Negative thermal expansion in ice I polytypes

TL;DR

This study investigates the negative thermal expansion behavior in different ice I polytypes, revealing similarities between cubic and hexagonal ice and quantifying the metastability of cubic ice through enthalpy calculations.

Contribution

It provides the first experimental density measurements of cubic ice across temperatures and compares its properties with hexagonal ice, highlighting their thermodynamic relationships.

Findings

Cubic ice exhibits negative thermal expansion at low temperatures.

Strong analogies exist between cubic and hexagonal ice properties.

Metastability of cubic ice is quantified through enthalpy calculations.

Abstract

The fundamental properties of ice have always attracted a lot of interest due to omnipresence of ice in many different natural contexts. Since cubic ice recently become experimentally accessible from a low-density gas hydrate precursor [1, 2], it has been possible to measure its density as a function of temperature in the whole thermodynamic range of metastability. We found strong analogies with respect to the other ice I polytype, i.e., hexagonal ice Ih [3], including the presence of a negative thermal expansion behavior at low temperature. Based on these results, a new enthalpy calculation quantifies the metastable nature of the cubic form and, consequently its inaccessibility from a "normal" ice Ih precursor.