Synergistic effects of Eu and Nb dual substitution on improving the thermoelectric performance of the natural perovskite CaTiO3

Driven by the development of sustainable and regenerable energy conversion materials, the mineral perovskite (CaTiO3) is considered to be a potential candidate for large-scale high-temperature applications owing to its abundance, light-weight, non-toxicity, and low cost. A series of compounds with the nominal composition Ca1-xEuxTi0.9Nb0.1O3 (0 ≤ x ≤ 0.4) has been synthesized and studied in this paper. The phase purities and crystal structures were evaluated by powder X-ray diffraction (XRD) and subsequent Rietveld analysis. Through X-ray photoelectron spectroscopy (XPS) characterization, the by far dominating valence states of Ti and Nb are confirmed to be +4 and + 5 in Ca1-xEuxTi0.9Nb0.1O3 compounds, respectively. Dual substitution by Nb and Eu yields a synergistic effect of improving electrical transport properties and simultaneously suppressing thermal conductivity. The former is mainly attributed to the d-f electron exchange induced by the strong hybridization of Eu 4f, Nb 4d, and Ti 3d orbitals. The latter is mostly attributed to the dominant phonon scattering by the mass fluctuation originating from the large mass contrast of Eu and Ca. The results demonstrate the evolution of insulating CaTiO3 to metallic-like conduction performance with increasing Eu content. Due to the largest power factor and lowest thermal conductivity, the sample Ca0.8Eu0.2Ti0.9Nb0.1O3 exhibits the maximum ZT of up to 0.3 at around 1173 K.