Thermoelectric performance in the binary semiconductor compound A2Se2(A = K, Rb) with host-guest structure

The host-guest structure compounds are potential candidates for high-efficiency thermoelectric (TE) materials because of their good electronic transport combined with ultralow thermal conductivity. In this paper, we theoretically investigate TE transport properties of the hexagonal ${A}_{2}{\mathrm{Se}}_{2}\phantom{\rule{4pt}{0ex}}(A$ = K, Rb) with strongly quartic anharmonicity based on an incorporation of first-principles calculations with self-consistent phonon theory and Boltzmann transport equations. The calculation results reveal that ${A}_{2}{\mathrm{Se}}_{2}\phantom{\rule{4pt}{0ex}}(A$ = K, Rb) have ultralow lattice thermal conductivities ${\ensuremath{\kappa}}_{L}$, e.g., 0.30--0.$34{\mathrm{W}\phantom{\rule{0.16em}{0ex}}\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at room temperature, which are only one third of quartz glass. The calculated results indicate that the coexistence of small phonon lifetime ${\ensuremath{\tau}}_{\mathrm{ph}}$ and phonon group velocity ${\ensuremath{\upsilon}}_{\mathrm{ph}}$ is the reason for the ultralow ${\ensuremath{\kappa}}_{L}$ in these two host-guest structure materials. Additionally, due to the anisotropic electronic structure, that is, the coexistence of high dispersion and flat band edges, these two materials capture a high TE power factor along the $c$ axis. As a consequence, the anomalous high $\mathit{ZT}$ values of 2.95 and 2.17 at 500 K along the $c$-axis direction are captured in $n$-type ${\mathrm{K}}_{2}{\mathrm{Se}}_{2}$ and ${\mathrm{Rb}}_{2}{\mathrm{Se}}_{2}$, which break the long-term record of $\mathit{ZT}\phantom{\rule{4pt}{0ex}}<2$ in most TE materials reported so far. These findings reveal that ${A}_{2}{\mathrm{Se}}_{2}\phantom{\rule{4pt}{0ex}}(A$ = K, Rb) has broad prospects in TE applications.