Impact of strain-induced electronic topological transition on the thermoelectric properties of PtCoO₂ and PdCoO₂

By a combination of first-principles calculations and semi-classical Boltzmann transport theory, we investigate the effect of epitaxial strain on the electronic structure and transport properties of PtCoO$_2$ and PdCoO$_2$. In contrast to the rather uniform elastic response of both systems, we predict for PtCoO$_2$ a high sensitivity of the out-of-plane transport properties to strain, which is not present in PdCoO$_2$. At ambient temperature, we identify a considerable absolute change in the thermopower from $-107\,\mu$V/K at $-5\,$\% compressive strain to $-303\,\mu$V/K at $+5\,$\% tensile strain. This remarkable response is related to distinct changes of the Fermi surface, which involve the crossing of two additional bands at a moderate compressive in-plane strain. Combining our transport results with available experimental data on electrical and lattice thermal conductivity we predict a thermoelectric figure of merit of up to $ZT$$\,=\,$$0.25$ at $T$$\,=\,$$600\,$K for strained PtCoO$_2$.