Thermoelectric Properties of CaMnO3 Doped with Gadolinium Ions

Abstract

Single-phase hollow spheres based on calcium manganite CaMnO₃ and gadolinium-doped Ca_0.96Gd_0.04MnO₃ with the perovskite structure Pnma are synthesized via the method of spray solution combustion synthesis. Spark plasma sintering leads to the production of a secondary phase of marokite CaMn₂O₄ with the Pbcm structure, while doping promotes to decrease the marokite content. A change in the parameters of the perovskite crystal lattice is established, which indirectly indicates the substitution of Ca²⁺ with Gd³⁺ and the formation of oxygen vacancies: both factors lead to a change in the Mn³⁺/Mn⁴⁺ ratio in the structure and, as a consequence, an increase in the electrical conductivity in the doped material due to a decrease in the activation energy of polaron hopping between localized states of Mn³⁺ and Mn⁴⁺. The resulting morphology enhances phonon scattering processes, which leads to a decrease in the thermal conductivity. The combination of the above factors leads to a significant increase in the thermoelectric efficiency: for Ca_0.96Gd_0.04MnO₃ the value zT reaches 0.26 at T = 975 K, which is 117% higher than that of the undoped sample (zT = 0.12). This confirms the promise of the chosen approaches for creating highly efficient thermoelectric ceramics.