Tuning the electronic ground state of perovskite nickelates with dynamic electrochemical doping

The rare earth nickelates have received renewed attention due to the discovery of superconductivity in infinite layered structures under appropriate electron filling via substitutional doping(1). Therefore, methods to study the electronic phase diagram in nickelate compounds are of interest. Here, we present work on doping PrNiO₃ using a dynamic electrochemical process. We construct electrochemical cells using epitaxial thin films as electrodes and then insert lithium using an ionic liquid electrolyte. For Li_xPrNiO₃, we find that the ground state evolves from an insulator (due to charge disproportionation) to a metal and finally back to an insulator under increasing x (or electron filling). This contrasts with previous work with substitutional doping where electron (or hole) dopants evolve the ground state only to the metallic state(2). We additionally report on the evolution of the metal insulator transition that occurs under these doping conditions. Based upon recent studies by resonant inelastic x-ray scattering(3, 4) we hypothesize that the differences with substitutional doping lie in the fact that lithium dopants introduce less strain and defects in the host lattice compared to other methods used to tune bulk electron filling(5).

1. D. Li et al., Superconductivity in an infinite-layer nickelate. Nature 572, 624-627 (2019).

2. J. L. Garcia-Munoz, M. Suaaidi, M. J. Martinez-Lope, J. A. Alonso, Influence of carrier injection on the metal-insulator transition in electron- and hole-doped R1-xAxNiO3 perovskites. Phys Rev B Condens Matter 52, 13563-13569 (1995).

3. V. Bisogni et al., Ground-state oxygen holes and the metal-insulator transition in the negative charge-transfer rare-earth nickelates. Nat Commun 7, 13017 (2016).

4. M. Hepting et al., Electronic structure of the parent compound of superconducting infinite-layer nickelates. Nat Mater 19, 381-385 (2020).

5. Y. Sun et al., Strongly correlated perovskite lithium ion shuttles. Proc Natl Acad Sci U S A 115, 9672-9677 (2018).