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

Abstract

The partial substitution of Co by Rh in the [Pb{sub 0-}bar {sub 7}Co{sub 0.4}Sr{sub 1.9}O{sub 3}]{sup RS}[CoO{sub 2}]{sub 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 CdI{sub 2}-like layer. Thus, a generic formula [Pb{sub 0.7}(Co{sub 0.4-z}Rh{sub z})Sr{sub 1.9}O{sub 3}]{sup RS}[Co{sub 1-y}Rh{sub y}O{sub 2}]{sub 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 +140{mu}V/K at room temperature. The Rh cation can adopt a mixed valency Rh{sup 3+}/Rh{sup 4+} (4d{sup 6}/4d{sup 5}) with low spin states t{sub 2g}{sup 6}/t{sub 2g}{sup 5} equivalent to the ones of low spin Co{sup 3+}/Co{sup 4+} (3d{sup 6}/3d{sup 5}). The large thermopower observed in the Rh substituted compounds is therefore a direct proof that the coexistence of low spin states t{sub 2g}{sup 6}/t{sub 2g}{sup 5} contributes to the thermoelectric power enhancement in these oxides.