We
report the phase evolution and thermoelectric properties of
a series of Co(Ge0.5Te0.5)3–x
Sb
x
(x = 0–0.20) compositions synthesized by mechanical alloying.
Pristine ternary Co(Ge0.5Te0.5)3 skutterudite
crystallizes in the rhombohedral symmetry (R3̅),
and Sb doping induces a structural transition to the cubic phase (ideal
skutterudite, Im3̅). The Sb substitution increases
the carrier concentration while maintaining a high thermopower even
at higher doping levels owing to an increased effective mass. The
exceptional electronic properties exhibited by Co(Ge0.5Te0.5)3 upon doping are attributed to the carrier
transport from both the primary and secondary conduction bands, as
shown by theoretical calculations. The enhanced electrical conductivity
and high thermopower increase the power factor by more than 20 times.
Because the dominant phonon propagation modes in binary skutterudites
are associated with the vibrations of pnictogen rings, twisting the
latter through the isoelectronic replacement of Sb4 rings
with Ge2Te2 ones, as done in this study, can
effectively reduce the thermal conductivity. This leads to an increase
in the dimensionless figure-of-merit (zT) by a factor
of 30, reaching 0.65 at 723 K for Co(Ge0.5Te0.5)2.9Sb0.1.
We have performed combined elastic neutron diffuse, electrical transport, specific heat, and thermal conductivity measurements on the quasi–one-dimensional Ba3Co2O6(CO3)0.7 single crystal to characterize its transport properties. A modulated superstructure of polyatomic CO32− is formed, which not only interferes the electronic properties of this compound, but also reduces the thermal conductivity along the c-axis. Furthermore, a large magnetic entropy is observed to be contributed to the heat conduction. Our investigations reveal the influence of both structural and magnetic effects on its transport properties and suggest a theoretical improvement on the thermoelectric materials by building up superlattice with conducting ionic group.
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