Verwey Transition under Oxygen Loading in RBaFe2O5+w (R=Nd and Sm)

In order to understand the origin of good thermoelectric (TE) properties in the transition metal oxides with the lattice structure isomorphous to the 232-structure, the bond nature between Co and O ions in Bi1.5Pb0.5Ca2-xMxCo2O8-delta-system has been tried to vary by replacing M with Sc3+, Y3+ or La3+ and by changing x from 0 to 0.3. The resistivity is minimum at x = 0.1 in Scand Y-systems, but very high in La-system. The large thermopower is obtained in every compound. The experimental TE properties have been discussed mainly within the framework of the charge-transfer scheme in which the ionic radii of Sc3+ and Y3+ smaller than Ca2+ reduce the energy between O 2p levels and Co eg parentages but the large ionic radius of La3+ expands it. The oxygen solubility in the compounds and the lattice distortion peculiar to the 232-structure are also likely to contribute somewhat to the experimental results.