On structural studies of high-density potassium and sodium
McBride, Emma Elizabeth
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The alkali elements at ambient conditions are well described by the nearly-free electron (NFE) model, yet show a remarkable departure from this “simple” behaviour with increasing pressure. Low-symmetry complex structures are observed in all, and anomalous melting has been observed in lithium (Li), sodium (Na), rubidium (Rb), and caesium (Cs). In this Thesis, static and dynamic compression techniques have been used to investigate the high-pressure high-temperature behaviour of the alkali elements potassium (K) and Na. Utilising diamond anvil pressure cells and external resistive heating, both in-air and in-vacuum, the melting curve of K has been determined to 24 GPa and 750 K, and is found to be remarkably similar to that of Na, but strikingly different to that reported previously. Furthermore, there is some evidence to suggest that a change in the compressibility of liquid-K occurs at lower pressures than the solid-solid phase transitions, perhaps indicating structural transitions occurring in the liquid phase, similar to those in the underlying solid. This could suggest a mechanism to explain the anomalous melting behaviour observed. Previous ab initio computational studies indicate that the unusual melting curve of Na arises due to structural and electronic transitions occurring in the liquid, mirroring those found in the underlying solid at higher pressures. The discovery that the melting curve of K is very similar to that of Na suggests that the same physical phenomena predicted for Na could be responsible for the high-pressure melting behaviour observed in K. The tI19 phase of K, observed above 20 GPa at 300 K, is a composite incommensurate host-guest structure consisting of 1D chains of guest atoms surrounded by a tetragonal host framework. Along the unique c-axis, the host and guest are incommensurate with each other. During the melting studies described above, it was observed that with increasing temperature, the weaker-bonded guest chains become more disordered while the host structure remains unchanged. To investigate and characterise this order-disorder transition, in situ synchrotron X-ray diffraction studies were conducted on single-crystal and quasi-single crystal samples of tI19-K. An order-disorder phase line has been mapped out to 50 GPa and 650 K. Perhaps the most striking departure from NFE behaviour in the alkali elements is observed in Na at pressures above 200 GPa where it transforms to a transparent electrical insulator. This phase is a so-called elemental “electride”, which may be thought of as being pseudo-ionically bonded. Electrides are predicted to exist in many elements, but at pressures far beyond the current capabilities of static pressure techniques. Utilising laser-driven quasi-isentropic compression techniques, dynamic compression experiments were performed on Na to see if it is possible to observe this electride phase under the timescales of dynamic compression experiment (ns). Optical velocimetry and reflectivity of the sample were measured directly to determine pressure and monitor the on-set of the transparent phase, respectively.