The Role of Intrinsic Defects in the Structural Transition of Superconducting La₃Ni₂O₇: A Many-Body DMC Prediction

The pressure-induced structural phase transitions of the superconducting Ruddlesden-Popper nickelate La₃Ni₂O₇ remain under active debate. While the Amam phase is generally identified as the non-superconducting ambient-pressure structure, several higher-symmetry phases have been proposed to emerge under compression; these phases would potentially host superconductivity. Adding to this complexity, experimental and theoretical studies consistently report oxygen vacancies in La₃Ni₂O₇, raising questions about the impact of oxygen vacancies and other intrinsic defects on the material's electronic and superconducting properties. Although vacancies are known to strongly affect the electronic properties of oxides in general, their influence on the relative stability of the high-pressure phases of La₃Ni₂O₇ has not been examined. To address this gap, we employ many-body diffusion Monte Carlo to investigate the role of oxygen vacancy defects in La₃Ni₂O₇. We apply this approach across the structural phases Amam, Fmmm, I4/mmm, and Cmmm at T = 0 to obtain a comprehensive understanding of their defect energetics. Our results show that departures from oxygen stoichiometry significantly alter the stability of the high-pressure phases. In particular, the oxygen vacancy stabilizes the experimentally observed Fmmm phase at higher pressures compared to the stoichiometric phases. Our findings indicate that vacancy formation under pressure may act as a driving mechanism for the phase transition, providing new insight into the stabilization pathways of La₃Ni₂O₇ and their connection to possible superconducting behavior.