Displacive Jahn–Teller Transition in NaNiO2

Below its Jahn–Teller transition temperature, TJT, NaNiO2has a monoclinic layered structure consisting of alternating layers of edge-sharing NaO6and Jahn–Teller-distorted NiO6octahedra. Above TJTwhere NaNiO2is 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 ab initiomolecular 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.