Edward B. Mitberg

Effect of oxygen ordering on the structural and magnetic properties of the layered perovskites PrBaCo2O5+δ

The influence of oxygen ordering on the structural and magnetic properties of the layered Co-based perovskites PrBaCo2O5+δ (0.16 0.7), could thus be arising from the percolation of oxygen-rich ferromagnetic clusters.

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.

The relation between high-temperature thermopower, conductivity and oxygen content in YBa2Cu3O6+x

Abstract Experimental data, more detailed and complete than previously available, for thermopower and conductivity in YBa 2 Cu 3 O 6+ x was obtained in the temperature range 600 to 850°C and oxygen content varying in the limits 0.1≤ x ≤0.8 under equilibrium pressure of oxygen. The concentration dependencies of conductivity and thermopower were derived using an equilibrium P(O 2 )–T–x diagram. Isothermal plots of thermopower versus the logarithm of conductivity show unusual features in the extrinsic hole regime, i.e. a small inward curvature, an anomalously small slope and a weak temperature dependence were observed. The decrease in the oxygen content at high temperature results in the expected transition to intrinsic behavior and the contribution of electrons to the conductivity. The results obtained have been interpreted in the framework of the band structure comprised of two narrow bands on the background of two broad bands. The main influence of the broad bands is on the position of the chemical potential, whereas the location of the narrow bands relative to the chemical potential of the electrons impacts the electron transport.

Thermodynamic and structural properties of PrBaCo2O5 + δ

AbstractThe oxygen content of the double cobaltite PrBaCo2O5 + δ was determined by coulometric titration in broad temperature and oxygen pressure ranges, and the partial thermodynamic functions of oxygen were evaluated. The results on the nonstoichiometry and structural properties of PrBaCo2O5 + δ demonstrate that, at 5 + δ < 5.3, this cobaltite has a high-temperature phase with lattice parameters a = 11.774 Å, b = 11.932 Å, and c = 7.609 Å. The composition dependences of

Intercalation thermodynamics and chemical diffusion of oxygen in the solid solution YBa2Cu3−xCoxO6+δ

\Delta \bar H(O)