Charge-Carrier Hopping in Highly Conductive CaMn_1–xM_xO_3−δ Thermoelectrics

Abstract

Highly dense CaMn_1–xM_xO_3−δ (with M = Nb, Mo, Ta, and W and 0 ≤ x ≤ 0.08) n-type thermoelectric materials with low electrical resistivities are prepared from nanocrystalline powders. Their room temperature power factors outperform the best reported results by 30% or more. In combination with the thermal conductivities, promising figure of merits of ZT_M=Ta,x=0.04 = 0.21 and ZT_M=W,x=0.04 = 0.20 were achieved at 1160 K. The relative changes and temperature dependencies of the Seebeck coefficient, the electrical resistivity, and the power factor are described with a small-polaron-hopping-based mechanism. In the limits of high temperatures and low substitution levels, the Seebeck coefficients are in good agreement with the Heikes formula. At high substitutions, the efficiency of the doping presumably decreases due to trapping states caused by the formation of bands from Jahn–Teller lowered e_g orbitals of Mn³⁺. Jahn–Teller distortion of Mn³⁺ also leaves its footprints in the orthorhombic distortion of the crystal structure along the b-axis.