Thermoelectric properties and first-principal calculations towards a donor-doped oxide CaMnO3–δ: electronic band structure and defect states

Journal of Alloys and Compounds. 2023.

Merkulov O. V., Shamsutov I. V., Ryzhkov M. A., Politov B. V., Baklanova I. V., Chulkov E., Zhukov V. P.

Crystal structure and thermoelectric characteristics of the donor-doped CaMnO3–δ are
significantly affected by doping elements and by oxygen exchange with ambient air occurring at
temperatures above 700 K. To investigate the role of dopants and oxygen nonstoichiometry in
thermoelectric phenomena, polycrystalline Ca0.95Bi0.025Y0.025MnO3–δ was synthesized and the
obtained temperature dependences of thermal and electronic parameters were studied by DFT
calculation method. The electrical conductivity has shown to possess a polaronic character
up to ~700 K at a low activation energy. An increase in temperature leads to formation of oxygen
vacancies resulting in the appearance of vacancy states inside the band gap and a decrease in its
width. The consequence of this is the observed gradual transition to the semiconductor behavior
of electrical conductivity explicitly manifested above ~1000 K. The obtained experimental figure
of merit is shown to be almost an order of magnitude less than the corresponding theoretical one
for bulk media, the main part of the deviation is related to the electrical conductivity of the ceramic
specimen having a polycrystalline nature. We conclude that the thermoelectric properties of the
donor-doped manganese oxide can be significantly improved employing bulk monocrystalline
media.

Keywords: DFT, Electrical conductivity, figure of merit, Seebeck coefficient, phonon transport, oxygen vacancies