One dimensional compounds with large thermoelectric power: Ca3Co2O6 and Ca3CoMO6 with M=Ir4+ and Rh4+

In the Ca–Co–O system, there exists at least two identified phases, Ca3Co4O9 and Ca3Co2O6 exhibiting 2D CoO2 planes and 1D Co–Co chains, respectively. Both oxides are characterized by large thermopower values at room temperature, the largest value being obtained for Ca3Co2O6 (Seebeck coefficient, S300 K=450 μV K−1). However, the electrical resistivity of the latter is too large (typically 50 Ω cm) for applications as thermopower materials. Interestingly, the substitutions of Ir4+ or Rh4+ for one cobalt out of two in the chain do not greatly affect the electronic properties in contrast to the substitution by Sc3+ (3d0). This emphasizes that the Ir or Rh cations can play a similar role to that of the Co cations. This opens a new route for the search of new thermoelectric materials alternative to the cobaltites.