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Title: Spin-ice physics and phase transitions in cadmium cyanide
Author: Coates, Chloe S.
ISNI:       0000 0004 8507 7559
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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There is increasing interest in the role of materials with correlated structural disorder in mimicking the exotic properties exploited in electronic condensed matter phases. Spin ices, for example, are important class of frustrated magnets in which the spin configuration obeys the `two-in two-out' ice rules that govern proton order in water-ice. Such ice rules are implicated in a wide range of functional responses in different materials, from heavy-fermion behaviour to superconductivity. Ice-rules-obeying states are characterised by an absence of long range order, leading to a manifold of degenerate ground states, extensive residual entropy and exotic excitations; in spin-ices, these excitations correspond to topological defects in the form of effective magnetic monopoles. The main barrier to experimentation and application of these topological defects is the extremely low temperatures (~ 1 K) at which the spin-ice correlations develop. This thesis explores the structural ice analogue cadmium cyanide, Cd(CN)2, a cubic negative thermal expansion (NTE) material, in which the orientation of cyanide ions maps onto the spin orientation in spin-ices. Cd(CN)2 is shown here to display spin-ice physics at room temperature. Chapter 3 outlines the extreme sensitivity of Cd(CN)2 to X-ray irradiation, including unit-cell contraction, and the selection or suppression of particular phase transitions. These results explain earlier discrepancies regarding the phase behaviour and negative thermal expansion in Cd(CN)2 and motivated the development of a synthesis of isotopically-enriched 114Cd(CN)2 suitable for neutron scattering studies. The synthesis of Cd(CN)2 directly from Cd metal forms the basis of Chapter 4. The structure of previously-unreported intermediate phase, Cd(NH3)2[Cd(CN)4], is characterised, as well as its thermomechanical response. This represents the first variable-temperature study of this type of framework (PtS), shown to display anisotropic NTE and negative linear compressibility. The main result of this thesis is to demonstrate that by replacing the magnetic dipole in spin-ice with the electric dipole of the cyanide ion, cadmium cyanide displays spin-ice behaviour at temperatures nearly sixty times higher that in its magnetic analogue. I present the results of neutron scattering measurements on 114Cd(CN)2 along with density functional theory calculations, Monte Carlo simulations, total scattering and nuclear magnetic resonance measurements. Having confirmed the importance of spin-ice physics, chapter 6 explores the pressure and temperature dependence of the ice-like correlations. At least two phase transitions are identied at low pressure, kinetic trapping is observed and the extreme compressibility shown to be a function of cyanide reorientation and ice-like coordination. It is hoped that increase in energy scale, identified here, will enable further exploration that may shed further light on the physics of the spin and water-ices themselves.
Supervisor: Goodwin, Andrew Sponsor: Leverhulme Trust
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Chemistry, Inorganic ; Materials Chemistry ; Condensed Matter Physics