Non-magnetic insulating phase induced by Jahn-Teller effect in RNiO₃

We propose a three-dimensional multi-orbital tight-binding model for rare-earth nickelates RNiOthat treats charge, spin, orbital, and lattice degrees of freedom on equal footing. All model parameters, including the on-site interactionsandand the electron-phonon (el-ph) coupling to the breathing mode, are extracted from hybrid-functional density functional theory calculations for the small-bandwidth nickelate LuNiO. The model describes three competing insulating phases governed by the interplay ofand el-ph coupling to the breathing and Jahn-Teller (JT) modes. For large, the insulating state is stabilized by local JT distortions on high-spin Nisites. For smaller, the system undergoes charge disproportionation,, resulting in the spin-polarized charge-ordered state observed experimentally below the Néel temperature in small-bandwidth RNiO. When the JT energy on the Nisite exceeds Hund's exchange, a distinct charge- and orbital-ordered insulating phase emerges in which the two-electrons occupy the same orbital with opposite spin. The stability of this phase is further confirmed by self-consistent calculations within the full three-dimensional tight-binding model. This newly predicted metastable state, characterized by JT distortions in a nonmagnetic charge-ordered RNiOphase, shows that the onset of magnetic order is not required for the metal-insulator transition in RNiO.