Modelling & Simulation


Thermoelectric properties of epitaxial LaNiO3/SrTiO3 superlattices and PdCoO2 & PtCoO2 delafossites under biaxial strain

Benjamin Geisler, University of Duisburg-Essen, Duisburg, Germany
Ariadna Blanca-Romero, Imperial College, London, United Kingdom
Markus Ernst Gruner, University of Duisburg-Essen, Duisburg, Germany
Ulrich Eckern, University of Augsburg, Augsburg, Germany
Rossitza Pentcheva, University of Duisburg-Essen, Duisburg, Germany

Thermoelectric materials which are at the same time abundant and environmentally friendly are highly relevant for the conversion of waste heat into electricity. A better microscopic understanding of the thermoelectric properties of existing nanoscale functional oxides is mandatory for designing novel materials and material combinations. In our ab initio approach we combine accurate DFT+U calculations to determine the atomic and electronic structure with Boltzmann transport theory in the constant relaxation time approximation.

First, we show how a targeted design of the interface composition can be used to optimize the thermoelectric properties of oxide superlattices like LaNiO3/SrTiO3(001). TiO2/LaO interfaces induce n-type doping, which results in a negative cross-plane Seebeck coefficient (-25 muV/K around room temperature), and tunneling transport through the SrTiO3 barriers. In contrast, NiO2/SrO interfaces lead to p-type doping and to a four times larger, positive cross-plane Seebeck coefficient, which is caused by the valence band of SrTiO3. In both cases, the in-plane Seebeck coefficient is quite small and negative with metallic conductance via NiO2 bands.

Second, we discuss the hexagonal delafossites PdCoO2 and PtCoO2, which exhibit highly anisotropic transport properties including large negative values of the thermopower perpendicular to the hexagonal plane. Despite their rather similar structural response, the isoelectronic compounds differ significantly in their transport and thermoelectric properties under tensile and compressive biaxial strain [1]. This is related to specific changes of the Fermi surface involving an electronic topological transition, which occurs in PtCoO2 at moderate compressive strain. Combining our first-principles results with available experimental data, we estimate a maximum figure of merit (ZT) of 0.25 obtained for PtCoO2 at 600 K and 4% compressive strain.

[1] M. E. Gruner, U. Eckern, and R. Pentcheva, Phys. Rev. B 92, 235140 (2015)

Funding by the DFG (TRR 80) is gratefully acknowledged.

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