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

Huan Wu, Yi‐Feng Zhao+1 more|arXiv (Cornell University)|Mar 17, 2026

Magnetic and transport properties of perovskites and related materials0 citations

TL;DR

The paper uses first-principles calculations to study La7Ni5O17-2323 under hydrostatic pressure and biaxial compressive strain, revealing a dynamically stable C2/c structure at ambient conditions that tetragonalizes under pressure and strain, and showing how d_z^2 and d_x2−y2 bands evolve with pressure and strain, including a pressure-induced trilayer d_z^2 pocket crossing the Fermi level.

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.

Motivation & Objective

• Determine the structural stability of the high-symmetry P4/mmm phase at ambient pressure.
• Identify lower-symmetry distortions that stabilize the ambient-pressure structure through phonon instabilities.
• Characterize how hydrostatic pressure and biaxial compressive strain modify crystal symmetry, tilts, and Ni–O–Ni angles.
• Analyze the evolution of the electronic structure, especially d_{x^2−y^2} and d_{z^2} bands, under pressure and strain.
• Compare the electronic changes to those observed in conventional bilayer and trilayer RP nickelates.

Proposed method

• Perform DFT calculations with PBE GGA exchange correlation and ultrasoft pseudopotentials.
• Relax structures until forces are below 10^-4 Ry/bohr.
• Compute phonon dispersions for P4/mmm using ALAMODE to identify dynamical instabilities.
• Use group-theory analysis of unstable irreps (A1u and Eu at R) to predict lower-symmetry distortions leading to C2/c.
• Explore pressure effects up to ~30 GPa, tracking lattice constants, Ni–O–Ni bond angles, and c-axis changes.
• Analyze electronic structure and Fermi surface contributions from Ni d_{x^2−y^2} and d_{z^2} orbitals under both pressure and strain.

Figure 1: Crystal structure of La 7 Ni 5 O 17 -2323 and phonon dispersion relation. The high-symmetry space group $P4/mmm$ (a), that displays no octahedral tilts, gives rise to unstable phonon modes (b). The structure obtained by applying distortions according to the irreducible representation of th

Experimental results

Research questions

• RQ1 How does the ambient-pressure high-symmetry P4/mmm phase of La7Ni5O17-2323 transform into a dynamically stable lower-symmetry structure?
• RQ2 What is the influence of hydrostatic pressure on octahedral tilts and the resulting electronic structure, particularly d_{z^2} contributions at the Fermi level?
• RQ3 How does biaxial compressive strain affect tilts, bond angles, and the orbital character of states near the Fermi level?
• RQ4 Do pressure and strain drive the system to electronic structures comparable to or distinct from conventional bilayer/trilayer RP nickelates, in terms of potential superconductivity indicators?
• RQ5 What is the role of the trilayer- and bilayer-block-derived d_{z^2} states in shaping the low-energy physics under different external perturbations?

Key findings

• The high-symmetry P4/mmm phase is dynamically unstable at ambient pressure, with A1u and E_u phonon modes at R driving distortions.
• Relaxation of distortions corresponding to these unstable modes leads to a dynamically stable C2/c structure that contains octahedral tilts (Ni–O–Ni angles ~160–170°).
• Under hydrostatic pressure, the structure becomes tetragonal as tilts are progressively suppressed, with c-axis and in-plane angles adjusting toward 180° around 20–25 GPa; complete tilt suppression occurs by ~30 GPa.
• At ~30 GPa, the electronic structure features both d_{z^2} and d_{x^2−y^2} states at the Fermi level, with a bonding d_{z^2} band from the bilayer crossing the Fermi surface at M and a trilayer bonding d_{z^2} state nearly crossing the Fermi level.
• Under -2% biaxial compressive strain, the d_{z^2} trilayer bonding band is pushed below the Fermi level, leaving only the bilayer bonding d_{z^2} band crossing near E_F, mirroring behavior seen in strained bilayer RP nickelates.
• The dominant hoppings at 30 GPa indicate strong bilayer/trilayer d_{z^2} and d_{x^2−y^2} hybridizations, consistent with a superposition of bilayer and trilayer physics in La7Ni5O17-2323.

Figure 2: Evolution of the structural parameters of La 7 Ni 5 O 17 -2323 under hydrostatic pressure and its electronic structure at 30 GPa. (a-d) Pressure-dependent evolution of (a) the absolute difference between in-plane lattice constants $|a-b|$ ; (b) the cross-plane lattice constant $c$ ; (c) th