Pressure Tuning the Jahn–Teller Transition Temperature in NaNiO₂

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 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 and 490 K. The 290 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 second-order Birch–Murnaghan equation of state, where B = 119.6(5) GPa at 290 K. We observe at 490 K that the fraction of the Jahn–Teller-distorted phase increases with 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 on removing pressure from 5.46(9) 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 the best of 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 focusing on the Jahn–Teller distortion in a nickelate.

Van Vleck analysis of angularly distorted octahedra using VanVleckCalculator

“…In the main text, we showed that the van Vleck parameter Figure S4: The pressure dependence of bond length distortion index, effective coordination, quadratic elongation, and the van Vleck ρ 0 parameter, for NaO 6 octahedra in NaNiO 2 as a function of pressure, using crystal structures previously reported in Ref. 13 However, we caution that ρ 0 may give a large result for coordination octahedra around Jahn-Teller-inactive cations when the symmetry of the distortion matches the…”

“…The structural effect of a rotation of within a range of 120 can be quite significant, as shown in Fig. 3; such changes can manifest as a JT-elongated (compressed) octahedron with four short (long) and two long (short) bonds [such as NiO 6 in NaNiO 2 (Nagle-Cocco et al, 2022)] or two short, two medium and two long bonds [such as LaMnO 3 (Rodriguez-Carvajal et al, 1998)].…”

“…We previously studied the effect of pressure on the JT distortion in NaNiO 2 (Nagle-Cocco et al, 2022) by performing Rietveld refinement of neutron diffraction data from the PEARL instrument (Bull et al, 2016) at the ISIS Neutron and Muon Source (Oxfordshire, UK). However, we did not utilize the Van Vleck distortion modes, instead quantifying the JT distortion using the bond-length distortion index (Baur, 1974) and the effective coordination number (Hoppe, 1979).…”

“…Here, we give the example of NaO 6 octahedra in NaNiO 2 , from the same pressure study referred to in the main text. 13 Figure S4 shows the pressure dependence of the distortion parameters of NaO 6 . It can be seen that ρ 0 follows the same trend as the other parameters despite the absence of the Jahn-Teller distortion, due to the NaO 6 octahedra having the same symmetry as its Jahn-Teller-distorted neighbouring NiO 6 octahedra.…”

“…In the literature, various techniques for parameterizing the JT distortion are used. An often-used example (Kimber, 2012;Lawler et al, 2021;Nagle-Cocco et al, 2022;Genreith-Schriever et al, 2023) is the bond-length distortion index, defined by Baur (1974) as…”