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

Hopeman 224

https://rochester.zoom.us/j/93079783534

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).