Calcium-manganate based thermoelectric (TE) oxides of the CaO(CaMnO₃)_m (m=1,2,3,∞)-form are good candidates for high-temperature power generation, and their TE performance strongly depends on their composition, as reflected by the m-value [1]. We study vibrational properties of these compounds applying the density functional theory (DFT) to elucidate their thermal transport properties. To shed light on the lattice thermal conductivity (κ) behavior, various contributions to the phonon relaxation time (τ) and vibrational spectrum were calculated and analyzed applying well-established approaches [2].

Our calculations reveal the existence of a strong scattering process between the dispersive acoustic phonons and non-dispersive optical modes. The dispersion of the acoustic phonons is interrupted in the frequency interval 3-5 THz.

We demonstrate that the low frequency parts of phonon spectrum are strongly dominated by Ca oscillations for all compounds within the CaO(CaMnO₃)_m (m=1,2,3, ∞) series, resulting in considerable decrease of κ during Ca substitution by La. Simultaneously, Mn substitution by Nb does not affect κ substantially,  and may even result in rise of thermal conductivity compared to both undoped and La-doped systems.

We found that the thermal conductivity of CaO(CaMnO₃)_m is governed by contributions from phonon scattering on CaO/CaMnO₃ layer boundaries (for m=1, 2, and 3) and Umklapp processes having characteristic relaxation times which are inversely proportional to the square Grüneisen parameter, γ [2]. Our calculated γ-values for the acoustic branches of phonon spectrum are as great as 8.5; a value which substantially exceeds the typical value of γ~2. The suggested computational approach enables us illuminate the peculiarities of thermal conductivity of these compounds.

This study is carried out with generous support from the Ministry of Immigrant Absorption Israel and the Gerald Schwartz & Heather Reisman Foundation (Waterloo – Technion corporation).

References:

1.        A. Graff, Y. Amouyal, Effects of Lattice Defects and Niobium Doping on Thermoelectric Properties of Calcium Manganate Compounds for Energy Harvesting Applications, J. Electron. Mater. 45, 1508–1516 (2015).

2.        S. H. Lo, J. He, K. Biswas, M. G. Kanatzidis, V. P. Dravid,  Phonon scattering and thermal conductivity in p-type nanostructured PbTe-BaTe bulk thermoelectric materials, Adv. Funct. Mater. 22, 5175–5184 (2012).