D.M. Sparlin

Structure and electrical properties of La1−xSrxCo1−yFeyO3. Part 1. The system La0.8Sr0.2Co1−yFeyO3

Abstract Crystal structure, thermal expansion, thermogravimetry, thermoelectricity, and electrical conductivity of compositions in the system La0.8Sr0.2Co1−yFeyO3 with 0 ≤ y ≤ 1 were studied as function of Co Fe ratio and temperature, in air. The electrical conduction mechanism is attributed to the adiabatic-hopping of p-type small polarons. At high temperatures, oxygen deficiency causes lattice expansion and a reduction in electrical conductivity. The observed temperature dependence of the Seebeck coefficient is attributed to changes in carrier concentration caused by a thermally excited charge disproportionation of Co3+ ions and by the ionic compensation of induced oxygen vacancies. The measured electrical conductivity and Seebeck coefficient as a function of the Co Fe ratio is interpreted using a two-site hopping and the site-percolation model. It is suggested that a preferential electronic compensation of Fe ions over Co ions may occur in this system.

Structure and electrical properties of La1 − xSrxCo1 − yFeyO3. Part 2. The system La1 − xSrxCo0.2Fe0.8O3

Abstract Crystal structure, thermal expansion, oxygen stoichiometry, thermoelectricity, and electrical conductivity of compositions in the system La 1 − x Sr x Co 0.2 Fe 0.8 O 3 with 0 ≤ x ≤ 0.6 were studied as function of temperature and Sr content, in air. The solubility of Sr in the sintered perovskite-type oxide (ABO 3 ) was limited to x ≤ 0.4. The observed p -type electrical conduction appeared to occur via a small-polaron hopping mechanism. The thermally-induced oxygen loss caused a lattice expansion plus decreases in both the carrier concentration and the carrier mobility. A semi-empirical model was developed which takes into account the thermally activated disproportionation of Co 3+ ions into Co 4+ and Co 2+ pairs, and the ionic compensation of oxygen vacancies formed at high temperatures. The concentrations of B-site ions (Co or Fe) in different valence states were calculated using this model and experimental data.

Oxidation-reduction behavior of undoped and Sr-doped LaMnO3 nonstoichiometry and defect structure

Undoped and Sr-doped LaMnO{sub 3} showed reversible oxidation-reduction behavior. These perovskites can be excess, stoichiometric or deficient in oxygen content depending on the specific conditions. Under very reducing conditions decomposition into new phases occurs. Phase stabilities for these oxides were determined. The results showed that Sr doping caused the LaMnO{sub 3} to dissociate at higher oxygen activities than those necessary for undoped LaMnO{sub 3}. Defects models are proposed to interpret the thermogravimetric results in which metal vacancies are assumed for the oxygen excess condition and oxygen vacancies are assumed for the oxygen deficient condition. Thermodynamic properties were calculated which support the model.

Oxidation-reduction behavior of undoped and Sr-doped LaMnO3: Defect structure, electrical conductivity, and thermoelectric power

Abstract Seebeck coefficient and electrical conductivity measurements were performed for undoped and Sr-doped LaMnO 3 as a function of temperature and oxygen partial pressure. The results of electrical conductivity showed typical p -type behavior. As reduction proceeded, the electrical conductivity of these LaMnO 3 -based perovskites decreased with P 1 4 O 2 . The analysis of the electrical conductivity data was performed by extending the defect model from a previous thermogravimetric (TG) study. The measured Seebeck coefficients were found to be positive except for the most reducing conditions when the decomposition into multiple phases occurred. The Heikes formula was adopted to interpret the Seebeck coefficient results and to calculate the mobility. These results indicated that the conduction was due to p -type carriers of a localized nature. It was also suggested that the conductivity for these perovskites was dominated by the mobility rather than by the carrier concentration.

Electrical conductivity, seebeck coefficient and defect chemistry of Ca-doped YCrO3

The electrical transport behavior and defect structure of Ca-doped YCrO3 were studied using electrical conductivity and Seebeck coefficient measurements as a function of oxygen activity and temperature. Defect models derived from the electrical conductivity data were found to adequately relate the concentration of charge carriers to the acceptor dopant and oxygen vacancy concentrations. Thermodynamic properties were calculated and found to be consistent with the model. Activation energy and carrier mobility data were also obtained. The analysis of the electrical conductivity, Seebeck and mobility data suggest that the conduction process in Ca-doped YCrO3 occurs via the small polaron hopping mechanism.

Electrical characterization of the (La,Ca)(Cr,Co)O3 system

Abstract The results of electrical conductivity and thermopower measurements are presented for selected (La,Ca)(Cr,Co)O 3 compositions. The La 1 − x Ca x CrO 3 compounds are typical p -type small polaron conductors. The La 1 − x Ca x CrO 3 compounds are found to be small polaron conductors at room temperature for x ≤ 0.20 with a higher electrical conductivity than the corresponding chromites. The electrical conductivity at room temperature is lower for Co-rich compositions than for Cr-rich compositions for compositions containing 10 mol% and 20 mol% Ca. This fact is attributed to the trapping of p -type small polarons at energetically lower Cr sites, resulting in a broad room temperature conductivity minimum. Above room temperature, the electrical conductivity increases with increasing Co content for all compositions. The thermopower for these compounds is interpreted using the Verwey electronic compensation mechanism including thermally activated disproportionation among Co ions.

Defect structure, nonstoichiometry, and phase stability of Ca-doped YCrO3

The dependence of the defect structure of Ca-doped YCrO{sub 3} on oxygen activity and temperature was investigated by high temperature thermogravimetric measurements. Defect models developed from electrical conductivity data obtained in a previous study were used to interpret the thermogravimetric data. A correlation was found between the electrical conductivity and the thermogravimetric data which suggested that these data were concomitantly dependent on the acceptor dopant and oxygen vacancy dependence of the thermodynamic parameters. Kroeger-Vink type diagrams showing the regions of stability with respect to oxygen activity and temperature were constructed. The TGA data show that Ca-doped YCrO{sub 3} is even more stable toward reduction than doped LaCrO{sub 3}.

High-temperature defect structure of Nb-doped LaCrO3

Electrical conductivity and Seebeck measurements on LaCr{sup 0.98}Nb{sub 0.02}CrO{sub 3} show that the defect structure of the material is mainly controlled by the extrinsic electrons formed by the Nb donors through the electronic compensation process. The experimental results also indicate that this material conducts electricity via a small polaron mechanism with an electron mobility around 0.004-0.01 cm{sup 2}/v sec between 1100 and 1300{degree}C. 8 refs., 10 figs.

Magnetic-field-induced pseudo phase transition in cadmium

Abstract Recent magnetostriction measurements in Cd showed anomalous behavior at about 25 kG. These data are shown to be consistent with a temperature and magnetic field version of Lifshitzs model for the ‘electron transitions’ associated with Fermi surface topology changes. The dependence of the model upon temperature and magnetic field direction is treated.