Two low-dimensional cobaltite series in the Sr-Ca-Co-O system have been investigated for their solid solution limit, structure, and compatibility phase relationships (850 deg C in air). Thermoelectric properties have been measured for selected members of these solid solutions. In (Ca, Sr)3Co4O9 which has a misfit layered structure, Sr was found to substitute in the Ca site to a limit of (Ca0.8Sr0.2)3Co4O9. Compounds in the homologous series, An+2ConCo O3n+3 (where A= Sr, Ca, (Ca,Sr) or (Sr,Ca)), consist of 1-dimensional parallel Co2O66- chains that are built from successive alternating face-sharing CoO6 trigonal prisms and n units of CoO6 octahedra along the hexagonal c-axis. In the Can+2ConCo O3n+3 series, only the n=1 phase (Ca3Co2O6 ) could be prepared under the present synthesis conditions. Sr substitutes in the Ca site of Ca3Co2O6 to a limit of (Ca0.9Sr0.1)3Co2O6. In the Srn+2ConCo O3n+3 series, Ca substitutes in the Sr site of the n = 2, 3 and 4 members to a limit of (Sr0.7Ca0.3)4Co3O9, (Sr0.67Ca0.33)5Co4O12, and (Sr0.725Ca0.275)6Co5O15, respectively. While the members of the Can+2ConCo O3n+3 and Srn+2ConCo O3n+3 series have reasonably high Seebeck coefficients and relatively low thermal conductivity, the electrical conductivity needs to be increased in order to achieve high ZT values.
Citation: Journal of Applied Physics
Pub Type: Journals
(Sr, Ca)-Co-O phase equilibira, Can+2ConCo O3n+3, Srn+2ConCo O3n+3, crystal structure, thermoelectric properties