I. A. Leonidov

Structure, ionic conduction, and phase transformations in lithium titanate Li4Ti5O12

The spinel structure of lithium titanate Li4Ti5O12 is refined by the Rietveld full-profile analysis with the use of x-ray and neutron powder diffraction data. The distribution and coordinates of atoms are determined. The Li4Ti5O12 compound is studied at high temperatures by differential scanning calorimetry and Raman spectroscopy. The electrical conductivity is measured in the high-temperature range. It is shown that the Li4Ti5O12 compound with a spinel structure undergoes two successive order-disorder phase transitions due to different distributions of lithium atoms and cation vacancies (□, V) in a defect structure of the NaCl type: (Li)8a[Li0.33Ti1.67]16dO4 → [Li□]16c[Li1.33Ti1.67]16dO4 → [Li1.33□0.67]16c[Ti1.67□0.33]16dO4. The low-temperature diffusion of lithium predominantly occurs either through the mechanism ... → Li(8a) → V(16c) → V(8a) → ... in the spinel phase or through the mechanism ... → Li(16c) → V(8a) → V(16c) → ... in an intermediate phase. In the high-temperature phase, the lithium cations also migrate over 48f vacancies: ... Li(16c) → V(8a, 48f) → V(16c) → ....

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.

Thermodynamics of the movable oxygen and conducting properties of the solid solution YBa2Cu3-xCoxO6+δ at high temperatures

High precision coulometric measurements of the equilibrium oxygen content in the solid solution YBa2Cu3-xCoxO6+δ, where x=0, 0.2, 0.4, 0.6 and 0.8, were carried out using a double-cell technique in the temperature range 600 – 850 °C and at oxygen pressure varying between 10−5 and 1 atm. The data were employed to determine the partial molar enthalpy and entropy of the movable oxygen depending on δ and x. The electrical conductivity and thermopower were also measured in the same range of the external parameters, and their dependence on the oxygen concentration was determined at different cobalt content. The data reveal several types of oxygen sites participating in the gas-solid equilibrium. The behavior of thermodynamic functions is indicative of the partial ordering of the complex species which form the structural layer Cu1-xCoxOδ with variable content of oxygen and cobalt. It was shown that replacement of copper by cobalt does not result in appearance of the electronic charge carriers. The behavior of the thermopower and electric conductivity was explained with a narrow band model. The energy change with δ and x of the p-band, which dominates the conductivity, was found to follow the respective change in the oxygen partial enthalpy. Thus, electronic carriers in the layered structure of the cuprate are strongly influenced by the labile oxygen ions.

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.

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.

Transport properties and defect structure of YBa2Cu7−δ☆

Abstract The influence of partial oxygen pressure on the electrical conductivity and the Seebeck coefficient of the high-temperature superconductor YBa 2 Cu 3 O 7−δ is investigated in the temperature range 220–900°C. The relation between non-stoichiometry, mechanism of conductivity and the type of electrical charge carriers is discussed.

Oxygen thermodynamics and ion conductivity in the solid solution La1−xSrxCoO3−δ at large strontium content

The equilibrium oxygen content was measured in the model system and important oxygen permeable material La1−xSrxCoO3−δ, where x=0.6, in the temperature range 650–900 °C and oxygen partial pressure range between 10−5 and 1 atm. The data were utilized to obtain changes in the partial entropy and enthalpy of oxygen in the solid as a function of the oxygen content. It is shown that the initially cubic perovskite undergoes to a phase transition to a tetragonal structure at δ >0.3. The oxygen permeation of L0.4Sr0.6CoO3−δ at 700–900 °C is found to be controlled by bulk solid state processes. The activation energy equals about 0.8 eV at high oxygen pressure and small oxygen nonstoichiometry. Increasing oxygen deficiency results in a rapid increase in the activation energy. In combination with thermodynamic data, these changes can be explained as resulting from the intrinsic spatial inhomogeneouty in oxygen vacancy distribution which varies both with temperature and oxygen nonstoichiometry. It is shown that, when the oxygen deficiency increases at constant temperature, the oxygen vacancies form locally ordered microdomains (clusters), which eventually results in a transition of the cubic perovskite structure to the tetragonal structure. The oxygen ion conductivity depends strongly on the development of the ordering.