Sr-Doping-Modulated Metal-Insulator Transition in NdNiO_3 Epitaxial Films

Perovskite-structured nickelates, ReNiO_3 (Re = rare earth), have long garnered significant research interest due to their sharp and highly tunable metal-insulator transitions (MITs). Doping the parent compound ReNiO_3 with alkaline earth metal can substantially suppress this MIT. Recently, intriguing superconductivity has been discovered in doped infinite-layer nickelates (ReNiO_2), while the mechanism behind A-site doping-suppressed MIT in the parent compound ReNiO_3 remains unclear. To address this problem, we grew a series of Nd_1-xSr_xNiO_3 (NSNO, x =0-0.2) thin films and conducted systematic electrical transport measurements. Our resistivity and Hall measurements suggest that Sr-induced excessive holes are not the primary reason for MIT suppression. Instead, first-principles calculations indicate that Sr cations, with larger ionic radius, suppress breathing mode distortions and promote charge transfer between oxygen and Ni cations. This process weakens Ni-O bond disproportionation and Ni^2+/Ni^4+ charge disproportionation. Such significant modulations in lattice and electronic structures convert the ground state from a charge-disproportionated antiferromagnetic insulator to a paramagnetic metal, thereby suppressing the MIT. This scenario is further supported by the weakened MIT observed in the tensile-strained NSNO/SrTiO_3(001) films. Our work reveals the A-side doping-modulated electrical transport of perovskite nickelate films, providing deeper insights into novel electric phases in these strongly correlated nickelate systems.