Water's polyamorphic transitions and amorphization of ice under pressure

Area of Science:

• Materials Science
• Physical Chemistry
• Geophysics

Background:

• Water exhibits unique amorphous phases, including high-density amorphous (HDA) and low-density amorphous (LDA) ice.
• Understanding the transitions between these phases and crystalline ice is crucial for various scientific disciplines.

Purpose of the Study:

• To investigate the transformations between HDA, LDA, and cubic ice (Ic) under high pressure.
• To elucidate the kinetic and thermodynamic pathways of water's phase transitions.

Main Methods:

• In situ thermal conductivity (kappa) measurements at high pressures.
• Controlled heating and densification experiments on amorphous ice samples.

Main Results:

• The HDA to LDA transformation becomes unobservable at 0.07 GPa, with increasing pressure merging HDA to LDA and LDA to Ic transitions.
• Amorphization of hexagonal ice (Ih) is kinetically controlled, evidenced by decreasing kappa over time.
• A reversible transition between HDA and LDA at ~135 K and ~0.2 GPa is not supported by findings.

Conclusions:

• The pressure-dependent merging of HDA to LDA and LDA to Ic transitions suggests HDA may convert directly to Ic under certain conditions.
• The kinetic control of ice Ih amorphization highlights the importance of time-dependent processes.
• The absence of a unique HDA challenges the hypothesis of a second critical point for water.