Realizing Modulations in Electron Correlations for Perovskite Mott‐System via Tunning A‐Site Covalency

ABSTRACT Although the electron correlation ( U ) within d‐ orbital perovskite Mott‐systems is the root‐cause for their unconventional functionalities, such as metal‐to‐insulator transitions (MIT), high‐ T _C superconductivity, and multiferroics, it yet lacks strategy to modulate their U . Herein, we enable the tunability in U for correlated perovskite nickelates ( RE NiO ₃ ) by manipulating their RE ‐site covalency via introducing partial Bi‐substitutions, based on which huge improvement in their electronic MIT abruptions beyond one order was achieved. The more covalent bonding between Bi‐6 s and O‐2 p enlarges the Ni‐3 d occupancy that enlarges U by 2–3 times, as indicated by synchrotron‐based X‐ray absorption spectroscopies and first principal calculations. Consequently, the ground‐state band gap ( E _g ) and resistivity are effectively increased, giving rise to significant enhancement in their resistive switches across adjustable critical temperatures ( T _MIT ) within 75–400 K, by up to 40 times. Simultaneously, the Bi‐substitutions concurrently descend T _MIT owing to their larger sizes than RE ³⁺ , indicating the prevailing dominance in the relative phase stability by the O‐2 p to Ni‐3 d charge transfer gap. This unravels the mystery in U that only electronically enlarges the ground‐state E _g and resistivity rather than determines the relative phase stability across MIT (or T _MIT ). Tuning U via A‐site covalency provides new freedom for optimizing functionalities of correlated perovskites.