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

Highlights: • Thermal transport is dominated by phonon scattering by CaO/CaMnO3 boundaries. • Acoustic phonon modes of CaO(CaMnO3)m system are dominated by Ca-atom oscillations. • U-scattering is governed by the Grüneisen parameters and sound velocities anisotropy. • Grüneisen parameter increases outstandingly when approaching at the Γ point. Vibrational properties of CaO(CaMnO₃)_m (m = 1, 2, 3, and ∞) 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, ∞) series, which predicts enhanced phonon scattering upon Ca-sublattice site substitution defects. Accordingly, laser flash analysis (LFA) indicates considerable decrease of thermal conductivity (κ) due to La- substitution for Ca, whereas Nb- substitution for Mn-sites does not affect κ 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 = ∞. Thermal transport in this system is strongly dominated by acoustic phonon modes possessing much larger Grüneisen parameters (γ) compared to optical ones.