Structural stability of calcium-manganate based CaO(CaMnO₃)_m(m = 1, 2, 3, ∞) compounds for thermoelectric applications

We investigate the crystal structure and bulk properties of calcium-manganate based TE oxides of the CaO(CaMnO₃)_m(m = 1, 2, 3, ∞) Ruddlesden-Popper (RP) class 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 m = ∞) or tetragonal I4/mmm structure (for m = 1, 2, and 3). The structure of Ca₂MnO₄(m = 1) is found to be tetragonal with the I4₁/acd space group symmetry, whereas the Ca₃Mn₂O₇(m = 2) and Ca₄Mn₃O₁₀(m = 3) compounds form orthorhombic structures having the Cmc2₁and Pbca symmetries, respectively; all of the above are formed by combinations of tilted MnO₆octahedra. These results are corroborated by X-ray diffraction experiments. This procedure provides us with a powerful tool predicting phase stability and polymorphism.