Displacive Jahn--Teller transition in NaNiO${}_2$

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.

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