Novel materials under extreme conditions
Kelsall, Liam Carl
Materials under extreme conditions often exhibit novel behaviours that differ widely from those observed at room pressure and temperature. These novel behaviours include metal to insulator transitions, insulator to metal transitions, pressure induced superconductivity, valency changes and structural phase transitions. Structural phase transitions are seen in many simple elemental materials when exposed to variations in pressure and temperature. In germanium, a number of well studied room temperature solid to solid phase transitions are known however few studies have been carried out on the high temperature behaviour of these phases. A study of high temperature germanium is presented at pressures up to 111 GPa using powder x-ray diffraction techniques showing evidence of a novel high temperature crystalline phase, Ge-VIII. Alongside this new phase, further extensions to the phase diagram are proposed, exploring the Ge-II/Ge-IV phase boundary. Another phenomenon often observed under extreme conditions is the introduction of novel reaction pathways. One such material group of interest within the area of novel reaction pathways is the formation of hydrides. These hydrides formed under high pressure often have high hydrogen densities and as such are of potential interest as potential superconducting materials as well as some hydrides having predicted behaviours analogous to those of atomic solid metallic hydrogen. Some experimental results are presented from investigations into the formation group XIV hydrides under high pressure which show no evidence of hydride formation up to 135 GPa along with evidence of the formation of a potentially superconducting hydride of Co.