Jahn-Teller distortions in layered nickel oxides

Abstract

This thesis presents the results of my work on Jahn-Teller distorted transition metal oxides. The Jahn-Teller distortion is a key factor in important phenomena and applications of materials with octahedrally-coordinated transition metal ions such as batteries, high-temperature cuprate superconductivity, and colossal magnetoresistance. This work advances both the methodology for parameterising Jahn-Teller-distorted octahedra in crystal structures, via calculation of the Van Vleck modes, and sheds new insight onto the Jahn-Teller distortion in layered nickel oxides LiNiO2 and NaNiO2. A new method for calculating the van Vleck modes is presented which, in contrast to the established method in the literature, does not assume octahedra are angularly-undistorted and hence allows for the acquisition of information on octahedral shear. A Python package, VanVleckCalculator, is presented which is available on GitHub for public use. This code is used in the analysis presented throughout this thesis. Variable-temperature studies on the crystal structure of NaNiO2, using synchrotron diffraction as a probe of the average structure, in conjunction with EXAFS and neutron total scattering as probes of the local structure, are presented. The neutron total scattering data is studied via modelling of large supercells. The analysis yields insight on the nature of the Jahn-Teller transitions. I find a displacive Jahn-Teller transition in NaNiO2, which is previously unseen with prior work using neutron PDF on the Jahn-Teller transition in LaMnO3 showing an order-disorder transition. Ab initio molecular dynamics calculations using density functional theory, which were performed by a collaborator, has been used to obtain many crystal structures at various temperature points, on LiNiO2 and NaNiO2. These are studied in-depth using VanVleckCalculator with the results for NaNiO2 indicating a displacive transition, consistent with the experimental work. The work on LiNiO2 shows a similar result, with the result resolving longstanding disagreement on the orbital behaviour of LiNiO2. Finally, variable-pressure studies on NaNiO2 using both neutron and x-ray synchrotron diffraction are presented. It is shown that in the range from ambient pressure to ~6 GPa, the Jahn-Teller distortion is reduced in magnitude while also remaining cooperative to higher temperatures (i.e. dTJT/dP > 0). At higher pressures, it is also shown that there is a transition from the monoclinic to rhombohedral structure which begins but is not observed to complete, potentially indicating a dome-shaped phase diagram.