Transition metal oxides showing the following chemical formula ABO₃ and A₂BO₄ are mostly known as perovskite-type and K₂NiF₄ type structure materials respectively. Their ability to undergo reversible oxydo-reductions chemical reaction are technologically very attractive as electrodes in fuel cells (SOFC), as oxygen storage materials in anaerobic processes and as three way catalysts (TWC’s).  In addition, it is the control of oxygen released and/or up taken that opens ways towards huge change of their physical properties such as metal / insulator transition and/or antiferromagnetic to ferromagnetic transition.

When a single transition metal is introduced on the B crystallographic site of the two chemical formula, its oxidation state is indeed in direct relationship with the oxygen content considering that there are no A cation deficiency. However, when two transition metal cations are introduced on the B crystallographic site, our recent results highlight that their relative oxidation states are closely related to a metal to metal charge transfer giving unexpected oxidation states distribution.
Following an overview of all the materials in which a distribution of oxidation state has been met considering magnetoresistant manganites, high Tc copper superconductor, non-centrosymmetric phosphate, battery materials and/or doped VO₂ compounds for thermochromics properties, our presentation will be focus on SrFe_1-xCo_xO_3-y materials. Our study coupling powder neutron diffraction, Mossbauer spectroscopy and Mohr Salt titration highlights that the distribution of cobalt and iron oxidation states is heterogeneous in the crystallographic sites, indicating that oxidation of cobalt cation requires higher oxidation potential than for the iron one. The correlations in between the exhibited paramagnetic to ferromagnetic transition around room temperature, lattice cell parameters and oxidation state distributions will be emphasized.