Displacive Jahn--Teller transition in NaNiO₂

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.