Displacive Jahn-Teller suppression on a triangular lattice with pressure

We have studied the variable-temperature (VT) structure in NaNiO2, a Jahn-Teller (JT) distorted battery cathode material with a triangular lattice, and uncovered a new and unusual phenomenon: the local JT distortion is suppressed at the same temperature as the cooperative JT (cJT) distortion in a displacive transition. This contrasts with the widely-studied perovskites, where JT distortions persist locally even when the cJT distortion disappears (an order-disorder transition). Displacive JT transitions are unreported and must be explored further. To investigate this under a different set of conditions, we performed variable-pressure (VP) diffraction on NaNiO2 and found that the cJT effect disappears at a symmetry-increasing transition above ~22 GPa. If the VT phenomenon is due to the triangular lattice, then the VP transition should coincide with a local JT vanishing, by analogy with the VT transition. To test this, we propose to study the Ni environment in NaNiO2 using VP-EXAFS.