Reconciling the ionic and covalent pictures in rare-earth nickelates

TL;DR

This paper uses first-principles calculations to reconcile ionic and covalent descriptions of the insulating state in rare-earth nickelates, revealing a complex interplay that influences their electronic and magnetic properties.

Contribution

It demonstrates that the insulating state of nickelates combines ionic charge disproportionation with covalent effects, providing a unified understanding of their ground state.

Findings

Insulating state resembles ionic charge disproportionation with distinct Ni sites.

Strong covalence effects involve oxygen electrons shifting toward Ni cations.

Results suggest strategies to control electronic and magnetic phases in transition metal oxides.

Abstract

The properties of AMO3 perovskite oxides, where M is a 3d transition metal, depend strongly on the level of covalency between the metal d and oxygen p orbitals. With their complex spin orders and metal-insulator transition, rare-earth nickelates verge between dominantly ionic and covalent characters. Accordingly, the nature of their ground state is highly debated. Here, we reconcile the ionic and covalent visions of the insulating state of nickelates. Through first-principles calculations, we show that it is reminiscent of the ionic charge disproportionation picture (with strictly low-spin 4+ and high-spin 2+ Ni sites) while exhibiting strong covalence effects with oxygen electrons shifted toward the depleted Ni cations, mimicking a configuration with identical Ni sites. Our results further hint at strategies to control electronic and magnetic phases of transition metal oxide perovskites.