Thermoelectric misfit-layered cobalt oxides with interlayers of hydroxide and peroxide species

• Parent m=0 [MmA2Om+2]qCoO2 misfit-layer oxides exhibit divergent chemical features.
• [(Ca,Sr)z(O,OH)2]qCoO2 is found to contain both peroxide and hydroxide species.
• Hydrothermal synthesis yields [(Ca1−xSrx)z(O,OH)2]qCoO2 samples up to x=0.2.
• With increasing x, the c axis expands but the misfit parameter q remains constant.
• Co valence remains intact, but peroxide and hydroxide contents may be affected.

Among the thermoelectric misfit-layered cobalt oxides, [MmA2Om+2]qCoO2, the parent m=0 phases exhibit divergent chemical features but are less understood than the more common m>0 members of the series. Here we synthesize Sr-for-Ca substituted [(Ca1−xSrx)z(O,OH)2]qCoO2 zero phases up to x=0.2 through low-temperature hydrothermal conversion of precursor powders of the m=1 misfit system, [Co(Ca1−xSrx)2O3]qCoO2. In the zero-phase [(Ca1−xSrx)z(O,OH)2]qCoO2 system, as the Sr content x increases the lattice expands anisotropically along the c axis such that the ab-plane dimension and the misfit parameter q remain essentially constant. X-ray absorption spectroscopy data suggest the presence of peroxide-type oxygen species in the (Ca1−xSrx)z(O,OH)2 rock-salt block and together with infrared spectroscopy, thermogravimetric and low-temperature resistivity and thermopower measurements evidence that the isovalent Sr-for-Ca substitution controls the balance between the peroxide and hydroxide species in the (Ca1−xSrx)z(O,OH)2 block but leaves the valence of Co essentially intact in the CoO2 block. The higher electrical conductivity of the Sr-substituted phases is explained as a consequence of increased carrier mobility.

Among the thermoelectric misfit-layered cobalt oxides, [MmA2Om+2]qCoO2, the parent zero (m=0) phases exhibit divergent chemical features. For [(Ca1−xSrx)z(O,OH)2]qCoO2, X-ray absorption spectroscopy data suggest the presence of peroxide-type oxygen species in the (Ca1−xSrx)z(O,OH)2 rock-salt block and together with thermogravimetric and low-temperature transport-property measurements evidence that the isovalent Sr-for-Ca substitution controls the balance between the peroxide and hydroxide species in the (Ca1−xSrx)z(O,OH)2 block but leaves the valence of Co essentially intact in the CoO2 block.Figure optionsDownload as PowerPoint slide