Ions Of Different Valences Research Articles

Magnetism of LiMn2O4 manganite in structurally ordered and disordered states

The specific features of the crystal structure and the magnetic state of stoichiometric lithium manganite in the structurally ordered Li[Mn2]O4 and disordered Li1 − δMnδ[Mn2 − δLiδ]O4 (δ = 1/6) states have been investigated using neutron diffraction, X-ray diffraction, and magnetic methods. The structurally disordered state of the manganite was achieved under irradiation by fast neutrons (E eff ≥ 1 MeV) with a fluence of 2 × 1020 cm−2 at a temperature of 340 K. It has been demonstrated that, in the initial sample, the charge ordering of manganese ions of different valences arises at room temperature, which is accompanied by orthorhombic distortions of the cubic spinel structure, and the long-range antiferromagnetic order with the wave vector k = 2π/c(0, 0, 0.44) is observed at low temperatures. It has been established that the structural disordering leads to radical changes in the structural and magnetic states of the LiMn2O4 manganite. The charge ordering is destroyed, and the structure retains the cubic symmetry even at a temperature of 5 K. The antiferromagnetic type of ordering transforms into ferrimagnetic ordering with local spin deviations in the octahedral sublattice due to the appearance of intersublattice exchange interactions.

The Thermodynamics of Some Transition Metals in Molten Slags

AbstractIn the present paper, the thermodynamic behaviour of transition metals, such as Cr, Ti, Nb and V in molten slags is systematically analysed based on a literature survey. These metals exist in molten slags with multi valences. Oxygen partial pressure, slag basicity, total content of each transition metal, content of each component in slag, and temperature are the influential factors on their thermodynamic properties in molten slags. Higher basicity and strong oxidative atmosphere are generally favourable for the stable existence of transition‐metal ions with higher oxidation states. Temperature is a factor that is less influential than the above‐mentioned ones. For a transition metal in molten slag, the concentration ratio of ions of different valences depends on the activity coefficient ratio of their oxides. The present paper summarizes the activity studies regarding the transition metal oxides in various molten slags. For chromium and titanium oxides, information on CaOSiO2 based systems is involved. For titanium oxides, its thermodynamic behaviour in MnOSiO2 based slags is introduced. For niobium and vanadium, the information in Na2OSiO2, CaOCaF2SiO2 systems is provided. Thermodynamic studies are described for Nb2O5MnOSiO2 molten slag equilibrated with liquid iron at 1828 K.

Effects of binding of counterions on adsorption and coalescence

In many industrial applications, ionic surfactants are used in aqueous media in the presence of a mixture of salts. The surface activity of a surfactant in such solutions depends strongly on the ions of different valence. In the present study, surface tensions of aqueous solutions of cetyltrimethylammonium bromide (CTAB) were measured in the presence of sodium bromide and potassium sulfate. The interfacial tension between water and toluene was also measured in the presence of CTAB and these salts. The effects of monovalent (bromide) and divalent (sulfate) counterions on the adsorption of CTAB at air–water and water–toluene interfaces were studied. Coalescence of air bubbles and toluene drops at flat interfaces was studied in these surfactant–salt systems. A stochastic model was used to fit the coalescence time distributions. The significance of the model parameters was discussed. The difference between air–water and water–hydrocarbon interfaces was analyzed in terms of the effects of counterion-binding on adsorption and coalescence. The mean values of the coalescence time distributions were compared with the predictions from seven film-drainage models.

Doping of LiNbO3 and (Na,Li)NbO3-based solid solutions

Doping of LiNbO3 and (Na,Li)NbO3 with ions of different valence above stoichiometry has been performed. The effect of modifiers on the grain structure and electrical properties of these compounds has been investigated. It is shown that the conductivity of ceramics can be controlled.

The structure of concentrated aqueous solutions of chromium nitrate and cerium chloride studied by X-ray diffraction and Raman spectroscopy

Concentrated aqueous solutions of chromium nitrate and cerium chloride were investigated by X-ray diffraction and Raman spectroscopy. The X-ray diffraction patterns of two sets of solutions (1.60 up to 2.48 mol dm − 3 for chromium nitrate and 0.98 up to 2.81 mol dm − 3 for cerium chloride) display an intense maximum, prepeak, in the angular region about Q 0 ∼ 0.9 Å − 1 ( Q = 4 π sin θ / λ, θ being the diffracting angle). The value of Q 0 corresponding to these maxima is proportional to the power 1/3 of the molar concentration. The proportionality coefficient has the value that should be observed for a close packing (fcc network) if assumed for cations. This study demonstrates, once more, the existence of an intermediate range order (up to ∼ 10 Å) in very concentrated aqueous solutions of salts constituted by small ions of different valences, range order previously reported by these authors in many articles. Raman experiments are in consonance with X-ray diffraction results.

Ion exchange phase transitions in water-filled channels with charged walls

Ion transport through narrow water-filled channels is impeded by a high electrostatic barrier. The latter originates from the large ratio of the dielectric constants of the water and the surrounding media. We show that "doping," i.e., immobile charges attached to the walls of the channel, substantially reduces the barrier. This explains why most of the biological ion channels are "doped." We show that at rather generic conditions the channels may undergo ion exchange phase transitions (typically of the first order). Upon such a transition a finite latent concentration of ions may either enter or leave the channel, or be exchanged between the ions of different valences. We discuss possible implications of these transitions for the Ca-vs-Na selectivity of biological Ca channels. We also show that transport of divalent Ca ions is assisted by their fractionalization into two separate excitations.

Condensation of DNA-actin polyelectrolyte mixtures driven by ions of different valences

Multivalent ions can induce condensation of like-charged polyelectrolytes into compact states, a process that requires different ion valences for different polyelectrolyte species. In this work we examine the condensation behavior in binary anionic polyelectrolyte mixtures consisting of DNA coils and F-actin rods in the presence of monovalent, divalent, and trivalent ions. As expected, monovalent ions do not condense either component and divalent ions selectively condense F-actin rods out of the polyelectrolyte mixture. For trivalent ions, however, we observe a microphase separation between the two polyelectrolytes into coexisting finite-sized F-actin bundles and DNA toroids. Further, by increasing the DNA volume fraction in the mixture, condensed F-actin bundles can be completely destabilized, leading to only DNA condensation within the mixture. We examine a number of possible causes and propose a model based on polyelectrolyte competition for ions.

Spectroscopic and electrical properties of Mg–Ti ferrite doped with different rare-earth elements

The dielectric constant ( ε′), and the electric conductivity ( σ) for Mg 1+ x Ti x R y Fe 2−2 x − y O 4, 0.025⩽ y⩽0.15 doped with different rare-earth ions, R=Er, Ce and Nd, were measured in the temperature range 300–750 K. The measurements were carried out as a function of frequency 50–1000 kHz X-ray diffractograms and IR spectra revealed that all the investigated samples posses the spinel structure. More than one conduction mechanism is used to interpret the electrical measurements. The most predominant one is the hopping mechanism that occurs between the ions of different valences existing on the same and different sites. All the ionic radii of the rare-earth used are too large to occupy the octahedral site. They form secondary phases on the grain boundaries. The electrical properties show that the pure sample has a larger dielectric constant as well as a larger valence exchange with respect to any doped one. This means that introducing rare-earth ions into the samples decreases ε′ and increases the resistivity, owing to the decreasing Fe–Fe interaction. This feature can help to obtain well applicable ferrites.

Conformational changes of DNA molecules in interactions with bioactive compounds. I. Influence of metal ions on the conformation of DNA molecules in solution

The influence of metal ions of different valence on the conformation of DNA molecule in solution has been studied. The influence of concentration and charge of counterions on the volume, persistent length and secondary structure of the macromolecules was analyzed. An assessment of the permeability of DNA coil for the solvent at different values of ionic strength of the solution was made. The state of the DNA molecule in solutions with high ionic strengths, when the presence of certain ions causes sharp changes in optical anisotropy of the macromolecule, is considered in detail.

Ferroelectric-Antiferroelectric Phase Transition in Li0.12Na0.88Ta0.4Nb0.6O3 Ceramics

Lattice defects and considerable diversity of the degree of ion ordering at substitution of ions of different valences in complex (A′A″)(B′B″)O 3 oxide structures generate wide variations in material properties. The complex perovskite structures of double oxides are currently known as relaxors and ion superconductors. Formation of BO 6 octahedrons and AO 12 polyhedrons is varied by the ratio of components in ferroelectric (Li, Na)(Ta, Nb)O 3 perovskite structures. The state of ionic superconductivity is attained at the Na:Li = 7:1 ratio. The regions of morphotropic transitions between the rhombic and rhombohedric phases are changed with the change of the Ta and Nb proportion. Raman studies of Li 0.12 Na 0.88 Ta 0.4 Nb 0.6 O 3 have shown the ferroelectric–antiferroelectric phase transition being of the ordering type and related to increasing anharmonicity of the symmetric vibration.

QUANTITATIVE ANALYSIS FOR Cr(III) AND Cr(VI) IONS IN ENVIRONMENTAL WATER SAMPLES BY USING PIXE

An enhanced sample preparation method for PIXE analysis is described allowing to separate and concentrate chromium ions of different valence in water samples. Cr 3+ ions are selectively adsorbed by ferric hydroxide colloids generated in the solution, and a PIXE target for analyzing the total concentration of chromium of hexavalent and trivalent states is prepared by depositing 0.15 ml of sample solution on a user-made thin polycarbonate film. PIXE analyses for the two kinds of targets reveal the fractions of chromium of different oxidation state. The standard method for collecting the colloids adsorbing Cr 3+ ions on Nuclepore filter of 0.2 μm pores is based on an investigation of the pH-dependence of the recovery of dissolved Cr 3+ ions and the calibration curve is measured. The prepared targets were examined for 5 to 10 minutes by 3 MeV proton beams (0.7-5 nA beam currents). The lower detection limit of chromium in a 25 ml aquatic sample is 1 ppb for the Cr 3+-precipitated targets and 3.4 ppb for the CrO 42--deposit targets based on the 3σ error of background counts integrated over the FWHM of chromium peak in the PIXE spectrum. The applicability of PIXE using these sample-preparation techniques for determining chromium oxidation states was confirmed using river water samples to which Cr 3+ and CrO 42- ions were added in 50 ppb concentrations.

Free energy of an ion system in porous media

Free energy for a system of point ions in porous ion matrix is calculated. The calculation is made in the random phase approximation. The ‘quenebedness' of porous ion medium is shown to provide for solely quantitative changes of the thermodynamic characteristics. In particular, due to the ‘quenching’, the free energy of the system decreases in comparison with that of the two-sort system with ions of different valence.

Screening effect of Ti 4+ ions on the electrical conductivity and thermoelectric power of Mg ferrite

The samples Mg 1+ x Ti x Fe 2−2xO 4 were prepared in a single phase spinel structure as indicated from X-ray analysis. The preference of Mg 2+ ions to the octahedral site decreases the ratio of the normal spinel in the investigated ferrite where the Mg 2+ increases on the expense of the Fe 3+ ions on the same site. The increase in the conductivity was found to be due to thermally activated mobility of charge carriers. The mobility data enhances the use of Verway model of conductivity which depends on the electron exchange between iron ions of different valences located on the same crystallographic sites. The existence of Ti 4+ ions on the octahedral site screens the polarization and decreases the conductivity of the samples. Peculiar behavior was obtained for Ti content of 0.7 and 0.8 due to the presence of secondary phases.

Growth and characterization of high purity epitaxial yttrium iron garnet films grown from BaO–B2O3–BaF2 flux

The growth of yttrium iron garnet (YIG) epitaxial films of excellent purity from a lead-free BaO-B 2 O 3 -BaF 2 flux and their characterization by the magnetic resonance methods are reported. The films were prepared by the standard dipping liquid-phase epitaxy onto gadolinium gallium garnet substrates of three crystallographic orientations (111), (110), and (100). The objective of utilization of a lead-free flux was to reduce the content of impurities and intrinsic defects in the garnet films and to obtain samples with minimum microwave losses. The samples exhibited very narrow NMR and spin wave linewidths. The diamagnetic and paramagnetic doping of YIG with ions of different valency and segregation of these ions between the garnet layer and the barium melt were also studied and the relevant distribution coefficients for doping into the tetrahedral (Ga, Al, Si Ge), octahedral (In) and dodecahedral (La, Ca) sites were determined.

Aging effect on the magnetic and transport properties of laser-deposited La0.5Sr0.5CoO3−δ films

Magnetic and transport studies are carried out on laser-deposited films of La0.5Sr0.5CoO3−δ and La0.65Sr0.35CoO3. It is shown that prolonged aging (for up to one year) of La0.5Sr0.5CoO3−δ films leads to a decrease of the oxygen concentration to a deficit δ≈0.08. The oxygen deficit that forms is accompanied not only by a redistribution of the charge between the cobalt ions of different valences but also by the formation of topological disorder in the Co–O–Co conducting channel as a result of the accumulation of oxygen vacancies. The observed growth of the resistivity with decreasing temperature is more accurately described in a model of weak localization of the carriers than by a thermally activated conduction mechanism. In addition to the usual ferromagnetic transition at TC≈240 K, in the “aged” La0.5Sr0.5CoO2.92 film an additional transition, typical of a magnetic transition in a spin glass, is observed at TM≈50 K. Analysis of the temperature behavior of the resistivity of La0.5Sr0.5CoO3 and La0.65Sr0.35CoO3 films found in the metallic state show that in addition to the quadratic term proportional to T2, the temperature dependence of the resistivity contains an exponential term of the form ∝exp(−T0/T), which is due to the opening of a spin gap in the conducting channel at low temperatures.

Phase separation in inhomogeneous mixed-valence systems: Application to manganites

Manganites are inhomogeneous mixed-valence systems in which the Mn ions coexist in two magnetic configurations, Mn4+ and Mn3+. We consider a simple alloy-analogy model with a charge interaction between nearest neighbor Mn ions. In addition there is an antiferromagnetic (AF) (superexchange) interaction between nearest neighbor ions of equal valence, while ions of different valence are coupled ferromagnetically (FM) (double exchange). Assuming classical spins, we show that the system is unstable to phase separation, i.e., to the coexistence of AF and charge ordered FM domains. In a magnetic field the FM domains increase at the expense of the AF ones.

Phase separation in inhomogeneous mixed-valence systems

A simple alloy-analogy model for inhomogeneous mixed-valence systems is considered in the context of manganites. It is assumed that the Mn ions coexist in two magnetic configurations, ${\mathrm{Mn}}^{4+}$ and ${\mathrm{Mn}}^{3+},$ and that they interact via a nearest-neighbor charge interaction. In addition, two nearest-neighbor ions with equal valence interact antiferromagnetically (superexchange), while ions of different valence are coupled ferromagnetically (double exchange). Assuming classical spins, we show that the system is unstable to phase separation, i.e., to the coexistence of antiferromagnetic and charge ordered ferromagnetic domains.

Donnan Dialysis with Ion-Exchange Membranes. III. Diffusion Coefficients Using Ions of Different Valence

Donnan dialysis with ion-exchange membranes was studied under various kinds of experimental conditions using ions of different valences. The diffusion coefficients (D d) of various kinds of ions in the ion-exchange membrane were obtained by curve fitting an equation derived from the mass balance to three kinds of Donnan dialytic experiments. It was found that the value of D d/D s using D d of monovalent ions in Donnan dialysis with a set of monovalent feed ions and bivalent driving ions was 1/175, where D s represents a diffusion coefficient in solution. D s was calculated from the Nernst-Einstein equation substituted by the ionic conductance of ions at infinite dilution in water. Using D d of bivalent ions in Donnan dialysis with the same set led to a D d/D s value of 1/438. Moreover, using D d in Donnan dialysis with the same set, the value of D d/D e was kept constant at 0.4 (D e expresses the diffusion coefficient in the membrane when the valences of the feed and driving ions are equal). On the other hand, both D d/D s and D d/D e using D d in Donnan dialysis with a set of bivalent feed ions and monovalent driving ions were not constant.

The Temperature Dependence of the Spin Susceptibility of High Temperature Superconductors

The opening of the so called pseudogap or spin-gap in underdoped high temperature superconductors (HTSC) is of crucial importance for the understanding of the phase transition and hence of the mechanism of superconductivity. There is conflicting evidence in literature as to whether the HTSC behave more or less like BCS-type superconductors but, unlike the conventional superconductors, not in the weak coupling but in the strong coupling limit. Or whether the mechanism and maybe even the superconducting pseudo-particles are different from those in the BCS theory. Up to now there is not even agreement in the literature if the pseudogap is directly related to the superconductivity or not. A direct probe of the opening of the gap as seen for instance by NMR is therefore very important. The temperature dependencies of the static (via Knight-shift K[sup s]) and dynamic (via spin-lattice relaxation time T[sub 1]) spin susceptibility of the conduction electrons give independent probes of this situation. In most cases data are obtainable for various NMR-active nuclei like [sup 1]70, [sup 6]3Cu, [sup 8]9Y etc. and thus superconductivity can be studied on a truly local basis. In this paper the authors summarize the results on Knight shift studies in Y-123, Y-124,more » La-2212, Bi-2201, Hg-2201, Hg-1223 HTSC with various doping levels. The doping has been achieved either by the variation of the oxygen content of the samples or by doping Zn, Ni, Pb and other ions of different valency into the structure in order to change their hole concentration p.« less

Metal-insulator transition induced by a magnetic field in the compounds Eu0.7 A 0.3MnO3 (A=Ca, Sr)

The temperature R(T) and field R(H) dependences of the electrical resistance in the compound Eu0.7A0.3MnO3 (A=Ca, Sr) are investigated in the temperature range 4.2–200 K in magnetic fields up to 14 T. Above the antiferromagnetic transition temperature Ta, the function R(T) is semiconducting in character. Application of a magnetic field H that exceeds a certain critical value Hc changes the character of the function R(T) for Eu0.7Sr0.3MnO3 to metallic (dR/dT>0). For T<Ta and H<Hc a jump in the resistance is observed indicating instability of the electronic state caused by competition between charge and spin ordering of the Mn ions of different valences.