Also shown in Fig. 3a are results for Ca3Co4O9 doped with transition metals. Copper, when replacing cobalt, is consistently reported to increase the electrical conductivity,but also to decrease the Seebeck coefficient.46,53,66,67 Copper is reported to occupy sites in the Ca2CoO3 rock salt layer rather than the CoO2 layer of Ca3Co4O9. 46 There is one report in which copper was used to replace calcium, rather than cobalt, and led to an increase in the Seebeck coefficient.68 However, Xray diffraction results showed the presence of a second phase, which suggests that copper is not stable on the calcium sites and the properties may be affected by the second phase. Other transition metal dopants, such as iron and manganese, occupy cobalt sites in the CoO2 layer.46 Although there are reports of increases in electrical conductivity due to doping of iron46 or nickel,67 in most cases the beneficial effect of transition metal dopants is an increase in thermopower, rather than increase