Thermoelectric figure-of-merit of metastable crystalline ST12 germanium allotrope

Abstract

Computational science has facilitated significant improvement in identifying stable crystal systems that are good thermoelectric materials with a high figure-of-merit (ZT). One approach for further discontinuous improvement is to explore the metastable phase of the crystals, particularly because there is an increasing need for thermoelectric materials operating at room temperature in energy harvesting applications. Here, using first-principles calculations, we find that the single-crystalline metastable ST12 germanium allotrope has an much higher ZT than DC8 stable phase. The optimal ZT for p- and n-type doping is 0.22 and 0.27 at room temperature, respectively, and the values reach 0.56 and 0.87 at 500 K. The electrical and thermal conductivities are higher and significantly lower than those of stable germanium, respectively. This is attributed to the smaller carrier effective mass and denser phonon bands with a lower group velocity and larger scattering phase space. The Seebeck coefficient remains high owing to the adequate bandgap and large derivative of the electron density of states with respect to the Fermi energy at the band edges. This work demonstrates that the metastable ST12 germanium allotrope, which has been recently synthesized, is a promising candidate to realize improvement in thermoelectric materials, and this case study encourages future exploration of metastable materials.