Partial substitution of rhodium for cobalt in the misfit [Pb0.7Co0.4Sr1.9O3]RS[CoO2]1.8 oxide

The partial substitution of Co by Rh in the [Pb0-bar 7Co0.4Sr1.9O3]RS[CoO2]1.8 family has been investigated. By transmission electron microscopy and X-ray powder diffraction, it is shown that the substitution of Rh for Co takes place at the two cobalt sites of the structure but for the low enough Rh contents, this substitution is made preferentially at the level of the CdI2-like layer. Thus, a generic formula [Pb0.7(Co0.4-zRhz)Sr1.9O3]RS[Co1-yRhyO2]b1/b2 (0=<y=<0.5 and 0=<z=<0.3) can be proposed for this new family of misfit phase. As observed for the pure misfit cobaltite, the thermoelectric power is also very large, close to +140μV/K at room temperature. The Rh cation can adopt a mixed valency Rh3+/Rh4+ (4d6/4d5) with low spin states t2g6/t2g5 equivalent to the ones of low spin Co3+/Co4+ (3d6/3d5). The large thermopower observed in the Rh substituted compounds is therefore a direct proof that the coexistence of low spin states t2g6/t2g5 contributes to the thermoelectric power enhancement in these oxides