Low lattice thermal conductivity in Zintl phases Na2AuBi and Na2AuSb : An ab initio study

In search of new prospects for room temperature thermoelectric applications, we computationally investigated the electronic structure and transport properties of Zintl phases ${\mathrm{Na}}_{2}\mathrm{AuBi}$ and ${\mathrm{Na}}_{2}\mathrm{AuSb}$. We find remarkably low room temperature lattice thermal conductivity of 0.46 and 0.73 W ${\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ for ${\mathrm{Na}}_{2}\mathrm{AuBi}$ and ${\mathrm{Na}}_{2}\mathrm{AuSb}$, respectively. The proposed values are competitive with the record figure of merit material SnSe. We have demonstrated that such low thermal conductivity finds its origin majorly in low phonon group velocities and short phonon lifetime, arising from atomic displacements and lattice anharmonicity. We further predict a high figure of merit, i.e., $ZT\ensuremath{\sim}$ 1 at 300 K for $p$-type ${\mathrm{Na}}_{2}\mathrm{AuBi}$. The value is further improved to $ZT\ensuremath{\sim}$ 1.8 with spin-orbit coupling. We hope through this work we could shed light on two potential Zintl phases which would motivate further experimental investigations.