[Paper Review] Dynamical structural instability and a new crystal-electronic structure of infinite-layer nickelates

Chengliang Xia, Jiaxuan Wu+2 more|arXiv (Cornell University)|Oct 24, 2021

Magnetic and transport properties of perovskites and related materials4 citations

TL;DR

This study reveals that infinite-layer nickelates RNiO₂ are dynamically unstable in the tetragonal P4/mmm phase for late lanthanides (R = Eu-Lu), favoring a lower-energy I4/mcm structure with out-of-phase NiO₄ square rotations. This distortion reduces Ni d-bandwidth, decreases Ni d-occupancy, and enhances Mott physics, making I4/mcm RNiO₂ a closer analog to superconducting cuprates than the P4/mmm phase.

ABSTRACT

We use first-principles calculations to find that in infinite-layer nickelates $R$NiO$_2$, the widely studied tetragonal $P4/mmm$ structure is only dynamically stable for early lanthanide elements $R$ = La-Sm. For late lanthanide elements $R$ = Eu-Lu, an imaginary phonon frequency appears at $A=(\pi,\pi,\pi)$ point. For those infinite-layer nickelates, condensation of this phonon mode into the $P4/mmm$ structure leads to a more energetically favorable $I4/mcm$ structure that is characterized by an out-of-phase rotation of `NiO$_4$ square'. Special attention is given to two borderline cases: PmNiO$_2$ and SmNiO$_2$, in which both the $P4/mmm$ structure and the $I4/mcm$ structure are local minimums, and the energy difference between the two structures can be fine-tuned by epitaxial strain. Compared to the $P4/mmm$ structure, $R$NiO$_2$ in the $I4/mcm$ structure has a substantially reduced Ni $d_{x^2-y^2}$ bandwidth, a smaller Ni $d$ occupancy, a `cleaner' Fermi surface with less contribution from lanthanide element $d$ orbitals, and a decreased critical $U_{ extrm{Ni}}$ to stabilize long-range antiferromagnetic ordering. All these features favor Mott physics and render $R$NiO$_2$ in the $I4/mcm$ structure a closer analogy to superconducting infinite-layer cuprates.

Motivation & Objective

• To investigate the dynamical stability of the tetragonal P4/mmm phase in infinite-layer nickelates RNiO₂ across the lanthanide series.
• To identify structural instabilities driving phase transitions in RNiO₂ beyond early lanthanides.
• To explore how epitaxial strain tunes the energy landscape between P4/mmm and I4/mcm phases in borderline cases (Pm, Sm).
• To evaluate electronic structure changes in the I4/mcm phase and their implications for Mott physics and antiferromagnetic ordering.

Proposed method

• Employing first-principles density functional theory (DFT) calculations to compute phonon dispersions and identify imaginary frequencies indicating dynamical instability.
• Using group theory to identify the soft phonon mode at the A=(π,π,π) point as a condensate leading to structural distortion into the I4/mcm space group.
• Performing structural relaxation and energy minimization to determine the ground-state structure for each R element.
• Analyzing electronic band structure, d-orbital occupancy, Fermi surface composition, and critical U values for antiferromagnetic ordering.
• Applying epitaxial strain in supercell models to tune the energy difference between P4/mmm and I4/mcm phases in PmNiO₂ and SmNiO₂.
• Comparing Ni d-orbital bandwidth, d-occupancy, and hybridization with lanthanide 4f/d states between the two phases.

Experimental results

Research questions

• RQ1 Why is the P4/mmm phase dynamically unstable for late lanthanide nickelates (R = Eu-Lu) despite being the ground state in early members?
• RQ2 What structural distortion emerges from condensing the soft phonon mode at A=(π,π,π), and what is its symmetry and atomic origin?
• RQ3 How does the I4/mcm structure alter the Ni d-bandwidth, d-occupancy, and Fermi surface topology compared to the P4/mmm phase?
• RQ4 What is the impact of the I4/mcm structure on the critical U_Ni required to stabilize long-range antiferromagnetic order?
• RQ5 Can epitaxial strain be used to control the relative stability of P4/mmm and I4/mcm phases in borderline cases like PmNiO₂ and SmNiO₂?

Key findings

• For R = Eu-Lu, the P4/mmm phase exhibits an imaginary phonon frequency at the A=(π,π,π) point, indicating dynamical instability.
• Condensation of this phonon mode leads to a lower-energy I4/mcm structure characterized by out-of-phase rotations of NiO₄ square planar units.
• The I4/mcm phase exhibits a substantially reduced Ni d_{x²−y²} bandwidth compared to the P4/mmm phase.
• Ni d-orbital occupancy is reduced in the I4/mcm structure, indicating a more Mott-insulating character.
• The Fermi surface in the I4/mcm phase is 'cleaner', with significantly reduced hybridization from lanthanide 4f and d orbitals.
• The critical U_Ni required to stabilize long-range antiferromagnetic order is decreased in the I4/mcm phase, further supporting Mott physics.