Insulator–Bad Metal Transition in RNiO3 Nickelates Beyond the Hubbard Model and Density Functional Theory

The insulator–bad metal transition observed in the Jahn–Teller magnets orthonickelates RNiO 3 (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 [NiO 6 ] 10– and spinless $$(S = 0)$$ hole [NiO 6 ] 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 NiO 6 centers. Model T –R phase diagram reproduces main features of the phase diagram found for RNiO 3 .