The effects of microstructure, Nb content and secondary Ruddlesden-Popper phase on thermoelectric properties in...

CaMn_1- Nb O₃ (x = 0, 0.5, 0.6, 0.7 and 0.10) thin films have been grown by a two-step sputtering/annealing method. First, rock-salt-structured (Ca,Mn_1- ,Nb )O thin films were deposited on 11̄00 sapphire using reactive RF magnetron co-sputtering from elemental targets of Ca, Mn and Nb. The CaMn_1- Nb O₃ films were then obtained by thermally induced phase transformation from rock-salt-structured (Ca,Mn_1- Nb )O to orthorhombic during post-deposition annealing at 700 °C for 3 h in oxygen flow. The X-ray diffraction patterns of pure CaMnO₃ showed mixed orientation, while Nb-containing films were epitaxially grown in [101] out of-plane-direction. Scanning transmission electron microscopy showed a Ruddlesden-Popper (R-P) secondary phase in the films, which results in reduction of the electrical and thermal conductivity of CaMn_1- Nb O₃. The electrical resistivity and Seebeck coefficient of the pure CaMnO₃ film were measured to 2.7 Ω cm and -270 μV K^-1 at room temperature, respectively. The electrical resistivity and Seebeck coefficient were reduced by alloying with Nb and was measured to 0.09 Ω cm and -145 μV K^-1 for x = 0.05. Yielding a power factor of 21.5 μW K^-2 m^-1 near room temperature, nearly eight times higher than for pure CaMnO₃ (2.8 μW K^-2 m^-1). The power factors for alloyed samples are low compared to other studies on phase-pure material. This is due to high electrical resistivity originating from the secondary R-P phase. The thermal conductivity of the CaMn_1- Nb O₃ films is low for all samples and is the lowest for x = 0.07 and 0.10, determined to 1.6 W m^-1 K^-1. The low thermal conductivity is attributed to grain boundary scattering and the secondary R-P phase.