Infrared spectroscopic study of the local structural changes across the metal insulator transition in nickel-doped GdBaCo2O5.5

Layered Oxygen-Deficient Double Perovskites as Promising Cathode Materials for Solid Oxide Fuel Cells

Development of new functional materials with improved characteristics for solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs) is one of the most important tasks of modern materials science. High electrocatalytic activity in oxygen reduction reactions (ORR), chemical and thermomechanical compatibility with solid electrolytes, as well as stability at elevated temperatures are the most important requirements for cathode materials utilized in SOFCs. Layered oxygen-deficient double perovskites possess the complex of the above-mentioned properties, being one of the most promising cathode materials operating at intermediate temperatures. The present review summarizes the data available in the literature concerning crystal structure, thermal, electrotransport-related, and other functional properties (including electrochemical performance in ORR) of these materials. The main emphasis is placed on the state-of-art approaches to improving the functional characteristics of these complex oxides.

Dual nature of high-temperature electronic transport in layered perovskite-like cobaltites: exhaustive consideration of experimental features observed

Complex oxides with the general formula Pr_1-xY_xBaCo_2-yNi_yO_6-δ (x = 0, 0.1, y = 0, 0.2) were successfully synthesized via combustion of organo-metallic precursors. The high-temperature dc conductivity of the obtained sintered materials was studied in a wide range of oxygen partial pressures and temperatures by means of the 4-probe method. The resulting dependencies were juxtaposed with the previously published data on oxygen non-stoichiometry for the oxides considered. The comprehensive analysis of these datasets in attempts to explain the observed trends has shown the large inadequacy of currently existing conduction models. Consequently, a new model approach was developed to account for the numerous experimental and theoretical peculiarities being characteristic for the cobaltites with a layered double perovskite structure. One of the key propositions made postulates mixed nature of the band structure for materials studied with spin states of Co ions acting as a spatial descriptor of a particular type of conductivity: semiconducting or a metallic one. The abovementioned hypothesis was validated by magnetic, thermodynamic and structural arguments obtained both theoretically and experimentally. The models suggested were shown to be adequate in describing large arrays of data collected. Additionally, the reasons behind doping and temperature/pressure influences on the respective conductivity changes in Pr_1-xY_xBaCo_2-yNi_yO_6-δ were uncovered. Transport and thermodynamic parameters determined were used to evaluate transference numbers and mobilities of different charge carriers which revealed the dominating role of metallic conductivity under oxidative conditions and the superiority of semiconducting charge transport in reducing environments. The obtained conclusions were further supported by utilizing the derived model equations for successful description of conductivity/non-stoichiometry data for other layered cobaltites. Also, interesting correlations between cation composition, thermodynamic and transport properties were found. Finally, general review of the formulated approach was made and further research directions were proposed.

Thermal expansion of layered cobaltites Y(Ba₁ - xSrx)Co₂O₅ + δ with x = 0, 0.05 and 0.10

The crystal structure and thermal expansion of Sr-doped layered cobaltites Y(Ba(1 - x)Sr(x))Co(2)O(5 + δ) (x = 0, 0.05 and 0.10) were studied by means of in situ neutron thermodiffraction in the temperature range 20 K ≤ T ≤ 570 K. The evolution with temperature of lattice parameters for the phases which crystallize in this system is presented, as well as their dependence on the oxygen non-stoichiometry δ. Each phase's volume has been fitted using available models based on the Grüneisen approximation to the zero-pressure equation of state and using the Ruffa model based on the Morse potential, both using a Debye model for the internal energy. The coefficient of volumetric thermal expansion, Debye temperature and other thermodynamic parameters are presented and compared with other perovskite compounds.