Jahn–Teller order in perovskites and Prussian blues: X-ray scattering and Monte Carlo modelling

Thesis

Prussian blue analogues (PBAs) have found applications in a variety of different areas, including batteries, gas storage, and cooperative magnetism. The functional versatility of PBAs arguably stems from the rich structural diversity of the PBA family. Hence, designing the next-generation of PBA functional materials requires understanding their many structure–property relationships. This thesis aims to see how well intentionally simple modelling can explain fairly complex phenomena.

This thesis first aims to get a better understanding of the local structure of relatively simple PBAs. The MPt(CN)6 PBAs—of interest due to their negative thermal expansion behaviour—are essentially defect-free and hence their local structures can be characterised in terms of thermal displacements away from their average structure. Exploiting this direct relationship between local structure and dynamics, a new big-box modelling technique of pair distribution function (PDF) measurements is developed based on the optimisation of empirical potentials to obtain atomistic representations of PBAs.

Next, this thesis progresses onto exploring the interplay between cooperative Jahn–Teller (CJT) distortions and vacancies in more complex PBAs. A simple model, based on the competition between strain and crystal-field effects, is developed to understand the nature and emergence of CJT effects in PBAs. The empirical potential PDF refinement approach developed earlier in the thesis is then used to develop atomistic representations of these PBAs for the first time.

Given the relative success of using simple considerations to understand the phenomenology of CJT order in PBAs, this thesis goes on to test whether simple geometric considerations of a similar kind developed for PBAs can be used to understand CJT effects in the manganite perovskites, where a complex interplay between orbital, lattice, charge, and spin degrees of freedom is known to drive the emergence of the useful property of colossal magnetoresistance.

Finally, this thesis finishes by exploring whether PDFs obtained using X-ray freeelectron laser (XFEL) technologies can be used to characterise JT/vacancy order in PBAs, with the eventual aim to use XFEL techniques to probe ultra-fast local structure changes in PBAs, e.g. during photoinduced charge transfer.

Authors: Harbourne, EA

• DOI: 10.5287/ora-kog8pmyog
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: total scattering; Monte Carlo; Jahn–Teller; Manganite perovskites; Prussian blue analogues; disorder; pair distribution function; x-ray free-electron laser; negative thermal expansion
• Subjects: Materials; Structural chemistry; Chemistry, Inorganic; Solid state chemistry