A. V. Fetisov

Anomalous currents under cyclic polarization of magnetite electrode in acidic medium

An anomalous phenomenon was observed in acidic solutions in cyclic voltammetric curves in the studies of electrochemical behavior of magnetite. It consisted in appearance of the reverse current: a cathodic response under the anodic potential sweep. It was shown that the direct and reverse currents follow the same regularities. Analysis of experimental data allows concluding that both currents correspond to the same process: Fe3O4 + 8H+ + 2e → 3Feaq2+ + 4H2O. Appearance of anomalous currents on polarization curves of magnetite occurs if the following conditions are observed: irreversibility of the given electrochemical reaction under the given mode experimental parameters; incompleteness of reaction at the cathodic halfcycle stage; layer-by-layer dissolution when the electrode surface is continually refreshed.

State of manganese atoms in the Nd1+xSr2−xMn2O7−δ solid solution

The oxidation state of manganese in the Nd2−xSr1+xMn2O7−δ solid solution was determined by X-ray photoelectron spectroscopy and by calculating the oxygen nonstoichiometry based on the gravimetric data. As a result of the heterovalent replacement of Nd3+ with Sr2+, the change in the oxidation state of manganese occurs in different ways, i.e., it increases at x > 0 and decreases at x < 0. In the latter case, some oxygen ions acquire the oxidation state of −1. The samples slowly cooled under oxidative conditions possess a significant positive oxygen non-stoichiometry, which tends to decrease after Nd3+ is replaced with Sr2+. An excess of oxygen stabilizes the crystal structure of Nd1+xSr2−xMn2O7−δ.

Properties of mechanically activated oxide NdMnO 3 + δ

Analysis of the behavior of mechanically activated oxides opens additional possibilities for studying their phase states and properties and also for controlling their functional characteristics. The rareearth metal- manganese-oxygen systems were chosen as objects of investigation because of the special interest in them in view of the colossal magnetoresistance effect and the promises of using materials based on these systems in electrochemical and catalytic applications. The fact that these oxides exhibit the cooperative Jahn-Teller effect significantly simplifies the theoretical interpre� tation of experimental results. The Jahn-Teller distor� tion and the Jahn-Teller transition temperature are directly related to the concentration of Jahn-Teller ions. This allows one to obtain more information on the changes in the nonstoichiometry and the size effects caused by the mechanical activation of oxide. Quite intriguing is also the possibility of relating the interactions between mechanically activated oxides and a gaseous medium to the diversity of Jahn-Teller phase states. In this context, the main purpose of this work was to study specific features of structural phase transfor� mations, isotope exchange, and redox processes in metastable states of the rareearth metal-Mn-O ( NdMnO 3+ δ) system that are caused by its mecha� nochemical activation.

Relationship Between Electronic and Crystal Structure in Nd 1 + x Ba 2 − x Cu 3 O 6 + δ

In accordance with a relation between the lattice parameter c and the concentration of holes p, which has been found earlier for the YBa2Cu3O6+δ compound, we obtain the p parameter in the same manner for the solid solution Nd1.2Ba1.8Cu3O6+δ with substitution of Nd3+ for Ba2+ in barium sites. For this, structural and superconducting properties of the Nd1.2Ba1.8Cu3O6+δ samples stored for specified times τ after quenching from annealing temperatures have been studied. The investigation shows that Nd1.2Ba1.8Cu3O6+δ stored for a long time has quite a low critical temperature Tc extending only to Tconset

Influence of Processing Techniques on the Surface Microstructure of V 85 Ni 15 Membrane Alloy

T_{\mathrm {c}}^{\text {onset}}

Regime of superhigh reactivity of the YBa2Cu3O6 + δ superconductor toward the components of air

= 55 K for the most oxidized sample with δ = 0.953. We find that the δ-dependence of the lattice parameter c significantly changes with time τ. When using the relation between c and p, the existence of electron doping of 0.045 per Cu in Nd1.2Ba1.8Cu3O6+δ can be inferred. This doping explains well-reduced Tc in Nd1.2Ba1.8Cu3O6+δ. In addition, we observe a local drop of Tc by up to 20 K for the samples with p˜0.095 (δ˜0.8) similar to decreasing Tc known as “anomaly 1/8”.

Effect of Cation Ordering on the Electrochemical Properties of (MgCr 2 O 4 ) x (Fe 3 O 4 ) 1 - x Solid Solutions

Vanadium-based alloys with a BCC structure are considered to be an alternative to palladium alloys for hydrogen purification. Since the performance of membranes is influenced by not only their bulk characteristics but also their surface condition, we have investigated the effect of different surface preparation techniques (abrasive grinding and polishing, electrolytic polishing, and ion etching) on the surface microstructure of V85Ni15 membrane alloy using X-ray photoelectron spectroscopy. After electrolytic polishing and ion etching, we observed an increase in the percentages of vanadium and nickel metals in the surface layer of the alloy in comparison with its state after abrasive polishing. Such changes are expected to be favorable for an increase in the rate of hydrogen dissociation and recombination on the surface of the membrane material, eventually improving hydrogen transport efficiency.

“Latent” oxygen-transport properties of YBa2Cu3O6 + δ

Physicochemical features of the YBa2Cu3O6 + δ superconductor synthesized via ceramic route and subjected to a kind of modification by long-term exposure to an atmosphere with low pH2O have been studied by X-ray diffraction, thermal analysis, and magnetometry. The resulting material had a high degree of saturation with air components at room temperature and 30% humidity, up to 1.5 wt % in 30 days, which is not inherent in YBa2Cu3O6 + δ.

Chemical state of the surface of mechanically activated MnmOn oxides

We have studied the influence of long- and short-range cation order on the crystallographic, electrical, and electrochemical properties of (MgCr2O4)x(Fe3O4)1 − x solid solutions. A mathematical model has been proposed which ensures an adequate description of the composition dependences of these properties. Short-range cation order on the octahedral site of the spinel structure has been shown to have a significant effect on these properties at x < 0.5. The results of this study demonstrate that the electrochemical reduction process is highly sensitive to structural features of the (MgCr2O4)x(Fe3O4)1 − x solid solutions.

XPS study of the chemical stability of DyBa2Cu3O6+δ superconductor

High-temperature superconductors possessing anastonishing and still not properly understood prop-erty, high-temperature superconductivity (HTSC),remain the focus of much attention [1]. At the sametime, another their advantage—relatively high oxy-gen ion conductivity—was noted already at the earli-est stage of the discovery of high-temperature super-conductors [2, 3].It is worth noting that diffusion coefficients foundby different authors for the same high-temperaturesuperconductors differ by many orders of magnitude.For example, the O