First principle investigation of electronic structure of CaMnO3 thermoelectric compound oxide

First principle calculations are employed to investigate the anti-ferromagnetic CaMnO3 with regard to its geometry, ground state electronic structure and charge distributions. The G-type anti-ferromagnetic CaMnO3 is found to be more stable via total energy minimization calculations; the calculated energy band structure reveals its band gap of 0.7 eV. There are combinations of light carriers in conduction bands and heavy carriers in valence bands that should favor high thermoelectric properties. The Mnd and Op orbitals are responsible for energy bands near Fermi level and they contribute to electronic property. There is strong hybridization between Mnd and Op orbitals and, the hybridization between Mn and O1 orbitals is stronger, it is indicated that the charge carriers are apt to transport along Mn–O1.

Research highlights▶ The more stable phase is found among G-type CaMnO3 and A-type CaMnO3. The G-type CaMnO3 is found to have lowest free energy and this type of Mn alignment is estimated to be more stable within perovskite compounds having O–Mn–O octahedron. ▶ The G-type CaMnO3 has band gap of 0.7 eV. The value maybe lower than its real band gap as a result of drawback of density functional theory, however the theoretical result is reported for the first time. The experimental semiconductor transport character agrees with our calculation result. ▶ The Mnd and Op orbitals are found to be responsible for bands near Fermi level; they are estimated to contribute to system electronic property. Combinations of light charge carriers in conduction bands and heavy carriers in valence bands are found, this finding theoretically explains why the CaMnO3 exhibits high thermal power. ▶ There is strong hybridization between Mnd and Op within O–Mn–O octahedron. The interaction with Mn is stronger for O atoms forming the rectangular plane within O–Mn–O octahedron than that between Mn and O atoms at the octahedral poles. The carriers would transport slightly favorably along the rectangular plane. This finding has not been reported until now. Further detailed investigations of transport coefficients confirm our estimation and the result would be reported in succession. ▶ The formation of O reduction phases during the fabrication of CaMnO3 thermoelectric oxides should be deleterious and the O vacancy should be prohibited in sample preparation process. This should be a meaningful guidance for further experiments to optimize the electrical properties as to sample preparation procedure control.