NaNiO2 is a layered material consisting of alternating layers of NaO6 and Jahn-Teller-active NiO6 edge-sharing octahedra. At ambient pressure it undergoes a broad phase transition from a monoclinic to rhombohedral structure between ∼465 K and ∼495 K, associated with the loss of long-range orbital ordering. In this work, we present the results of a neutron powder diffraction study on powdered NaNiO2 as a function of pressure and temperature from ambient pressure to ∼5 GPa between 290 K and 490 K. The 290 K and 460 K isothermal compressions remained in the monoclinic phase up to the maximum pressures studied, whereas the 490 K isotherm was mixed-phase throughout. The unit-cell volume was fitted to a 2nd-order Birch-Murnaghan equation of state, with B=113(1) GPa. We observe at 490 K that the fraction of Jahn-Teller-distorted phase increases with increasing pressure, from 67.8(6)% at 0.71(2) GPa to 80.2(9)% at 4.20(6) GPa. Using this observation, in conjunction with neutron diffraction measurements at 490 K from 5.46(9) GPa to 0.342(13) GPa, we show that the Jahn-Teller transition temperature increases with pressure. Our results are used to present a structural pressure-temperature phase diagram for NaNiO2. To our knowledge, this is the first diffraction study of the effect of pressure on the Jahn-Teller transition temperature in materials with edge-sharing Jahn-Teller-distorted octahedra, and the first variable-pressure study on a JT-active edge-sharing nickelate.
@article{arxiv.2202.09127,
title = {Pressure tuning the Jahn-Teller transition temperature in NaNiO$_2$},
author = {Liam A V Nagle-Cocco and Craig L Bull and Sian E Dutton and Christopher J Ridley},
journal= {arXiv preprint arXiv:2202.09127},
year = {2022}
}