Evidence for superionic H2O and diffusive He-H2O at high temperature and high pressure.

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.