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

TL;DR

This study investigates how applying pressure influences the Jahn-Teller transition temperature in NaNiO$_2$, revealing that pressure increases the transition temperature and providing a detailed pressure-temperature phase diagram.

Contribution

First neutron diffraction study to examine pressure effects on Jahn-Teller transition in NaNiO$_2$, establishing a pressure-dependent phase diagram for this edge-sharing nickelate.

Findings

Jahn-Teller transition temperature increases with pressure.

Unit-cell volume fitted to Birch-Murnaghan equation of state.

Pressure stabilizes the Jahn-Teller-distorted phase.

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 $\sim$465 K and $\sim$495 K, associated with the loss of long-range orbital ordering. In this work, we present the results of a neutron powder diffraction study on powdered NaNiO$_2$ as a function of pressure and temperature from ambient pressure to $\sim$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 NaNiO$_2$. 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.