Displacive Jahn--Teller transition in NaNiO$_2$

TL;DR

This study provides direct evidence of a displacive Jahn-Teller transition in NaNiO$_2$, combining experimental local probes and ab initio simulations, marking a first in observing such a transition in any material.

Contribution

It demonstrates, for the first time, a displacive Jahn-Teller transition using local probe techniques and ab initio simulations.

Findings

Displacive Jahn-Teller transition observed in NaNiO$_2$

Transition characterized by structural change without disorder

Supported by neutron scattering, NMR, EXAFS, and AIMD simulations

Abstract

Below its Jahn--Teller transition temperature, $T_\mathrm{JT}$, NaNiO$_2$ has a monoclinic layered structure consisting of alternating layers of edge-sharing NaO$_6$ and Jahn-Teller-distorted NiO$_6$ octahedra. Above $T_\mathrm{JT}$ where NaNiO$_2$ is rhombohedral, diffraction measurements show the absence of a cooperative Jahn-Teller distortion, accompanied by an increase in the unit cell volume. Using neutron total scattering, solid-state Nuclear Magnetic Resonance (NMR), and extended X-ray absorption fine structure (EXAFS) experiments as local probes of the structure we find direct evidence for a displacive, as opposed to order-disorder Jahn-Teller transition at $T_\mathrm{JT}$. This is supported by \textit{ab initio} molecular dynamics (AIMD) simulations. To our knowledge this study is the first to show a displacive Jahn-Teller transition in any material using direct observations with local probe techniques.