Influence of the cation Ca on the structural, magnetic, and thermoelectric properties of Na1−xCaxCoO2 (x = 0.20, 0.35, 0.50) cobaltite

The influence of Ca substitution on the structural, magnetic, electrical, and thermoelectric properties of layered Na 1- x Ca x CoO 2 (x = 0.20, 0.35, and 0.50) cobaltites synthesized by the conventional solid-state reaction method was systematically investigated. X-ray diffraction combined with Rietveld refinement confirmed that all compositions crystallize predominantly in the hexagonal P6 3 /mmc structure, accompanied by a minor Co 3 O 4 secondary phase with nearly constant concentration. Microstructural analysis revealed homogeneous grains with increasing average grain size as the Ca content increased. Electrical transport measurements showed a systematic increase in electrical resistivity, whereas the Seebeck coefficient remained positive over the entire temperature range and exhibited a slight enhancement with increasing Ca concentration, confirming stable p -type conduction. Simultaneously, thermal conductivity decreased due to enhanced phonon scattering associated with substitutional disorder and microstructural defects. The observed transport behavior is consistent with the combined effects of charge compensation, carrier scattering, and lattice disorder introduced by Ca substitution. Within the framework of the generalized Heikes–Koshibae model for strongly correlated cobalt oxides, the moderate increase in the Seebeck coefficient can be qualitatively associated with modifications of the effective carrier concentration and the Co 3+ /Co 4+ distribution, while the pronounced increase in electrical resistivity reflects the dominant influence of reduced carrier mobility. Consequently, at room temperature the power factor (PF) and figure of merit (ZT) for x = 0.20 showed maximum values of 0.7x10 −2 W/mK 2 , and 0.43, respectively. These findings demonstrate that Ca substitution provides an effective strategy for tailoring the coupled electronic and thermal transport properties of layered sodium cobaltites and offers valuable insights for the optimization of oxide thermoelectric materials.