Phase transition kinetics of superionic H2O ice phases revealed by Megahertz X-ray free-electron laser-heating experiments
Rachel J. Husband, Hanns‐Peter Liermann, J. McHardy, R. S. McWilliams, Alexander F. Goncharov, Vitali B. Prakapenka, Eric Edmund, S. Chariton, Zuzana Konôpková, C. Strohm, Carmen Sanchez‐Valle, Mungo Frost, L. Andriambariarijaona, Karen Appel, Carsten Baehtz, Orianna B. Ball, Ronald Briggs, Johannes Buchen, Valerio Cerantola, Jinhyuk Choi, A. L. Coleman, Hyunchae Cynn, Ashish Ranjan Dwivedi, H. Graafsma, Huijeong Hwang, Egor Koemets, Torsten Laurus, Yongjae Lee, Xinyu Li, Hauke Marquardt, Anshuman Mondal, M. Nakatsutsumi, S. Ninet, Edward J. Pace, Charles Pépin, Clemens Prescher, Stephan Stern, J. Sztuk-Dambietz, U. Zastrau, M. I. McMahon
Abstract
Abstract H 2 O transforms to two forms of superionic (SI) ice at high pressures and temperatures, which contain highly mobile protons within a solid oxygen sublattice. Yet the stability field of both phases remains debated. Here, we present the results of an ultrafast X-ray heating study utilizing MHz pulse trains produced by the European X-ray Free Electron Laser to create high temperature states of H 2 O, which were probed using X-ray diffraction during dynamic cooling. We confirm an isostructural transition during heating in the 26-69 GPa range, consistent with the formation of SI-bcc. In contrast to prior work, SI-fcc was observed exclusively above ~50 GPa, despite evidence of melting at lower pressures. The absence of SI-fcc in lower pressure runs is attributed to short heating timescales and the pressure-temperature path induced by the pump-probe heating scheme in which H 2 O was heated above its melting temperature before the observation of quenched crystalline states, based on the earlier theoretical prediction that SI-bcc nucleates more readily from the fluid than SI-fcc. Our results may have implications for the stability of SI phases in ice-rich planets, for example during dynamic freezing, where the preferential crystallization of SI-bcc may result in distinct physical properties across mantle ice layers.