  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.
