  The shandite family of solids, with hexagonal structure and composition
A3M2X2 (A = Ni,Co,Rh,Pd; M = Pb,In,Sn,Tl; X = S,Se), has attracted recent
research attention due to promising applications as thermoelectric materials.
Herein we discuss the electron and phonon transport properties of
shandite-structured Ni3Sn2S2, based on a combination of density functional
theory (DFT), Boltzmann transport theory, and experimental measurements.
Ni3Sn2S2 exhibits a metallic and non-magnetic groundstate with Ni$^0$ oxidation
state and very low charge on Sn and S atoms. Seebeck coefficients obtained from
theoretical calculations are in excellent agreement with those measured
experimentally between 100 and 600 K. From the calculation of the ratio
$\sigma$/$\tau$ between the electronic conductivity and relaxation time, and
the experimental determination of electron conductivity, we extract the
variation of the scattering rate (1/$\tau$) with temperature between 300 and
600 K, which turns out to be almost linear, thus implying that the dominant
electron scattering mechanism in this temperature range is via phonons. The
electronic thermal conductivity, which deviates only slightly from the
Wiedemann-Franz law, provides the main contribution to thermal transport. The
small lattice contribution to the thermal conductivity is calculated from the
phonon structure and third-order force constants, and is only ~2
Wm$^{-1}$K$^{-1}$ at 300 K (less than 10% of the total thermal conductivity),
which is confirmed by experimental measurements. Overall, Ni$_3$Sn$_2$S$_2$ is
a poor thermoelectric material (ZT~0.01 at 300 K), principally due to the low
absolute value of the Seebeck coefficient. However, the understanding of its
transport properties will be useful for the rationalization of the
thermoelectric behavior of other, more promising members of the shandite
family.
