Displacive Jahn–Teller transition in NaNiO_2

Below its Jahn–Teller transition temperature, T_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_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_JT. This is supported by 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.