New misfit cobaltites [Pb0.7A0.4Sr1.9O3][CoO2]1.8 (A = Hg, Co) with large thermopower

New misfit cobaltites, containing lead and mercury, have been synthesized, [Pb0.7Sr1.9Co0.4O3][CoO2](1.8) and [Pb0.7Hg0.2Sr1.9Co0.2O3][CoO2](1.8). Their structural study, using high resolution electron microscopy and X-ray powder diffraction, shows that the samples are perfectly homogeneous and consist of a regular stacking of CdI2-type [CoO2](infinity) layers stacked with triple rocksalt type layers [Pb(0.7)A(0.4)Sr(1.9)O(3)] (A = Hg, Co), similarly to the misfit structure of Ca3Co4O9. The intermediate rock salt layer contains lead, strontium mercury, and cobalt cations distributed at random, like in the 1201 and 1212 structures of cuprates and cobaltites. The investigation of the transport properties at 300 K shows very large thermopower values, S-300K approximate to 105 and 115 muV K-1 for the Pb and (Pb/Hg)-based cobaltites, respectively, small resistivity values, rho(300K) approximate to 2 x 10(-2) Omega cm, and the thermal conductivity measurements yield kappa approximate to 2 x W K-1 m(-1). The figure of merit (Z) of these oxides is close to that of NaCo2O4.