A postdoctoral stay in the group of Gerard Demazeau began in may 1997 and ended in october 1998 making it a total of 18 months that I have been there. It was financed by the European Union (TMR, Marie-Curie research fellowship) the general subject being the chemistry of mixed-metal oxides containing Rhodium in high oxidation states, i.e. Rh⁴⁺, Rh⁵⁺ and Rh⁶⁺.

One should be able to stabilize these high oxidation states by the aid of high pressure preparation techniques, namely

1. preparation under high oxygen pressure (up to 2 kbar i.e. 20 MPa) in a gasous atmosphere,
2. preparation under very high static pressure (up to 150 kbar i.e. 1.5 GPa) with the possibility to introduce oxygen by the in-situ decomposition of KClO₃.

The mentioned oxidation states of Rhodium are expected to exhibit low-spin electronic configurations and should therefore lead to interesting properties of mixed-metal oxides, especially concerning electronic and magnetic behaviour.

As claimed above, we concentrated on the stabilization of SrRhO₃ and CaRhO₃, but neither one seems to exist. A phase, that is probably isostructural to Sr₆Co₅O₁₅ exists in the Sr - Rh - O system. The Rietveld-refinement of the powder diffractogramme is a bit tricky, because it is not yet clear whether this compound obeys a so called "composite structure" or simply incorporates a modulation vector. This work is still ongoing in collaboration with FranÁois Weill (ICMCB).

We have successfully prepared La₂M²⁺Rh⁴⁺O₆ (M = Co, Mg, Mn, Ni, Zn). Most results for theses perovskites are published. Those containing diamagnetic M (Mg, Zn) are paramagnetic insulators with a reduced magnetic moment on Rh⁴⁺. The Ni-compound is a spin-glass with a freezing temperature at 10 K. The Mn-compound is ferromagnetic due to partial B-site ordering. Structural survey, magnetic and electrical studies of the other compounds are ongoing.

You may wish to have a look into the intermediate report or the final report to the European Comission. Both are made readable in HTML.