Effect of dual-doping on the thermoelectric transport properties of CaMn1−xNbx/2Tax/2O3

Abstract

The dual-substituted CaMn_1−xNb_x/2Ta_x/2O₃ is synthesized by solid-state reaction and it's crystal structure-thermoelectric property relationship is established. Rietveld refinement confirms the formation of a single phase orthorhombic structure with a gradual increase of cell parameters and bond lengths with doping level. The electrical resistivity (ρ) shows non-metal like temperature dependence. The ρ-value decreases with increasing doping level indicating an increase in charge-carrier concentration through formation of Mn³⁺ ions with e¹_g electron in the Mn⁴⁺ matrix of CaMn_1−xNb_x/2Ta_x/2O₃. The shallow region observed around 500 K in the resistivity curve is interpreted as the formation of local charge-ordering clusters due to the presence of oxygen vacancies in CaMn_1−xNb_x/2Ta_x/2O₃. The Seebeck coefficient initially decreases with temperature as expected from increasing charge carrier concentration. Above 600 K, the Seebeck coefficient increases with temperature as oxygen vacancies start playing the dominant role. The relatively low thermal conductivity of CaMn_1−xNb_x/2Ta_x/2O₃ results from the damping of local vibration through substitution of heavier ions of Nb and Ta as well as crystallographic distortion. The dual-substituted CaMn_1−xNb_x/2Ta_x/2O₃ shows a maximum power factor of 200 μW m⁻¹ K⁻² and dimensionless figure-of-merit (ZT) of 0.15 at x = 0.04, arising from low electrical resistivity of 15 mΩ cm, a moderate Seebeck coefficient of −176 μV K⁻¹ and low thermal conductivity of 1.2 W m⁻¹ K⁻¹.