Phase transformation and transport in sulphur at high pressure and temperature.

Abstract

Sulphur has one of the most complex high pressure-temperature phase diagrams of the elements, with many solid phases and multiple liquid phases reported up to 40 GPa (400 kbar). The high temperature stability of solid phases is poorly understood and the melting curve above 12 GPa has been largely unstudied. As pressure and temperature increase, optical and electronic characteristics of sulphur change significantly in both solid and liquid phases. At present there are four generally accepted solid phases up to 83 GPa, above which solid sulphur becomes metallic. Sulphur is thought to be present in multiple planetary cores and is a constituent of H3S, a high pressure-temperature superconductor (TC = 203 K), so understanding its behaviour at extremes is of broad interest. In this thesis, multiple probe techniques have been used to examine phase transformations and transport properties of sulphur at high pressure-temperature conditions, including optical spectroscopy and x-ray diffraction measurements. High pressure conditions have been achieved using the diamond anvil cell, and three heating techniques have been implemented; resistive heating, near-IR (1070 nm) laser heating and mid-IR (10.6 µm) laser heating. Pressures were determined using the ruby fluorescence effect, the diamond edge scale, the equation of state of gold during x-ray studies and by comparing Raman band frequencies to the literature. Stokes to anti-Stokes intensity ratio behaviour was used to measure temperatures alongside thermal emission measurements, and a thermocouple during resistive heating experiments. Sulphur samples were heated to temperatures of ∼1900 K, and compressed to 49.9 GPa. Solid and liquid phase changes were identified using Raman spectra, optical observations, anomalies in laser power vs. temperature dependencies, and x-ray diffraction patterns. The melting curve has been extended up to 45.6 GPa, and an extensive study of the stability regions of solid phases has been made. Multiple liquid phases have been detected, exhibiting different optical absorption properties. The time-domain thermoreflectance technique was also investigated for high pressure samples heated using laser pulses to measure transport (thermal conduction).