Rare-earth nickelates are intringuing perovskite oxides showing metal-insulator transition tuneable by the rare-earth size, and complex antiferromagnetic order at low temperature. Yet, a complete theoretical description of their rich phase diagram was  missing. In this work just out in NPJ Quantum Materials, we have used first-principles simulations to describe their electronic and magnetic experimental ground state. We show that the insulating phase is characterized by a split of the electronic states of the two Ni sites (i.e. resembling low-spin 4+ and high-spin 2+) with a concomitant shift of the oxygen-2p orbitals toward the depleted Ni cations. Therefore, from the point of view of the charge, the two Ni sites appear nearly identical whereas they re in fact distinct. Performing such calculations for several nickelates, we have built a theoretical phase diagram that reproduces all their key features, namely a systematic dependence of the MIT with the rare-earth size and the crossover between a second to first order transition for R=Pr and Nd. Our results hint at strategies to control the electronic and magnetic phases of perovskite oxides by fine tuning of the level of covalence.

This work was performed thanks to collaboration with LIST.

Complete phase diagram of rare-earth nickelates from first-principles
J. Varignon et al ; NPJ Quant. Mater. 2, 21 (2017)