Temperature-induced amorphisation of hexagonal ice.

Area of Science:

• Materials Science
• Physical Chemistry
• Geophysics

Background:

• Polymorphic transformations and amorphisation of hexagonal ice are critical phenomena under specific temperature and pressure conditions.
• Understanding these transitions is key to comprehending ice phase behavior in various environments.

Purpose of the Study:

• To systematically investigate the competition between polymorphic transformations and amorphisation of hexagonal ice.
• To elucidate the mechanisms of temperature-induced amorphisation (TIA) and its distinction from pressure-induced amorphisation (PIA).

Main Methods:

• In situ dilatometry and ex situ X-ray diffraction and calorimetry were employed.
• Volume-temperature data were analyzed using a novel fitting approach.
• Hexagonal ice was subjected to isobaric heating from 77 K to 170 K at pressures between 0.50 and 1.00 GPa.

Main Results:

• Hexagonal ice transformed to ices IX/III and IV at lower pressures (0.50-0.80 GPa).
• Amorphisation to very high-density amorphous ice (VHDA) became dominant at higher pressures (0.85-1.00 GPa).
• This study provides the first diffraction-based observation of TIA in hexagonal ice.

Conclusions:

• TIA is proposed as a first-order phase transition, involving immediate vitrification after melting, without a precursor process.
• The activation energies for amorphisation and polymorphic transformation were found to be equal around 0.75 GPa.
• At 1.00 GPa, the activation energy for amorphisation of hexagonal ice is significantly lower than for polymorphic transitions.