  In this work, both thermal and electrical transport properties of
diamond$-$cubic Si (Si$-$I) and metastable R8 phase of Si (Si$-$XII) are
comparatively studied by using first$-$principles calculations combined with
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 the electronic transport properties, although Si$-$XII with
smaller band gap (0.22 eV) shows lower Seebeck coefficient, the electrical
conductivities of anisotropic $n$$-$type Si$-$XII show considerable values
along $x$ axis due to the small effective masses of electron along this
direction. The peaks of thermoelectric figure of merit ($ZT$) in $n$$-$type
Si$-$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 $x$ axis at
carrier concentration of $2.6\times10^{19}$, $4.1\times10^{19}$, and
$4.8\times10^{19}$~cm$^{-3}$, respectively. The reported results elucidate that
the metastable Si could be integrated to the thermoelectric power generator.
