Pressure tuning the Jahn-Teller transition temperature in NaNiO$_2$

Abstract

NaNiO 2 is a layered material consisting of alternating layers of NaO 6 and Jahn-Teller-active NiO 6 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 NaNiO 2 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 mixedphase 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