Structural transitions, octahedral rotations, and electronic properties of A3Ni2O7 rare-earth nickelates under high pressure

Motivated by the recent observation of superconductivity with T_c ~ 80 K in pressurized La₃Ni₂O₇¹, we explore the structural and electronic properties of A₃Ni₂O₇ bilayer nickelates (A = La-Lu, Y, Sc) as a function of pressure (0–150 GPa) from first principles including a Coulomb repulsion term. At ~ 20 GPa, we observe an orthorhombic-to-tetragonal transition in La₃Ni₂O₇ at variance with x-ray diffraction data, which points to so-far unresolved complexities at the onset of superconductivity, e.g., charge doping by variations in the oxygen stoichiometry. We compile a structural phase diagram that establishes chemical and external pressure as distinct and counteracting control parameters. We find unexpected correlations between T_c and the in-plane Ni-O-Ni bond angles for La₃Ni₂O₇. Moreover, two structural phases with significant c⁺ octahedral rotations and in-plane bond disproportionations are uncovered for A = Nd-Lu, Y, Sc that exhibit a pressure-driven electronic reconstruction in the Ni e_g manifold. By disentangling the involvement of basal versus apical oxygen states at the Fermi surface, we identify Tb₃Ni₂O₇ as an interesting candidate for superconductivity at ambient pressure. These results suggest a profound tunability of the structural and electronic phases in this novel materials class and are key for a fundamental understanding of the superconductivity mechanism.