We present a model approach to describe charge fluctuations and different charge phases in strongly correlated 3d oxides. As a generic model system we consider that of centers each with three possible valence states M0,± described in the framework of an S=1 pseudospin (isospin) formalism by an effective anisotropic non-Heisenberg Hamiltonian which includes two types of single-particle correlated hopping and also the two-particle hopping. Simple uniform mean-field phases include an insulating monovalent M0 phase, mixed-valence binary (disproportionated) M± phase, and a mixed-valence ternary (“under-disproportionated”) M0,± phase. We consider the first two phases in more detail, focusing on the problem of electron–hole states and different types of excitons in the M0 phase and the formation of electron–hole Bose liquid in the M± phase. The pseudospin formalism provides a useful framework for revealing and describing different topological charge fluctuations, such as, in particular, domain walls or bubble domains in antiferromagnets. Electron–lattice polarization effects are shown to be crucial for the stabilization of either phase. All the insulating systems such as M0 phase are subdivided to two classes: stable and unstable ones with respect to the formation of self-trapped charge transfer (CT) excitons. The latter systems appear to be unstable with respect to the formation of CT exciton clusters, or droplets of the electron–hole Bose liquid. The model approach suggested is believed to apply to the description of the physics of strongly correlated oxides such as cuprates, manganites, bismuthates, and other systems with charge transfer excitonic instability and/or mixed valence. We briefly discuss an unconventional scenario of the essential physics of cuprates which implies their instability with respect to the self-trapping of charge-transfer excitons and the formation of electron–hole Bose liquid.

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