Measuring the high-pressure melting curve of platinum using in-situ x-ray diffraction and optical pyrometry
Mary Kate Ginnane, PhD Qualifying Exam, Advised by Professor Gilbert 'Rip' Collins and Ryan Rygg
Tuesday, June 21, 2022
Platinum is valuable to both static and dynamic high-pressure experiments due to mechanical and thermal properties that make it an ideal pressure calibrant. It has a high melting temperature, relative chemical inertness, and exhibits structural stability over a wide range of pressure-temperature states. However, platinum is predicted to undergo a solid-solid phase transition from face-centered cubic (fcc) to a randomly disordered hexagonal close-packed (rhcp) phase at pressures P and temperatures T of 35 < P < 300 GPa and 3500 < T < 14000K [Burakovsky 2014]. Additionally, there remain significant discrepancies in the melting curve of platinum above 10 GPa. When extended to the Hugoniot, the range of predicted melting temperatures and pressures span well over 3400 K and 150 GPa [Liu 2010; Jeong 1999; Anzellini 2019]. In this work, we explore the phase diagram of laser-driven Pt using x-ray diffraction, velocimetry and optical pyrometry to infer phase, density, pressure, and temperature. Preliminary results show no evidence of a solid-solid phase transition in shock-ramped Pt, with initial shocks of P = 80, 110, and 155 GPa and isentropic compression up to P = 530 GPa. Diffraction measurements of shocked platinum indicate that Pt is fully melted at 500 GPa on the Hugoniot. These results serve to help resolve the discrepancies for Pt shock-melting and within the more widely-studied pressure ranges (P < 110 GPa in static experiments).