Below its Jahn--Teller transition temperature, TJT, NaNiO2 has a monoclinic layered structure consisting of alternating layers of edge-sharing NaO6 and Jahn-Teller-distorted NiO6 octahedra. Above TJT where NaNiO2 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 TJT. 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.
@article{arxiv.2408.01267,
title = {Displacive Jahn--Teller transition in NaNiO$_2$},
author = {Liam Agostino Vincenzo Nagle-Cocco and Annalena R. Genreith-Schriever and James M. A. Steele and Camilla Tacconis and Joshua D. Bocarsly and Olivier Mathon and Joerg C. Neuefeind and Jue Liu and Christopher A. O'Keefe and Andrew L. Goodwin and Clare P. Grey and John S. O. Evans and Siân E. Dutton},
journal= {arXiv preprint arXiv:2408.01267},
year = {2024}
}