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Workshop / Seminar

Physical Chemistry Seminar – Sohan Ahmed, Chemistry Graduate Student

Fulmer Hall
Fulmer 150
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About the event

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

Abstract: H2O is the major constituents in our planet and other “icy” Giant planets. Upon solidification at ambient temperature, it forms a wide range of ice phases in various hydrogen bonding networks at various pressure-temperature (PT) conditions. Some of them are orientationally ordered, some are disordered, and some are partially disordered. At high PT conditions the disordering in H2O can give rise to the superionic (SI) H2O phase where H becomes fully disordered and making the conductivity very high. It has been predicted that the H2O-He mixture produces two SI phases at high PT.

Our data suggests that superionic phase forms in pure H2O and for the first time in He-H2O mixture at high PT conditions. The diffraction results signify a similar bcc-like structure of superionic H2O and diffusive He–H2O, while following different transition dynamics. Based on time-resolve ramp laser heated diamond anvil cell (LH-DAC) data, pure H2O forms SI phase over the temperature range from 1350 K – 1400 K at 23 GPa with a gradual transition to the H-disordered SI phase but in He-H2O mixture SI phase forms abruptly at 1300 K at 26 GPa. The lower transition temperature of H2O-He mixture to the SI phase is because of the much higher diffusion rate of He than strongly bonded H in the H-bonded network in H2O.