The insulator–bad metal transition observed in the Jahn–Teller magnets orthonickelates RNiO3 (R = rare earth or yttrium Y) is considered to be a canonical example of the Mott transition, traditionally described in the framework of the Hubbard \(U\) – \(t\) -model and the density functional theory. However, actually the real insulating phase of nickelates is the result of charge disproportionation (CD) with the formation of a system of spin-triplet \((S = 1)\) electron [NiO6]10– and spinless \((S = 0)\) hole [NiO6]8– centers, equivalent to a system of effective spin-triplet composite bosons moving in a nonmagnetic lattice. Taking account of only charge degree of freedom we develop a novel minimal \(U\) – \(V\) – \({{t}_{b}}\) -model for nickelates making use of the charge triplet model with the pseudospin formalism and effective field approximation. We show the existence of two types of CD-phases, high-temperature classical CO-phase with the \(G\) -type charge ordering of electron and hole centers, and low-temperature quantum CDq-phase with charge and spin density transfer between electron and hole centers, uncertain valence and spin value for NiO6 centers. Model T–R phase diagram reproduces main features of the phase diagram found for RNiO3.
