Facile synthesis of morphology-controlled La:BaSnO3 for the enhancement of thermoelectric power factor

Abstract

Donor-doped BaSnO₃ (BSO) ceramics are promising n-type oxide materials for high-temperature thermoelectric applications. In the present work, nanostructured La-substituted BSO (Ba_1−xLa_xSnO₃, x = 0, 0.03, 0.05, 0.08, and 0.10) samples were prepared by the polymerization complex method and compacted by the vacuum-processed spark plasma sintering system. X-ray diffraction patterns confirmed the single-phase cubic structure of BSO for x = 0 (BSO), 0.03 (BSOLa3) and 0.05 (BSOLa5) samples. A secondary phase of La₂Sn₂O₇ was observed in the sample with x = 0.08 (BSOLa8) and 0.10 (BSOLa10) in addition to the cubic structure of BSO. The high-resolution transmission electron microscopy images confirmed the cubic morphology of the sample and the effect of La substitution revealed the morphology of grain size variations. The chemical valence states of Ba, La, Sn and O were confirmed by X-ray photoelectron spectroscopy (XPS). Density functional theory calculations of band structure confirmed that the Sn 5s orbital majorly contributed for the conduction mechanism with the partial inclusion of the La 5d near the Fermi energy for the La-substituted BSO. The theoretical study and XPS results revealed that the substitution of La atom for Ba atom effectively shifted the position of the Fermi level (E_F) toward the conduction band minimum. From O 1s spectra of the samples, oxygen-vacancy related peaks were detected for each sample. The carrier concentration of BSO increased from 1.2 × 10¹⁹ to 3.7 × 10¹⁹ cm⁻³ when the La content was increased from x = 0.05 to 0.10. The addition of La³⁺ atoms enhanced the electrical conductivity of BSO. A high Seebeck coefficient of −58.5 μV K⁻¹ was achieved for BSOLa5 at 573 K. The electrical conductivity of BSOLa5 was 1.19 × 10³ S m⁻¹, which was relatively higher than that of BSOLa10 La-substituted BSO (1.03 × 10³ S m⁻¹). The BSOLa5 La-substituted BSO sample showed highest power factor of 4.07 μW m⁻¹ K⁻².