On the origin of vibrational properties of calcium manganate based thermoelectric compounds

Vibrational properties of CaO(CaMnO3)(m) (m = 1, 2, 3, and infinity) thermoelectric (TE) oxides for high-temperature energy conversion applications are studied both experimentally and computationally. Density functional theory (DFT) calculations reveal strong scattering events involving dispersive acoustic phonons and non-dispersive optical modes in the frequency range of 3-5 THz. We demonstrate that the low frequency parts of the phonon spectra are strongly dominated by Ca-sublattice oscillations for all compounds of the CaO(CaMnO3)(m) (m = 1,2,3, infinity) series, which predicts enhanced phonon scattering upon Ca-sublattice site substitution defects. Accordingly, laser flash analysis (LFA) indicates considerable decrease of thermal conductivity (kappa) due to La-substitution for Ca, whereas Nb-substitution for Mn-sites does not affect kappa noticeably. It is found that thermal conductivity of CaO(CaMnO3)(m) compounds is governed by phonon scattering on CaO/CaMnO3 boundaries for m = 1, 2, and 3 and by Umklapp processes for m = infinity. Thermal transport in this system is strongly dominated by acoustic phonon modes possessing much larger Gruneisen parameters (gamma) compared to optical ones.