Ga4C-family crystals, a new generation of star thermoelectric materials, achieved by band degeneracies, valley anisotropy, and strong phonon scattering among others

Abstract

Using density functional theory combined with the Boltzmann transport equation, we uncover a new class of high-performance thermoelectric materials, i.e., supertetrahedral Ga₄C-family materials characterized by an ultrahigh thermoelectric figure of merit (ZT), which is produced both by a large power factor and ultralow lattice thermal conductivity. The former is contributed by multiple electronic band degeneracies, flat bands and valley anisotropy, while the latter is contributed both by strong phonon scattering and low phonon group velocity. Furthermore, we find that tensile strain engineering may further suppress lattice thermal conductivity, while well retaining the multi-band degeneracies, valley anisotropy and flat bands. In addition to the band degeneracy, the scattering mechanism of the system can also dominate the power factor in the process of strain implementation. Consequently, a novel thermoelectric phenomenon, i.e., a high thermoelectric figure of merit, is observed in Ca₄C with a ZT value larger than 4.4 under a small strain ratio (0.75%). Our theoretical studies not only uncover a new class of high-performance thermoelectric materials integrating multiple effective mechanisms, but also establish a realistic material plateau to investigate the competitive effect of multiple factors on enhancing the thermoelectric figure of merit.