Pressure and strain tuning of the alternating bilayer-trilayer Ruddlesden-Popper nickelate: crystal and electronic structure

TL;DR

This study uses first-principles calculations to explore how pressure and strain influence the crystal and electronic structures of a hybrid bilayer-trilayer nickelate, revealing structural stabilization and electronic band shifts.

Contribution

It provides new insights into the effects of pressure and strain on the structure and electronic properties of the hybrid nickelate, highlighting similarities and differences with conventional Ruddlesden-Popper nickelates.

Findings

Octahedral tilts are suppressed under pressure and strain, leading to a tetragonal structure.

The $d_{z^2}$ bonding band crosses the Fermi level at 30 GPa pressure.

Strain modifies the electronic structure similarly to pressure, but with key differences.

Abstract

We use first-principles calculations to investigate the crystal and electronic structure of the hybrid bilayer-trilayer Ruddlesden-Popper (RP) nickelate La$_7$Ni$_5$O$_{17}$ under hydrostatic pressure and biaxial compressive strain. By analyzing the irreducible representations of the dynamically unstable phonon modes in the high-symmetry $P4/mmm$ structure, we identify a dynamically stable lower-symmetry $C2/c$ structure containing octahedral tilts. The application of both pressure and compressive strain tends to suppress the octahedral tilts, effectively tetragonalizing the structure, in analogy with the conventional RPs. The electronic structure under hydrostatic pressure and strain has similarities, but it differs in the position of the $d_{z^2}$ bonding band from the trilayer block. This band crosses the Fermi level at a pressure of 30 GPa, but it remains below it for any level of compressive strain. This strain-induced modification mirrors the electronic structure changes observed in the conventional bilayer nickelate.