New Misfit Cobaltites [Pb_0.7A_0.4Sr_1.9O₃][CoO₂]_1.8 (A = Hg, Co) with Large Thermopower

New misfit cobaltites, containing lead and mercury, have been synthesized, [Pb 0.7 Sr 1.9 Co 0.4 O 3 ][CoO 2 ] 1.8 and [Pb 0.7 Hg 0.2 Sr 1.9 Co 0.2 O 3 ][CoO 2 ] 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 CdI 2 -type [CoO 2 ] ∞ 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 Ca 3 Co 4 O 9 . 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 ≈ 105 and 115 μV K - 1 for the Pb and (Pb/Hg)-based cobaltites, respectively, small resistivity values, ρ 300K ≈ 2 × 10 - 2 Ω cm, and the thermal conductivity measurements yield κ ≈ 2 × W K - 1 m - 1 . The figure of merit ( Z ) of these oxides is close to that of NaCo 2 O 4 .