In this work, both thermal and electrical transport properties of diamond-cubic Si (Si-I) and metastable R8 phases of Si (Si-XII) are comparatively studied by using first-principles calculations combined with the Boltzmann transport theory. The metastable Si-XII shows one magnitude lower lattice thermal conductivity than stable Si-I from 300 to 500 K, attributed from the stronger phonon scattering in three-phonon scattering processes of Si-XII. For electronic transport properties, although Si-XII with smaller bandgap (0.22 eV) shows a lower Seebeck coefficient, the electrical conductivities of anisotropic n-type Si-XII show considerable values along the x axis due to the small effective masses of electrons along this direction. The peaks of the thermoelectric figure of merit (ZT) in n-type Si-XII are higher than that of p-type ones along the same direction. Owing to the lower lattice thermal conductivity and optimistic electrical conductivity, Si-XII exhibits larger optimal ZT compared with Si-I in both p- and n-type doping. For n-type Si-XII, the optimal ZT values at 300, 400, and 500 K can reach 0.24, 0.43, and 0.63 along the x axis at carrier concentrations of 2.6×1019,4.1×1019, and 4.8×1019 cm−3, respectively. The reported results elucidate that the metastable Si could be integrated to the thermoelectric power generator.

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