Dome-like pressure-temperature phase diagram of the cooperative Jahn–Teller distortion in NaNiO₂

NaNiO₂is a Ni³⁺-containing layered material consisting of alternating triangular networks of Ni and Na cations, separated by octahedrally-coordinated O anions. At ambient pressure, it features a collinear Jahn-Teller distortion belowTonsetJT≈480 K, which disappears in a first-order transition on heating toTendJT≈500 K, corresponding to the increase in symmetry from monoclinic to rhombohedral. It was previously studied by variable-pressure neutron diffraction (Nagle-Cocco<i>et al</i>2022<i>ACS Inorg. Chem.</i><b>61</b>4312) and found to exhibit an increasingTonsetJTwith pressure up to ∼5 GPa. In this work, powdered NaNiO₂was studied via variable-pressure synchrotron x-ray diffraction up to pressures of ∼67 GPa at 294 K and 403 K. Suppression of the collinear Jahn-Teller ordering is observed via the emergence of a high-symmetry rhombohedral phase, with the onset pressure occurring at ∼18 GPa at both studied temperatures. Further, a discontinuous decrease in unit cell volume is observed on transitioning from the monoclinic to the rhombohedral phase. These results taken together suggest that in the vicinity of the transition, application of pressure causes the Jahn-Teller transition temperature,TonsetJT, to decrease rapidly. We conclude that the pressure-temperature phase diagram of the cooperative Jahn-Teller distortion in NaNiO₂is dome-like.