Examples of inhomogeneous mixed valence

In contrast, to homogeneous mixed-valence the inhomogeneous mixed-valence case involves a mixture of different integer valence ions which occupy inequivalent lattice sites in a static charge-ordered array. Examples of this are provided by Fe3O4, Eu3O4 and Eu3S4.

The following material and figure is copied from the chemexplore web site

Magnetite (Fe3O4) has the AM₂X₄ spinel structure, of the "inverse" type :

Magnetite has the empirical formula Fe₃O₄, or Fe²⁺(Fe³⁺O₂)₂, “ferrous ferrite”. Its formula as a spinel would be Fe³⁺_tetFe²⁺_octFe³⁺_octO₄ , where "tet" and "oct" stand for tetrahedral and octahedral coordinations by the oxide anions. In the above model, the blue spheres represent the tetrahedral iron(III) cations , and the red spheres are the octahedrally coordinated iron(II) and (III) cations. The oxide anions are shown as the green spheres. Because of the fortuitous inverse nature of the magnetite structure, ferrous and ferric cations are both in the similar octahedral coordination by oxides. In "normal" spinels, such as the mineral spinel itself (magnesium aluminate), the A cation is tetrahedral and the M cations are both octahedral:

However, this inverse-spinel charge ordering has recently been challenged in favour of the normal spinel charge structure where the Fe3+ ions exclusively occupy all the octahedral sites while the Fe2+ ions reside in the tetrahedral sites.

What happens in higher order oxides of cerium is not so simple, as discussed here.