Structural phase diagram and optical properties of correlated A₃Ni₂O₇ rare-earth nickelates under high pressure

Motivated by the recent observation of superconductivity with T_C∼80 K in pressurized La₃Ni₂O₇ [1], we explore the structural and electronic properties in A₃Ni₂O₇ bilayer nickelates (A=La-Lu, Y, Sc) as a function of hydrostatic pressure (0-150 GPa) from first principles including a Coulomb repulsion term [2]. At ∼20 GPa, we observe an orthorhombic-to-tetragonal transition in La₃Ni₂O₇ at variance with recent 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 two distinct and counteracting control parameters. Unexpected correlations between T_C and the in-plane Ni-O-Ni bond angles are found for La₃Ni₂O₇. Moreover, two novel structural phases with significant c⁺ octahedral rotations and in-plane bond disproportionations are uncovered for A=Nd-Lu, Y, Sc that exhibit a surprising pressure-driven electronic reconstruction in the Ni e_g manifold. We identify Tb₃Ni₂O₇ as an interesting candidate for superconductivity at ambient pressure. Finally, we assess the role of electronic correlations in La₃Ni₂O₇ by comparing experimental and simulated optical spectra.

[1] H. Sun et al., Nature 621, 493 (2023)

[2] B. Geisler, J. J. Hamlin, G. R. Stewart, R. G. Hennig, P. J. Hirschfeld, arXiv:2309.15078 [cond-mat.supr-con]