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