V. L. Kozhevnikov

Band structure and the magnetic and elastic properties of SrFeO3 and LaFeO3 perovskites

The band structure and the magnetic and elastic characteristics of SrFeO3 and LaFeO3 perovskites with ferromagnetic and antiferromagnetic collinear spin configurations (of the A, C, and G types) are investigated using the ab initio pseudopotential method (the VASP program package) with the inclusion of the single-site Coulomb correlations (the LSDA + U formalism). It is shown that, in the pressure range 0–50 GPa, the most stable states are the ferromagnetic metal state for the SrFeO3 compound and the antiferromagnetic insulator state of the G type for the LaFeO3 compound.

High-temperature electrical conductivity and thermopower in nonstoichiometric La1−xSrxCoO3−δ (x=0.6)

From 650 to 900°C in situ conductivity and thermopower measurements it was determined that La1−xSrxCoO3−δ (x=0.6) is a small polaron conductor over the range 0.12<δ<0.38 with a hopping energy of approximately 0.08 eV. Cobalt’s valence distribution was calculated from the thermopower data complemented with the site balance and electroneutrality relations. Three valence states of cobalt appear to be present with the equilibrium constant of disproportionation, 2Co3+=Co2++Co4+ independent of temperature and dependent of oxygen content. As δ decreases, a p-to-n transition occurs indicating a near half-filling of the conductance band. It is concluded also that the simple polaronic description of conductance becomes insufficient when approaching the stoichiometric oxygen composition.

Conductivity and carrier traps in La1−xSrxCo1−zMnzO3−δ (x=0.3; z=0 and 0.25)

Abstract Measurements of the equilibrium oxygen content, electrical conductivity and thermopower in the perovskite-like solid solution La 0.7 Sr 0.3 Co 1– z Mn z O 3− δ ( z =0 and 0.25) as a function of the temperature and oxygen partial pressure are used to determine the temperature dependence of the conductivity and thermopower at different values of the oxygen deficiency. A model for a hopping conductor with screened charge disproportionation is applied for the data analysis in combination with trapping reactions of n- and p-type carriers on local oxygen vacancy clusters and manganese cations, respectively. Changes in the ratio of n-type to p-type mobility are due to variations in oxygen vacancy concentration and manganese content, while the energetic parameters governing charge disproportionation of the trivalent cobalt cations and formation of vacancy associates are shown to be essentially invariable. These calculated charge carrier site occupancies are used to model temperature variations of the electrical properties in La 0.7 Sr 0.3 Co 1− z Mn z O 3− δ in favorable correspondence with experimental observations.

High-temperature electrical conductivity of Sr0.7La0.3FeO3-δ

Abstract The electrical properties of the lanthanum-doped strontium ferrite Sr 0.7 La 0.3 FeO 3− δ are studied within the temperature range 750–950°C and the oxygen partial pressure range between 10 −19 and 0.5 atm. The ferrite undergoes a transition from perovskite-like to brownmillerite-like structure at an oxygen pressure of about 10 −4 atm in the studied temperature range. The observed pressure and temperature dependencies of the conductivity in the brownmillerite-like form of the ferrite are related to the transition of the electron conductivity from the intrinsic regime, which is governed by the band gap of about 2 eV, to the extrinsic regime, which is controlled by the extra-stoichiometric oxygen in the brownmillerite-like phase. The brownmillerite-like phase is shown to be a mixed conductor with the oxygen conductivity level rising to about 0.3 S/cm at about 900°C.

Electrical characterization of the intergrowth ferrite Sr4Fe6O13+δ

Abstract Experimental data for thermopower, conductivity and oxygen content in Sr 4 Fe 6 O 13+ δ were obtained in the temperature range 750–1000°C and oxygen partial pressure between 10 −17 and 0.5 atm. The results were utilized to obtain partial contributions of hole- and electron-like carriers in the total conductivity, to evaluate respective mobility parameters and to carry out model calculations of oxygen content in the oxide. The hole mobility is shown to be independent of temperature while mobility of electrons has temperature-activated character. The charge compensation of the electronic carriers occurs due to appearance of doubly charged oxygen interstitial ions and oxygen vacancies in the crystalline structure of the oxide. The conductivity evolves mainly over perovskite layers in the oxide at high pressure of oxygen while all structurally different positions of iron ions become available for electron-like carriers in the low-pressure limit. The oxygen ion contribution in total conductivity is shown to be essentially smaller than the contribution of electronic carriers.

Effect of disordering on properties of high-temperature superconductors

Abstract The influence of radiation-induced disorder on the properties of high-T c superconductors is investigated. The results show that effects of localization are essential even in the samples with high T c .

Electronic structure and Fermi surface of the superconductors LaNiBiO and LaCuBiO from first principles

Based on first-principles FLAPW-GGA calculations, we have investigated the electronic structure of newly synthesized novel superconductors LaONiBi and LaOCuBi, the first bismuth-containing compounds from the family of quaternary oxypnictides which attract now a great deal of interest in search for novel 26-55K superconductors. The band structure, density of states and Fermi surfaces are discussed. Our results indicate that the bonding inside of the (La-O) and (Ni(Cu)-Bi) layers is covalent whereas the bonding between the (La-O) and (Ni(Cu)- Bi) blocks is mostly ionic. For both oxybismuthides, the DOSs at the Fermi level are formed mainly by the states of the (Ni(Cu)-Bi) layers, the corresponding Fermi surfaces have a twodimensional character and the conduction should be strongly anisotropic andhappen only on the (Ni(Cu)-Bi) layers. As a whole, the new oxybismuthides may be described as low-TC superconducting non-magnetic ionic metals.

Oxygen non-stoichiometry and defect equlibria in CaMnO3 − δ

A defect equilibrium model is suggested for CaMnO3 − δ based on oxygen non-stoichiometry and conductivity data. The model includes reactions of oxygen exchange and thermal excitation of electrons. The respective equilibrium constants, enthalpies and entropies for the reactions entering the model are obtained from the fitting of the calculated and experimental data for oxygen non-stoichiometry. The energy parameters obtained from the model and variations in the concentration of manganese species enable explanation of temperature and oxygen pressure dependencies of thermopower and conductivity within frameworks of a small polaron type model.

New superconductor with a layered crystal structure: Nickel oxybismuthide LaO1−δNiBi

The results of synthesizing a new layered phase—nickel oxybismuthide LaO1−δNiBi in a series of superconducting oxypnictides—and its properties in the superconducting and normal states are reported. Although the temperature of the transition of this phase to the superconducting state, Tc ∼ 4 K, is much lower than the value Tc = 55 K reached at present in oxyarsenide SmO1−δFeAs, the similarity of the crystal structures and ρ(T) dependencies indicates that the mechanism responsible for the appearance of the semiconducting state is the same in lanthane oxybismuthide and samarium oxyarsenide.

The oxygen permeation through YBa2Cu3O6+x

The ionic conductivity of oxygen was studied in the dense ceramic YBa2Cu3O6+x using the oxygen permeation measurements. The temperature interval of measurements was 600–850°C and the limits of the partial pressure of oxygen were 0.06-1.00 atm. The method was based on the direct determination of the steady flow of oxygen through the specimen in the conditions of constant temperature with a small gradient of oxygen chemical potential along the sample. The oxygen diffusion coefficient was calculated. The activation energy (U) of the oxygen conductivity (diffusion coefficient) was determined in the interval 0.35<x<0.6. The dependence of the activation energy on the oxygen content in the tetragonal phase, U=2.80-1.45x (eV), was found. This dependence was explained by the interaction of the mobile oxygen ions.