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In the landscape of π-conjugated polymers, poly(3,4-ethylenedioxythiophene) doped with iron(III) p-toluenesulfonate (PEDOT:Tos) has shown promise as a thermoelectric material for near room temperature applications. Such properties are inherent to its semi-metallic nature when optimally doped leading to high electrical conductivity and a relatively good Seebeck coefficient. Nevertheless, the final thermoelectric properties of PEDOT:Tos are highly influenced by the polymerization pathways and a thorough understanding of the interplay between polymerization processes and thermoelectric properties is needed. Here, PEDOT:Tos thin films with a doping level of 22 ± 2% were produced by in situ polymerization and vapor-phase polymerization and a comparative study was performed in order to investigate the subtle correlations between morphological features and electronic signatures for both types of samples. Accordingly, optimized in situ polymerized PEDOT:Tos films were demonstrated to exhibit higher electrical conductivities (up to 4398 ± 68 S cm−1) and power factors (up to 148 ± 37 μW m−1 K−2), highlighting the importance of the polymerization process on the final thermoelectric properties.

The magnetism, magnetotransport, and Seebeck coefficients $(S)$ for three ruthenates ${\mathrm{Ba}}_{1\ensuremath{-}\ensuremath{\delta}}{M}_{2+x}{\mathrm{Ru}}_{4\ensuremath{-}x}{\mathrm{O}}_{11}$ $(\ensuremath{\delta}=0.06;$ $M=\mathrm{Mn}, \mathrm{Co}; x=0.4)$ and ${\mathrm{Sr}}_{1\ensuremath{-}\ensuremath{\delta}}{M}_{2+x}{\mathrm{Ru}}_{4\ensuremath{-}x}{\mathrm{O}}_{11}$ $(\ensuremath{\delta}=0.02; M=\mathrm{Fe}; x=0.7)$ compositions have been studied. Their crystallographic structures contain three metal sites, edge-sharing octahedra forming kagome lattices, face-shared octahedra with the shortest $\mathrm{Ru}(M)\text{\ensuremath{-}}\mathrm{Ru}(M)$ distance, and ${M\mathrm{O}}_{5}$ trigonal bipyramids. These three compositions have been selected for their transport behavior exhibiting small resistivity values (\ensuremath{\sim}m\ensuremath{\Omega} cm) together with a complex ferrimagnetic behavior, with localization increasing from $M=\mathrm{Co}$ to $M=\mathrm{Fe}$. This enabled the thermopower to be measured in hexagonal ruthenates in which the conducting kagome layers are more or less diluted by three different magnetic cations substituted for Ru. The positive Seebeck coefficient of the three compounds is found to increase up to 750 K to values in the range of 22 to $35\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{4pt}{0ex}}{\mathrm{K}}^{\text{--}1}$. Such values, similar to those of perovskite ruthenates, reveal a Seebeck coefficient dominated by the Ru network at high temperature whatever the foreign magnetic cation is. In addition, below about 50 K, the values of $S$ are very small for $M=\mathrm{Mn}$ and Co, and the $S(T)$ curves of the ${\mathrm{Ba}}_{1\ensuremath{-}\ensuremath{\delta}}{M}_{2.4}{\mathrm{Ru}}_{3.6}{\mathrm{O}}_{11}$ compounds exhibit similarities with that of ruthenium metal. This is interpreted by shorter Ru-Ru distances as compared with perovskite ruthenates allowing a metallic direct exchange. The ferrimagnetism associated with the $M$ cation does not seem to play a major role in transport, as there is almost no impact of the magnetic ordering on thermopower and electrical resistivity and the values of magnetoresistance remain very small, reaching at most \ensuremath{-}1% in 9 T at 5 K for $M=\mathrm{Mn}$, and \ensuremath{-}0.4% at ${T}_{\mathrm{C}}$ for $M=\mathrm{Co}$. The present results obtained in these phases containing hexagonal Ru networks show that Hund's metal model developed to describe the thermopower of perovskite ruthenates with a Ru square lattice can have a broader range of validity.

Investigation into the thermoelectric properties of oxides in the last 20 years has shown that optimization of thermoelectric power depends on original parameters that cannot be simply described by Boltzmann transport equations, as the entropy part of the Seebeck coefficient (S) plays a major role. The optimization of S strongly depends on carrier doping and also on the specific orbital filling of these carriers, on the presence of electronic correlations, and on carrier spins and magnetic exchanges between them. The peculiarities observed mostly in p-type oxides are presented here, and the Seebeck coefficient in different ruthenium oxides will be discussed. In these ruthenates with different crystallographic structures and electronic and magnetic ground states, and for which spin-orbit coupling plays a major role, the Seebeck coefficient is shown to converge toward a common value at very high temperatures, which depends on the spin entropy only. This highlights the dominant role of entropy in these p-type oxides, and demonstrates how spin and magnetism can strongly affect the thermopower of oxides. Recent results have shown that apart from oxides, magnetism can also be a tuning parameter in other chalcogenides such as sulfides.

International audience; The magnetic and transport properties of two hollandites, Sr1.5Ru6.1Cr1.9O16 and Ba1.5Ru6.1Cr1.9O16, crystallizing in the I4/m space group, have been investigated. The structural study shows a lack of cation ordering, and the mixed Cr and Ru occupation in the edge shared MO6 octahedra is responsible for magnetic and electrical transport properties dominated by the localized nature of the carriers, in clear contrast with the metallic and Pauli magnetism observed in ruthenium hollandites. In particular, a complex magnetic behavior together with a cluster-glass behavior and semi-conducting resistivity are observed. Interestingly, both compounds exhibit negative magneto-resistance, reaching −18.6% at 5 K and 9 T in Ba1.5Ru6.1Cr1.9O16, and −4% in Sr1.5Ru6.1Cr1.9O16. This magneto-resistance is directly associated with the variable-range hopping nature of transport and hence with the existence of localized carriers induced by the Cr for Ru substitution. In contrast, the thermopower at high temperature shows a behavior dominated by the Ru cation network, with S ∼ 30 μV K−1, consistent with those measured for other ruthenates such as SrRuO3 and CaCu3Ru4O12 quadruple perovskites, or in the KRu4O8 hollandite. The comparison of these two hollandites with the KRu4O8 hollandite enables a direct investigation of the impact of localization on the thermopower in these materials.