Tuning The Insulator Metal Transition in Rare Earth Nickelates through Dynamic Electrochemical Ion Insertion

The rare earth nickelates have received renewed attention due to the discovery of superconductivity in infinite layered structures under substitutional doping[1] and the observation of widely tunable electronic behavior in perovskite structures for use in analog memory devices[2]. Recent work has demonstrated that interstitial dopants (H, alkali metals) can be introduced into nickelates to change the room temperature resistance by 10⁶ -10⁸. However, the evolution of the bond disproportionation transition as a function of interstitial dopants has not been reported and the doping fraction leading to rich correlated electronic behavior is often unknown. Therefore, the electronic phase diagram in nickelate compounds as a function of interstitial doping is of interest. Here, we carried out lithium doping of PrNiO₃ using a dynamic electrochemical process. We constructed electrochemical cells using epitaxial thin films as electrodes and then insert lithium using an electrolyte. For Li_xPrNiO₃, we find that increased lithium doping interrupts bond disproportionation causing a reduction in the ground state resistivity at small fractions 0&lt;x&lt;0.25 with a successively smaller ON/OFF ratio. At larger fractions x&gt;0.25 we observe the disproportionation transition to be destroyed and fully insulating type behavior is observed over T= 5-300K. Raman spectroscopy reveals that lithium introduces structural changes that affect A1g modes which are a sensitive probe of bond disproportionation. Density functional theory calculations confirm the disruption to bond disproportionation with an initial reduction in the bandgap at small fractions and an increase at larger fractions. The results point to interstitial doping as a powerful method to synthesize new phases in strongly correlated systems and for next generation memory devices.<br/>[1] D. Li<i> et al.</i>, "Superconductivity in an infinite-layer nickelate," <i>Nature, </i>vol. 572, no. 7771, pp. 624-627, Aug 2019, doi: 10.1038/s41586-019-1496-5.<br/>[2] H. T. Zhang<i> et al.</i>, "Reconfigurable perovskite nickelate electronics for artificial intelligence," <i>Science, </i>vol. 375, no. 6580, pp. 533-539, Feb 4 2022, doi: 10.1126/science.abj7943.