The effects of microstructure, Nb content and secondary Ruddlesden–Popper phase on thermoelectric properties in perovskite CaMn1−xNbxO3 (x = 0–0.10) thin films

CaMn1−xNbxO3 (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,Mn1−x,Nbx)O thin films were deposited on 100 sapphire using reactive RF magnetron co-sputtering from elemental targets of Ca, Mn and Nb. The CaMn1−xNbxO3 films were then obtained by thermally induced phase transformation from rock-salt-structured (Ca,Mn1−xNbx)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 CaMn1−xNbxO3. 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 CaMn1−xNbxO3 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.