Ca-for-Sr substitution in the thermoelectric [(Sr,Ca)2(O,OH)2]q[CoO2] misfit-layered cobalt-oxide system

Calcium-for-strontium substituted samples of the misfit-layered cobalt-oxide system, [(Sr1−xCax)2(O,OH)2]q[CoO2], were successfully synthesized up to x=0.2 with a sample-encapsulation technique originally developed for the x=0 end phase. While the x=0 sample has a commensurate match between the two layer blocks (i.e. q=0.5), isovalent Ca-for-Sr substitution induces lattice misfit (i.e. q>0.5). At the same time the Seebeck coefficient gets increased, but the increase in resistivity results in suppressing the thermoelectric power factor. The magnetic anomaly in the x=0 sample gets released upon the Ca substitution for the x=0.2 sample to exhibit an almost Curie–Weiss behavior. It is concluded that with increasing x in [(Sr1−xCax)2(O,OH)2]q[CoO2] the properties smoothly evolve towards those previously reported for the x=1.0 end member, [Ca1.7O2.1H2.4]0.58[CoO2].

Graphical abstractIn the misfit-layered [(Sr1−xCax)2(O,OH)2]q[CoO2] (0.0≤x≤0.2) system the x=0 phase has a commensurate match between the two layer blocks (i.e. q=0.5), while isovalent Ca-for-Sr substitution induces lattice misfit (i.e. q>0.5). At the same time Seebeck coefficient gets increased. Simultaneous increase in resistivity however outweighs this benefit, and accordingly the thermoelectric power factor is decreased.Figure optionsDownload full-size imageDownload as PowerPoint slide