The compound Cu
12
Sb
4
S
13
, known as tetrahedrite, is an eco-friendly p-type thermoelectric material with earth-abundant, low-cost, and less toxic constituents. This contemporary work studied the ...thermoelectric properties of Gd and Se double-substituted tetrahedrites. The samples with nominal composition Cu
11.95
Gd
0.05
Sb
4
S
13−
x
Se
x
(where
x
= 0, 0.2, 0.4, 0.6, and 0.8) were prepared using the solid-state synthesis and sintered using high vacuum hot pressing. The structural analysis using X-ray diffraction revealed a successful tetrahedrite phase formation, and the systematic increase of lattice parameters with the selenium content (
x
) indicated successful Se substitution. The electron probe microanalysis revealed the presence of secondary phases Cu
3
SbS
4
, CuSbS
2
, and CuGdS
2
. The Raman spectroscopy showed a weakening Sb-S/Se bond with increased Se content in the samples. The +3 and −2 oxidation states of Gd and Se, respectively, were confirmed from the XPS study. Gd
3+
, a higher valence substituent at the Cu
+
/Cu
2+
tetrahedral site, helped reduce the carrier concentration. On the other hand, the isoelectronic substitution of Se
2−
for S
2−
enhanced the thermopower and power factor by introducing resonant energy states near the Fermi level. Consequently, a maximum power factor of ∼1.35 (±0.08) mW m
−1
K
−2
at ∼729 K was obtained for the sample
x
= 0.4. The sample
x
= 0.4 also exhibited the lowest thermal conductivity ∼ 1.18 (±0.04) W m
−1
K
−1
at 729 K. The Callaway model indicates that the lowering in the lattice thermal part of conductivity of this sample is due to the combined effect of point defect and Umklapp scattering. The simultaneous improvement of carrier concentration and thermal conductivity resulted in a relatively high
zT
of ∼0.83 (±0.09) in the
x
= 0.4 sample.
Simultaneous reduction of carrier concentration and enhancement of density of states near Fermi energy leading to enhanced thermoelectric figure of merit in Cu
12
Sb
4
S
13
tetrahedrite.
The present study reports the structural, electronic, elastic, and thermoelectric properties of Cu12Sb3.9Te0.1S13-xSex (x = 0, 0.1, 0.5, 0.75 and 1) tetrahedrite. The thermal conductivity was reduced ...due to charge carrier compensation (by Te), weakening of the Sb-S bond and local chemical disorder (induced by Se), resulting in a low mean sound velocity. Simultaneously, the power-factor was maintained high from an enhancement of the density of states near the Fermi level due to Se. Consequently, a maximum figure of merit ~0.84 at 673 K was achieved for the Cu12Sb3.9Te0.1S12.5Se0.5 sample due to concomitant optimisation of the power-factor and thermal conductivity.
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Filling the voids of a skutterudite material using suitable electropositive elements majorly affects the power factor by altering the carrier concentration. The mass difference of the filler with ...constituent elements leads to a lower lattice thermal conductivity and consequently increases the figure of merit. The combination of doping and filling can further improve thermoelectric properties. Usually, the type of bonding between the fillers and host atoms is decided by the difference in their electronegativity values. Electropositive elements have been successfully used as n-type fillers in Co4Sb12. The doping of electron donors at the Co or Sb site helps to incorporate electronegative elements into the voids by the charge compensation between a n-type dopant and a p-type filler. In this study, a p-type dopant Fe has been substituted in the Co site of S0.15Co4Sb12. A series of S0.15Fe x Co4–x Sb12 (x = 0.05–0.9) samples were prepared by a solid-state reaction. CoSbS, Sb secondary phases were noticed in the samples with x = 0.05 and 0.1 and FeS, Sb secondary phases were observed in the samples with x ≥ 0.2. The filling content of S into the void was ∼0.04 for S0.15Co4Sb12, which decreased with the increasing Fe content. The Seebeck coefficient was positive, indicating that holes are the majority of charge carriers. An increase in carrier concentration was observed with the increase in the Fe content, which reduced the Seebeck coefficient and electrical resistivity. The thermal conductivity was reduced for the samples with x = 0.1 and 0.9 compared to the sample S0.15Co4Sb12 due to Fe doping in the Co site and S filling in the void site. The highest zT of ∼0.30 at 773 K was obtained for S0.15Fe0.9Co3.1Sb12.
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