Ultralow thermal conduction and impurity scattering in Cu2HgSnS4 : An Hg-harnessed diamondlike semiconductor for thermoelectric devices

Among the most sought-after chalcogenides, Hg-based diamondlike semiconductors have recently been realized for ultralow low heat conduction and potential thermoelectric devices. We present a less well-known but promising quaternary Hg-based chalcogenide, ${\mathrm{Cu}}_{2}{\mathrm{HgSnS}}_{4}$ (CHTS), for its electrical and thermal transport properties and compare it to its parent compound ${\mathrm{Cu}}_{2}{\mathrm{ZnSnS}}_{4}$ (CZTS) for better insight. CHTS is observed with distinct Hg-S and other bond pair strengths, but CZTS has similar cation-anion bond strengths, signifying anharmonicity caused by the heavy element Hg. Phonon dispersion in the Hg-dominated top acoustic frequency zone is nearly flat, indicating reduced phonon group velocity. The avoided-crossing characteristic, a sign of coupling in both high-frequency acoustic and lower-frequency optical modes, is also evident in the same region. The Hg atom's weak binding and considerable vibration with avoided crossing resemble cagelike skutterudite rattling. An ultralow glasslike average lattice thermal conductivity (${\ensuremath{\kappa}}_{l}$) of $\ensuremath{\sim}0.53\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 300 K is noticed and associated with overall lower phonon group velocity, mild anharmonicity, and a lower avoided-crossing frequency. At higher temperatures, ${\ensuremath{\kappa}}_{l}$ decreases to $0.23\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 573 K, comparable to typical Hg-based tellurides such as ${\mathrm{Cu}}_{2}{\mathrm{HgGeTe}}_{4}$ and ${\mathrm{Cu}}_{2}{\mathrm{HgSnTe}}_{4}$ (${\ensuremath{\kappa}}_{l}<0.25\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$) and noticeably smaller than the ${\ensuremath{\kappa}}_{l}$ of known ${\mathrm{Cu}}_{2}{\mathrm{HgSnSe}}_{4}$ nanoparticles. A large power factor for $p$-type doped structures is ascribed to an increased Seebeck coefficient because of the high-density-of-state effective mass close to the valence-band edge. The isoelectronic environment of CZTS and CHTS band edges should make them conduct similarly. This is further confirmed while comparing the estimated electrical conductivity of CHTS with the experimental data of CZTS for moderate doping concentration. Furthermore, in modest concentrations, holes with Ionized impurity scattering as their principal limiter have lifetimes of $\ensuremath{\sim}10$ fs. A $p$-type Figure of Merit (ZT) value $>2.0$ is obtained with moderate doping of $\ensuremath{\sim}{10}^{18}$ to ${10}^{19}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ at 700 K. These remarkable ZT and ultralow lattice thermal conductivity characteristics suggest CHTS's promise for intermediate-temperature thermoelectricity, making it easier to implement devices using the same material.