Studies of phase and strength in high pressure vanadium

Abstract

High temperature and pressure states of matter are ubiquitous in nature. It is therefore of the upmost importance that they are understood. Extreme pressures can be obtained using both static and dynamic compression. Static compression using a diamond anvil cell (DAC) can reach pressures into the multi-100 GPa range and, combined with resistive and laser heating techniques, can simultaneously access high-temperature regimes. Dynamic compression using either impactors or lasers can be employed to simultaneously generate high temperature-pressure states into the TPa regime. The differing pressure and temperature states accessed make static and dynamic compression methods ideal for complimentary studies on a material’s behaviour. To probe material conditions under extreme conditions x-ray diffraction is a powerful tool, allowing the different structures adopted by samples to be studied directly. In this thesis in situ x-ray diffraction was combined with static and dynamic compression techniques to probe the structural behaviour of vanadium (V), the high-pressure behaviour of which is unique amongst the elements. At 120 GPa, V has one of the highest known superconducting transition temperatures in the elements, Tc=17.2 K. Subsequent lattice dynamic calculations aimed at understanding the high Tc noted dramatic softening of a transverse acoustic mode, suggestive of a structural phase transition. A bcc-rhombohedral structural transition has been reported by x-ray diffraction studies using DACs at 30-70 GPa, and the same transition has also been reported in several computational studies. These latter studies predict a transition to a second rhombohedral phase at 120 GPa, and a re-entrant transition to the bcc phase at 280 GPa. Neither of these higher-pressure phases have been observed experimentally. Dynamic compression studies of V have reported evidence of a phase transition starting at 32 GPa and completing at 60 GPa, but no x-ray diffraction data were collected. Despite near universal agreement on there being a bcc-rhombohedral phase transition in V between 30 and 70 GPa, the experimental evidence is surprisingly weak. Indeed, close analysis of the published diffraction data reveals that they do not fit a rhombohedral structure, thereby inspiring the current study of V using both static and dynamic compression techniques. For direct comparison with previous studies, V was compressed with no pressure transmitting medium (PTM) to 139 GPa and with a mineral oil PTM to 118 GPa. The diffraction data from these non-hydrostatic experiments were completely consistent with previous studies, showing splitting of the bcc peaks starting at 45 GPa. However, these splittings could not be fitted by the reported rhombohedral structure. V samples were also compressed in He to 154 GPa, reproducing previous quasi-hydrostatic studies. The peaks of the bcc phase were observed to start splitting at pressures as low at 20 GPa, but again a rhombohedral structure was unable fit the diffraction profiles from the high-pressure phase. Difficulties in studying the bcc-rhombohedral transition arise from an inherent limitation of the powder-diffraction method, that is the overlap of peaks with similar d-spacings. This was overcome by making studies of [001]-oriented single crystals of V compressed in a mineral oil PTM to 118 GPa. The bcc peaks were found to split at 40 GPa, and, in contrast to the powder studies, the high-pressure phase could be fitted by a rhombohedral structure. The pressure dependence of the rhombohedral angle is suggestive of two different rhombohedral phases, but the distortions are much smaller than those predicted by theory. The excellent fit of the rhombohedral phase to the single crystal data when compared with the powder data suggests that texture of the powdered sample, which would not be present in the single crystal may cause the misfits to the reported structures. For comparison with the DAC studies, shock compression studies were carried out on V foils up to 180 GPa along the principal Hugoniot. The data collected below 40 GPa were well fitted by the bcc structure but above that pressure a rhombohedral structure gave a better fit. The fit was much better than that obtained with statically compressed powder samples, and the rhombohedral structure agrees with that obtained from the single-crystal data. For comparison with the shock compression study, high-temperature static compression was conducted using a resistive heating set up and a KCL PTM. The rhombohedral transition was seen to occur at higher pressures under these conditions, with the fitted rhombohedral lattice appearing more bcc-like, suggesting the rhombohedral transition was suppressed by high temperatures.