A FIRST-PRINCIPLES STUDY OF THE CRYSTAL STRUCTURE AND ELECTRONIC PROPERTIES OF CALCIUM-MANGANATE COMPOUNDS CAO(CAMNO₃)_M (M=1,2,3,∞) FOR THERMOELECTRIC APPLICATIONS

One of the directions applied in the search for new alternative energy sources is capturing waste heat and directly converting it into electrical power using thermoelectric (TE) generators, which are based on the Seebeck effect. Among the various TE compounds, oxides demonstrate many advantages due to their structural and chemical stability at high temperatures, non-toxicity, and low-cost. We investigate the crystal structure and electronic properties of calcium-manganate based TE oxides of the CaO(CaMnO₃)_m (m=1, 2, 3, ∞)-form applying the density functional theory (DFT) approach.

We find that the crystal structures of all compounds in this series tend to deviate from the high-symmetry cubic perovskite structure (for the case of CaMnO₃ compound) or tetragonal I4/mmm structure (for CaO(CaMnO₃)_m ; m=1, 2, 3). The structure of Ca₂MnO₄ is found to be tetragonal with the I4₁/acd space group symmetry, where the Ca₃Mn₂O₇ and Ca₄Mn₃O₁₀ compounds form orthorhombic structures having the Cmc2₁ and Pbca symmetries, respectively. These results are corroborated by x-ray diffraction experiments.

The electronic properties of the above compounds are analyzed in terms of band structure. It is shown that the total density of states (DOS) values at E_F increase with increasing the m-value, which implies on increase of the electrical conductivity, as observed experimentally. The calculated values of the relative slopes of the DOS(E_F) correlate with the experimentally-measured Seebeck coefficients. The variation of TE properties within the CaO(CaMnO₃)_m series is governed by the spread of d-electronic band and by reduction of the pseudo-gap at E_F due to growth of relative concentration of Mn. We show how first-principles calculations can be employed and serve as a predictive tool for further materials design for energy harvesting.