  Electrical and thermal transport properties of disordered materials have long
been of both theoretical interest and engineering importance. As a new class of
materials with an intrinsic compositional disorder, high/medium-entropy alloys
(HEAs/MEAs) are being immensely studied mainly for their excellent mechanical
properties. By contrast, electrical and thermal transport properties of
HEAs/MEAs are less well studied. Here we investigate these two properties of
silicon (Si)-germanium (Ge)-tin (Sn) MEAs, where we keep the same content of Si
and Ge while increasing the content of Sn from 0 to 1/3 to tune the
configurational entropy and thus the degree of compositional disorder. We
predict all SiyGeySnx MEAs to be semiconductors with a wide range of bandgaps
from near-infrared (0.28 eV) to visible (1.11 eV) in the light spectrum. We
find that the bandgaps and effective carrier masses decrease with increasing Sn
content. As a result, increasing the compositional disorder in SiyGeySnx MEAs
enhances their electrical conductivity. For the thermal transport properties of
SiyGeySnx MEAs, our molecular dynamics simulations show an opposite trend in
the thermal conductivity of these MEAs at room temperature, which decreases
with increasing compositional disorder, owing to enhanced Anderson localization
and strong phonon-phonon anharmonic interactions. The enhanced electrical
conductivity and weakened thermal conductivity make SiyGeySnx MEAs with high Sn
content promising functional materials for thermoelectric applications. Our
work demonstrates that HEAs/MEAs not only represent a new class of structural
alloys but also a novel category of functional alloys with unique electrical
and thermal transport properties.
