Resolving dynamic JT distortion with EXAFS in NaNiO2

NaNiO2 is a candidate layered cathode material. It exhibits electron degeneracy in the Ni3+ eg orbitals, resulting in a Jahn-Teller (JT) distortion. At lower temperatures, it is both static and co-operative, resulting in a monoclinic unit cell until ~480 K, where the material transitions to a rhombohedral unit cell without a co-operative JT distortion. A recent theoretical study predicts that the high-temperature phase is in fact dynamically stabilised, with the JT distortion surviving the transition at a local level, but appearing undistorted via diffraction methods due to its non-cooperative nature. Using EXAFS at the Ni K edge, in conjunction with recently-obtained neutron PDF, we propose to look at the Ni environment as a function of temperature, studying the bond disproportionation in NiO6 octahedra to see if the JT distortion survives the transition. This will provide evidence for a dynamic JT distortion at high-temperatures.