From oxides to selenides and sulfides: The richness of the CdI₂ type crystallographic structure for thermoelectric properties

In this paper, we show how the thermoelectric properties can be modified in crystallographic structures based on the CdI₂ type layer, by changing the block layers between these CdI₂ type layers or by going from oxides to selenides and sulfides. In the case of oxides, the unique properties generated in these structures will be demonstrated in Bi-based misfit compounds. By combining Hall effect, resistivity, and Seebeck coefficients in single crystals of this family, the importance of doping and of spin and orbital degeneracy term on the Seebeck coefficient will be shown. From this single crystal investigation, the power factor at 300 K is found to be unexpectedly constant as a function of doping. To further enhance the power factor and thus ZT, it is necessary to modify either the block layer or to perform anionic substitutions. By going from oxides to selenides and sulfides, the decrease of the ionic character can induce a decrease of electrical resistivity. Compared to oxides, the properties can generally be described in a more classical way using Boltzmann transport theory. For these materials, the critical parameter is then thermal conductivity and this quantity can be decreased as shown here by intercalating Cu between the layers (Cu_xTiS₂), or by making solid solution such as TiS_{2 - x}Se_x. These two approaches will be described here, leading to ZT close to 0.5 and 0.4, at 800 and 700 K, respectively. The CdI₂ type crystallographic structure is a very rich family in which oxides and non-oxide materials can be synthesized. For oxides, Na_xCoO₂ has already been found to be a promising p-type thermoelectric material. The aim of this Feature Article is first to analyze the thermoelectric properties of misfit cobalt oxides, showing the relationship between this layered structure which stabilizes Co³⁺ and Co⁴⁺ in low spin states, and transport properties. By changing the separating block layer, or by modifying oxides into selenides or sulfides, it is possible to strongly modify the transport properties. The second part of this Feature Article reports the interesting thermoelectric properties of sulfides and selenides, with ZT up to 0.5 at 800 K, and shows the differences between these properties and that of oxides.