What drives the charge ordering in the rare-earth nickelates?

Usual examples of charge ordered systems are the doped transition metal oxides, where strong correlation effects at the transition metal site lead to the charge carriers being preferentially localized on some transition metal sites over others. The rare-earth (RE) nickelates of the form RENiO₃ are unusual in that the undoped parent compounds are found to be charge ordered insulators for all rare earth atoms with the exception of La _. Structural distortions accompany the charge ordering, with two Ni sites with different local environments of the oxygens emerging. Although these aspects have been identified for several decades now, the basic mechanism of what drives the charge ordering is still debated.

We have explored the origin of the charge ordering by using an ab-initio approach to introduce an additional potential on the Ni d states, and thereby vary the effective charge transfer energy for the electron between the Ni and oxygen sites. The changes on the structure as well as the electronic structure are examined. We identify the regime in which the system is charge ordered, and when it is not. This enables us to address the question of what drives the charge ordering, as well as explain why it is not ubiquitous among a larger class of systems beyond the nickelates.