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

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 (RENiO₃) 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-6s and O-2p enlarges the Ni-3d 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 RE³⁺, indicating the prevailing dominance in the relative phase stability by the O-2p to Ni-3d 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.