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Title: Crystal structures and phase transitions of bismuth-tellurides under high pressure
Author: Freeman, Kenneth Nicol
ISNI:       0000 0004 8508 6930
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 2019
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Bismuth-tellurides are binary compounds of bismuth (Bi) and tellurium (Te) which form an infinitely adaptive series, (Bi2)m(Bi2Te3)n, within certain compositional limits. Members of this series exhibit a number of interesting and useful physical properties: they are among the most widely known thermoelectric materials and they have been shown to exhibit superconductivity and topological insulation. Many of these properties have been reported to be induced or enhanced by the application of high pressure. Despite this, the current understanding of the crystal structures of the (Bi2)m(Bi2Te3)n series under high pressure remains fragmentary. Knowledge of these crystal structures is the first necessary step towards further investigation of the properties of these materials through, for example, electronic structure calculations. This thesis presents the analysis of high-pressure x-ray diffraction data collected for several members of the (Bi2)m(Bi2Te3)n series. Angle-dispersive x-ray powder diffraction experiments were performed at synchrotron facilities, utilising diamond-anvil pressure cells to generate pressures up to 26 GPa. Several structural phases of the (Bi2)m(Bi2Te3)n series are investigated in detail including a previously unreported complex host-guest structure that forms in several of the series members at high pressures. This is a similar structure to the host-guest phase of elemental bismuth (Bi-III) with a host framework enclosing linear guest chains which lie along the c-axis direction. The guest chains are found to be disordered along their lengths, contributing only diffuse features to the measured x-ray powder diffraction profiles. Along with this structural disorder, this structure is found to include chemical ordering with the guest chains composed primarily of bismuth Other investigated phases include the layered structure found at ambient conditions, consisting of regular Bi2 and Bi2Te3 blocks stacked along the c-axis. Where appropriate, Rietveld refinement of these structures found the block compositions to differ from the idealised structure. For certain compositions, a four-dimensional modulated structure is the more appropriate description; le Bail fits were performed in these cases to provide the pressure-evolution of the lattice parameters and modulation vector. All investigated samples were found to adopt high-symmetry cubic phases at the highest pressures investigated here. This, along with various similarities between the other pressure-induced phases, suggests a universal behaviour in the (Bi2)m(Bi2Te3)n series on pressurisation. Composition was found to have a significant influence on the behaviour under pressure and individual structures of bismuth-tellurides. This work represents the first systematic, high-pressure structural study of these materials and explores the pressure-behaviour of the series as a whole and as a function of composition. This provides a first necessary step towards an improved understanding of these materials and their properties at high pressures.
Supervisor: Loa, Ingo ; Loveday, John Sponsor: Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: bismuth-tellurides ; pressure ; diamond anvil cells ; x-ray diffraction ; atomic structure ; Bi:Te ratios