E. I. Nikulin

Conductivity, magnetoresistance, and specific heat of oxygen-deficient La0.67Sr0.33MnO3−α (0≤α≤0.16)

Oxygen-deficient La0.67Sr0.33MnO3−α solid solutions have been studied. A comparison is made with the results obtained in an earlier study of a similar lanthanum-calcium manganite series. The physical characteristics of both series are accounted for as being due to a change in the Mn3+/Mn4+ ratio caused by oxygen removal. The differences between the strontium and calcium series originate from differences in both the bulk properties of the original oxygen-stoichiometric materials and their texture. In the strontium series, the texture manifests itself in intergrain magnetoresistance, which exceeds in magnitude the colossal magnetoresistance caused by bulk properties of the material. Study of the oxygen-deficient La0.67Sr0.33MnO3−α compound revealed specific features in the dependence of the electrophysical parameters on temperature and the Mn4+ fractional content that were not observed in the La0.67Ca0.33MnO3−α compound studied by us earlier and in La1−xSrxMnO3 samples described in the literature. The physics underlying these differences is discussed. A modified phase diagram relating the phase transition temperature to the Mn4+ fraction is proposed.

A new ionic heterostructure of the palladium-solid hydroxide proton conductor type and its electrical characteristics in the region of the phase transition in electrolyte

A protonic PdHx|KOH · 0.5H2O heterojunction is obtained upon electrochemical activation of a new heterostructure based on metallic palladium and an equimolar mixture of solid hydroxides KOH and KOH · H2O. The contributions of the obtained heterojunction (∼0.8 V) to the emf of the fuel cell, as well as to the cell conductivity, exhibit significant variations at temperatures below 358 K. These variations are correlated with a phase transition in the system and with sharp changes in the diffusion and the chemical activity of protons in the individual KOH · 0.5H2O eutectic. The observed correlation is explained by the determining role of the order-disorder transition in orientation of the O-H bonds both in the volume of electrolyte and in the protonic heterojunction.

Well-known perovskites (cerates, cuprates, manganates): Problems and facts; A review of nontraditional approaches

Phenomena and processes either new or not studied in-depth occurring in perovskite oxides and in systems which contain these compounds are reported and summarized. The materials under consideration are well-known for specific properties, e.g., cerates of alkaline earths are high-temperature proton conductors, manganates are electrode materials of SOFCs and show the effect of colossal magnetoresistance or YBa2Cu3O7 cuprates YBa2Cu3O7 are “high-temperature” superconductors. The presented nontraditional approach consists of considering physical and chemical phenomena and processes, which do not relate to the indicated main properties of the compounds. For instance, BaCeO3 undoped and slightly doped has partially cerium in the form of Ce+3, which provides the effect of blue luminescence which is controlled by thermochemical treatment; H2YBa2Cu3O7 manifests mixed proton-electron conduction at 400–550 K under water vapor of 100–300 Pa; La0.67Ca0,33MnO2.84 can dissolve some part of water vapor as this occurs in traditional HTPC. The electrochemical cells Pd | HTPC | hydrocuprate | HTPC | Pd and the more simple one Pd | HTPC | Pd show e.m.f. values near 1 V under certain conditions after polarization by d.c. currents at 420–520 K under different atmospheres. The system water vapor — membrane of undoped BaCeO3-hydride is described. Defect chemistry equations are used to describe some of the phenomena which are employed for the production and storage of pure hydrogen.

High-temperature superconductors YBaCu2O5 and Tl1.5BaCa2Cu2.5O8 with a decreased content of heavy metals (Ba and Tl)

New high-temperature superconductors YBaCu2O5 and Tl1.5BaCa2Cu2.5O8 have been synthesized. They can be considered as a result of the removal of barium cuprate BaCuO2 from YBa2Cu3O7 and barium cuprate-tallate BaCu0.5Tl0.5O2 from Tl2Ba2Ca2Cu{ia3}O10. The synthesized compounds contain a smaller amount of toxic Ba and Tl. Investigation of electrical conductivity has demonstrated that the values of Tc of synthesized materials are close to Tc of starting materials. The Meissner effect observed for these materials has confirmed their belonging to high-temperature superconductors.

Effect of excess oxygen on the electrical and magnetic properties of La1 − xAgxMnO3 (x = 0.05, 0.1, 0.2) at 77 K < T < 300 K

A study has been made of the magnetic and electrical properties of oxygen-enriched La1 − xAgxMnO3.1 manganites at 77 K < T < 300 K. The samples have been produced by the sol-gel technique. The study has revealed an increase of the Curie temperature and a shift of the maximum in magnetoresistance toward high temperatures.

Electrical and magnetic properties of manganites La1 − xAgxMnO3 (x = 0.05, 0.1, and 0.2) at 77 K < T < 300 K

The electrical conductivity, magnetization, and magnetoresistance of manganites La1 − xAgxMnO3 have been investigated in the temperature range 78–300 K. The samples have been synthesized by the sol-gel method. At room temperature, the magnetic field of 0.6 T has no effect on the electrical conductivity. As the temperature decreases, an abrupt jump is observed in the magnetization curve due to the semiconductor-metal phase transition. This transition hardly affects the temperature dependence of the resistance.

Effect of oxygen deficiency on the electrical and magnetic properties of the manganites La1 − xCaxMnO3 − α (x = 0.5, 0.6, and 0.7)

The electrical conductivity of La1 − xCaxMnO3 − α (x = 0.5) alloys in a magnetic field of 0.6 T is investigated in the temperature range 78–300 K. According to the phase diagram, this compound at α = 0 corresponds to an intermediate region between the ferromagnetic conducting (x < 0.5) and antiferromagnetic semiconducting (x s> 0.5) phases and contains islands of the metallic phase. Under oxygen-deficient conditions, the fraction of the metallic phase increases and the semiconductor-metal transition takes place. The formation of bound islands of the metallic phase is observed under oxygen-deficient conditions in the La0.4Ca0.6MnO3 − α compounds.

Electrical conductivity, magnetoresistance, and specific heat of oxygen-deficient La0.67Ca0.33MnO3−α(0≤α<0.4)

This paper reports on a first study of the effect of oxygen content in La0.67Ca0.33MnO3−α on the electrical conductivity, magnetoresistance, and specific heat within a broad range of oxygen deficiency variation, 0≤α<0.4. Preparation of a large quantity of stable samples with an easily controllable oxygen deficiency was made possible by using a “soft” hydrogen treatment (870 K, 1–2 kPa). The properties of samples with 0≤α≤0.06 are shown to differ from those with α≥0.1. For instance, samples studied in the semiconducting phase at 300 K revealed, on the average, Eg=0.28 eV in the first group and Eg=0.43 eV in the second, and only samples of the first group exhibit giant negative magnetoresistance and specific-heat anomalies in the range 150–200 K. The latter anomalies correlate with magnetoresistance peaks observed in all samples of the first group (with α≤0.06). The metal-semiconductor transition was found to disappear abruptly for α>0.006. The effects of a variation in the cation composition and the oxygen content on the physical properties are compared.

Proton conductivity and phase transition in potassium hydroxide monohydrate

The ionic conductivity of a proton conductor, namely, potassium hydroxide monohydrate, has been studied in the temperature range of 200–410 K. It has been established that the temperature dependence of the conductivity has the Arrhenius form with an activation enthalpy of ∼0.4 eV. The pre-exponential factors for the intervals above and below room temperature differ by a factor of ∼2.5. The anomalous temperature behavior observed in the range of 285–345 K indicates a phase transition with Tc ∼ 295 K. The mechanism of proton transport has been discussed.

Conductivity of Ce x Sr1−x MnO3 manganites in a magnetic field within the temperature range 78–300 K

The electrical conductivity of the family of CexSr1−xMnO3 (x = 0.50, 0.67) alloys is studied in magnetic fields of up to 0.6 T in the temperature range 78–300 K. The semiconductor-metal phase transition is observed in unannealed samples with x = 0.5 and in both annealed and unannealed samples with x = 0.67. All samples exhibit giant negative magnetoresistance. The temperature dependence of the giant negative magnetoresistance effect, the dependence of the electrical resistivity on the magnetic field at 78 K, and the time dependence of the magnetoresistance at 78 K are measured for the first time. Some samples reveal the properties of spin glass and strong ferromagnets. The reproducibility of the data obtained for these samples depends on the prehistory of the samples, specifically on the conditions of annealing and exposure to a magnetic field.