr_0.2Ni_0.4O₄ has more uniform cubic-structure morphology than that of LiMn₂O₄. EDX reveals that a small amount of Mn³⁺ still exists in LiMn_1.4Cr_0.2Ni_0.4O₄. The galvanostatic charge-discharge test indicates that the initial discharge capacities for the LiMn_1.4Cr_0.2Ni_0.4O₄ and LiMn₂O₄ at 0.15 C discharge rates are 130.8 and 130.2 mAh g⁻¹, respectively. After 50 cycles, their capacity are 94.1% and 85.1%, respectively. The CV curve implies that Ni and Cr dual substitutions are beneficial to the reversible intercalation and deintercalation of Li⁺, and suppress Mn³⁺ generation at high temperatures and provide improved structural stability.     

Optical investigation of ferromagnetic Ga1−xMnxN layers grown on sapphire (0 0 0 1) substrates

We have investigated the temperature-dependent photoluminescence (PL) spectra in Ga_1−xMn_xN layers (where x≈0.1–0.8%) grown on sapphire (0 0 0 1) substrates using the plasma-enhanced molecular beam epitaxy technique. All the layers doped with manganese exhibited n-type conductivity with Curie temperature over 350 K. The efficient PL are peaked in the red (1.86 eV), yellow (2.34 eV), and blue (3.29 eV) spectral range. It was found that the blue band at 3.29 eV is mostly associated with the formation complexes between donors (e.g., N vacancy) and Mn acceptors, which results in forming donor levels at 0.23 eV below the conduction band edge. The yellow band is attributed to intrinsic gallium defects. The broad band at 1.86 eV is attributed to inner ⁵D state transition (T₂ to E) of Mn ions.     

An XPS study on the surface reduction of V2O5(0 0 1) induced by Ar ion bombardment

The effect of the irradiation with Al Kα X-rays during an XPS measurement upon the surface vanadium oxidation state of a fresh in vacuum cleaved V₂O₅(0 0 1) crystal was examined. Afterwards, the surface reduction of the V₂O₅(0 0 1) surface under Ar⁺ bombardment was studied. The degree of reduction of the vanadium oxide was determined by means of a combined analysis of the O1s and V2p photoelectron lines. Asymmetric line shapes were needed to fit the V³⁺2p photolines, due to the metallic character of V₂O₃ at ambient temperature. Under Ar⁺ bombardment, the V₂O₅(0 0 1) crystal surface reduces rather fast towards the V₂O₃ stoichiometry, after which a much slower reduction of the vanadium oxide occurs.     

Structural properties and Raman spectroscopy of orthorhombic (Eu1−xPrx)0.6Sr0.4MnO3 (0≤x≤1.0)

The effect of Pr doping on structural properties and room temperature Raman spectroscopy measurements is investigated in manganites (Eu_1−xPr_x)_0.6Sr_0.4MnO₃ (0≤x≤1.0) with fixed carrier concentration. The result of the Rietveld refinement of x-ray powder diffraction shows that these compounds crystallize in an orthorhombic distorted structure with a space group Pnma. It is evident that, with increasing Pr substitution, three types of orthorhombic structures can be distinguished. The phonon frequencies of the three main peaks, in room temperature Raman-scattering measurements, have been discussed together with their structural characteristics, such as bond-length, bond-angles, and the change of orthorhombic structure type. With the increase of Pr content, the mode at 491  cm⁻¹ also shows a corresponding change. A step effect becomes evident, which seems to indicate the close dependence between the frequency shift of this mode and the change in crystal symmetry. This further supports the notion that the mode at 491  cm⁻¹ is closely correlated with the Jahn–Teller distortion. Moreover, we have found that the lowest frequency peak (assigned as an A_1g phonon mode) depends linearly on the tolerance factor t.