{"id":{"repo_id":"cambridge","oai_identifier":"oai:www.repository.cam.ac.uk:1810/372432"},"canonical_url":"https://search.dev.ndltd.org/etd/cambridge/oai:www.repository.cam.ac.uk:1810/372432","repository":{"repo_id":"cambridge","name":"Cambridge University","base_url":"https://api.repository.cam.ac.uk/server/oai/request"},"display":{"title":"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 LiNiO<sub>2</sub> and NaNiO<sub>2</sub>. 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 NaNiO<sub>2</sub>, 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 NaNiO<sub>2</sub>, which is previously unseen with prior work using neutron PDF on the Jahn-Teller transition in LaMnO<sub>3</sub> 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 LiNiO<sub>2</sub> and NaNiO<sub>2</sub>. These are studied in-depth using VanVleckCalculator with the results for NaNiO<sub>2</sub> indicating a displacive transition, consistent with the experimental work. The work on LiNiO<sub>2</sub> shows a similar result, with the result resolving longstanding disagreement on the orbital behaviour of LiNiO<sub>2</sub>. Finally, variable-pressure studies on NaNiO<sub>2</sub> 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. *dT*<sub>JT</sub>/*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.","abstract_html":"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 LiNiO&lt;sub&gt;2&lt;/sub&gt; and NaNiO&lt;sub&gt;2&lt;/sub&gt;. 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 NaNiO&lt;sub&gt;2&lt;/sub&gt;, 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 NaNiO&lt;sub&gt;2&lt;/sub&gt;, which is previously unseen with prior work using neutron PDF on the Jahn-Teller transition in LaMnO&lt;sub&gt;3&lt;/sub&gt; 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 LiNiO&lt;sub&gt;2&lt;/sub&gt; and NaNiO&lt;sub&gt;2&lt;/sub&gt;. These are studied in-depth using VanVleckCalculator with the results for NaNiO&lt;sub&gt;2&lt;/sub&gt; indicating a displacive transition, consistent with the experimental work. The work on LiNiO&lt;sub&gt;2&lt;/sub&gt; shows a similar result, with the result resolving longstanding disagreement on the orbital behaviour of LiNiO&lt;sub&gt;2&lt;/sub&gt;. Finally, variable-pressure studies on NaNiO&lt;sub&gt;2&lt;/sub&gt; 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. *dT*&lt;sub&gt;JT&lt;/sub&gt;/*dP* &gt; 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.","abstract_has_math":false,"creators":["Nagle-Cocco, Liam"],"institution":"University of Cambridge","degree_name":"Doctor of Philosophy (PhD)","degree_level":"Doctoral","degree_discipline":null,"degree_department":null,"school":null,"contributors":[],"advisors":["Dutton, Sian"],"committee_chairs":[],"committee_members":[],"year":2024,"date_issued":"2024-04-30","date_published":"2024-04-30","updated_at":"2026-07-22T22:24:31Z","subjects":["Crystallography","Jahn-Teller","Layered nickelates"],"languages":["eng"],"rights":[],"rights_urls":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/fde392d1-f094-47d8-a60f-962f6b59a3ad/download","https://www.rioxx.net/licenses/all-rights-reserved/"],"identifier_entries":[{"key":"dc:creator.authoridentifier","label":"Author Identifier","values":["0000000192651588"],"render_values":[{"text":"0000-0001-9265-1588","href":"https://orcid.org/0000-0001-9265-1588","code":true}]}]},"links":{"outbound_url":"https://doi.org/10.17863/CAM.111303","outbound_label":"DOI","outbound_source":"dc:identifier.doi"},"metadata_groups":[{"id":"people","label":"People","entries":[{"key":"dc:contributor.advisor","label":"Advisor","values":["Dutton, Sian"]},{"key":"dc:contributor.sponsor","label":"Sponsor","values":["UK Engineering and Physical Sciences Research Council (EPSRC) (EP/R513180/1). 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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 LiNiO<sub>2</sub> and NaNiO<sub>2</sub>. 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 NaNiO<sub>2</sub>, 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 NaNiO<sub>2</sub>, which is previously unseen with prior work using neutron PDF on the Jahn-Teller transition in LaMnO<sub>3</sub> 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 LiNiO<sub>2</sub> and NaNiO<sub>2</sub>. These are studied in-depth using VanVleckCalculator with the results for NaNiO<sub>2</sub> indicating a displacive transition, consistent with the experimental work. The work on LiNiO<sub>2</sub> shows a similar result, with the result resolving longstanding disagreement on the orbital behaviour of LiNiO<sub>2</sub>. Finally, variable-pressure studies on NaNiO<sub>2</sub> 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. *dT*<sub>JT</sub>/*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."]},{"key":"dc:format.checksum.md5","label":"Dc Format Checksum Md5","values":["d5bce82280b2484902ea636478f78ab8","87eda9de84448d1f82354d60eee3eb5f"]},{"key":"dc:title","label":"Title","values":["Jahn-Teller distortions in layered nickel oxides"]}]}],"canonical_facts":{"dc:contributor.advisor":["Dutton, Sian"],"dc:contributor.sponsor":["UK Engineering and Physical Sciences Research Council (EPSRC) (EP/R513180/1). Additional funding from the Cambridge Philosophical Society."],"dc:creator":["Nagle-Cocco, Liam"],"dc:creator.authoridentifier":["0000000192651588"],"dc:date.issued":["2024-04-30"],"dc:description.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 LiNiO<sub>2</sub> and NaNiO<sub>2</sub>. 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 NaNiO<sub>2</sub>, 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 NaNiO<sub>2</sub>, which is previously unseen with prior work using neutron PDF on the Jahn-Teller transition in LaMnO<sub>3</sub> 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 LiNiO<sub>2</sub> and NaNiO<sub>2</sub>. These are studied in-depth using VanVleckCalculator with the results for NaNiO<sub>2</sub> indicating a displacive transition, consistent with the experimental work. The work on LiNiO<sub>2</sub> shows a similar result, with the result resolving longstanding disagreement on the orbital behaviour of LiNiO<sub>2</sub>. Finally, variable-pressure studies on NaNiO<sub>2</sub> 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. *dT*<sub>JT</sub>/*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."],"dc:format.checksum.md5":["d5bce82280b2484902ea636478f78ab8","87eda9de84448d1f82354d60eee3eb5f"],"dc:identifier.doi":["https://doi.org/10.17863/CAM.111303"],"dc:identifier.uri":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/ad3f0339-34c4-4697-9ff3-b09bd1844c0a/download"],"dc:language":["eng"],"dc:publisher.institution":["University of Cambridge"],"dc:relation.isreferencedby.uri":["https://www.repository.cam.ac.uk/handle/1810/372432"],"dc:rights":["https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/fde392d1-f094-47d8-a60f-962f6b59a3ad/download","https://www.rioxx.net/licenses/all-rights-reserved/"],"dc:rights.embargodate":["2025-08-15"],"dc:rights.embargotype":["embargo"],"dc:subject":["Crystallography","Jahn-Teller","Layered nickelates"],"dc:title":["Jahn-Teller distortions in layered nickel oxides"],"dc:type":["Thesis"],"dc:type.qualificationlevel":["Doctoral"],"dc:type.qualificationname":["Doctor of Philosophy (PhD)"]},"updated_at":"2026-07-22T22:24:31Z"}