Evidence for superionic H2O and diffusive He–H2O at high temperature and high pressure

In this work, we present the evidence of superionic phase formed in H2O and, for the first time, diffusive H2O–He phase, based on time-resolved x-ray diffraction experiments performed on ramp-laser-heated samples in diamond anvil cells. The diffraction results signify a similar bcc-like structure of superionic H2O and diffusive He–H2O, while following different transition dynamics. Based on time and temperature evolution of the lattice parameter, the superionic H2O phase forms gradually in pure H2O over the temperature range of 1350–1400 K at 23 GPa, but the diffusive He–H2O phase forms abruptly at 1300 K at 26 GPa. We suggest that the faster dynamics and lower transition temperature in He–H2O are due to a larger diffusion coefficient of interstitial-filled He than that of more strongly bound H atoms. This conjecture is then consistent with He disordered diffusive phase predicted at lower temperatures, rather than H-disordered superionic phase in He–H2O.