The recent discovery of a superconducting transition temperature $T_{\mathrm{c}}$ of 96 K in the Sm-doped Ruddlesden-Popper bilayer nickelate $\mathrm{L}{\mathrm{a}}_{3-x}\mathrm{S}{\mathrm{m}}_x\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_{7-d}$ under high pressure has attracted considerable attention [F. Li et al., Nature (London) 649, 871 (2026)]. However, the underlying mechanism of the effects of chemical pressure induced by such doping remains unclear. In this work, we systematically investigate the crystal and electronic structures of $\mathrm{L}{\mathrm{a}}_2\mathrm{SmN}{\mathrm{i}}_2{\mathrm{O}}_7$ under pressure using first-principles calculations and extend our study to $\mathrm{L}{\mathrm{a}}_{2}R\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_7$ ($R=\mathrm{Pr}$, Nd) to elucidate the general influence of chemical pressure. Our results indicate that chemical pressure intensifies octahedral rotational distortion, inducing anisotropic in-plane lattice evolution and elevating the critical pressure for phase transition, while it compresses the out-of-plane apical Ni-O bond length and reduces octahedral regularity, thereby enhancing the density of states of the Ni $d_{z^2}$ orbital at the Fermi level, which is favorable for superconductivity. Chemical pressure adjusts the electron occupation of the $d_{z^2}$ and $d_{x^2-y^2}$ orbitals by creating a crystal field with smaller $e_g$ splitting.

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• Electronic structure
• High-pressure studies
• Pressure effects
• Superconductivity
• Density functional theory
• First-principles calculations