Smith, Rachael Louise; (2024) Stacking Disorder as a Critical Tuning Parameter for the Properties of Materials. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Stacking disorder is a complex yet vital phenomenon that in principle, can occur in every layered material.1-9 The need for a thorough understanding of stacking disorder has only become apparent within the last 12 years, 10 together with the extensive impact this has on material properties. This thesis builds upon stacking disorder research explored previously for diamond 2, 7, 11, 12 in addition to new studies on AgI, CuI, AgI-CuI, and provides insight into the theoretical aspects of stacking disorder and third-order memory effects. This thesis focuses initially on stacking disorder in AgI, where it has been found that the dependence of the Ag+ and I- ratios allows for targeted precipitation of cubicities in the 0–81% range. AgI samples exhibiting varying polarity, originating from preparation method, have been shown to experience surface ion adsorption effects with cations. A link between AgI cubicity and associated band gap energies has also been found. In addition, when heating AgI to the high temperature α-phase, samples exhibited memory effects, and this is hypothesised to relate to the surface stabilisation by ions. CuI did not exhibit any stacking disorder, however this research has given greater insight into the characteristics of this material. Stacking disorder in AgI-CuI was explored by adjusting the starting ratios of ions, showing a tendency of the resultant material to form cubic polytypes amongst potential new phases and stacking disordered structures when a deficiency of I- was exploited. Investigations into Popigai diamond samples showed hexagonalities of 0-39%. Additional peaks in XRD patterns at 2θ=12 ° in combination with DIFFaX simulations and HRTEM suggested the presence of extended graphitic regions resulting in both type 1 and 2 diaphite. The final chapter focuses on evolving theoretical aspects of stacking disorder, developing MCDIFFaX to incorporate third-order stacking probabilities and fitting of high-order memory effect simulations.

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