Specialist #5

Thermal conductivity, shear modulus, and transverse sound speed of ice decrease with pressure

For most solids these properties increase with pressure -- for ice they decrease.

Scientific Explanation

In most crystalline solids, increasing pressure brings atoms closer together, stiffens the lattice, and causes heat and sound waves to propagate faster. Ice Ih behaves in the opposite way: its thermal conductivity, shear modulus, and transverse sound speed all decrease with rising pressure. This behavior is extremely rare among crystalline materials.

The explanation lies in the open, tetrahedral structure of ice Ih. Each oxygen atom is hydrogen-bonded to exactly four neighbors, creating a lattice with large cavities. Under pressure, these cavities are not simply compressed uniformly — instead, the hydrogen bond angles become distorted. These angular distortions weaken the intermolecular couplings through which heat is transported and mechanical forces are transmitted.

Additionally, the phonons — the vibrational modes that carry heat through the crystal — are scattered more strongly by the increasing disorder in the lattice. The mean free path of phonons shortens, and the thermal conductivity drops. Simultaneously, the material becomes less stiff under pressure because the distorted hydrogen bonds offer less resistance to shearing. This anomaly is yet another expression of the unique hydrogen-bond-dominated architecture of ice.

It should be noted that this behavior has been studied primarily in ice Ih. High-pressure ice phases (ice II, III, and others) show partly different trends because their crystal structures differ fundamentally from the open ice Ih framework. The precise atomic-level mechanisms remain a subject of active research, particularly through molecular dynamics simulations.

Ice Thermal Conductivity vs Pressure Line chart showing how the thermal conductivity of ice Ih decreases as pressure increases from 0 to about 0.3 GPa, contrary to the behavior of most crystalline solids where thermal conductivity increases with pressure. Thermal Conductivity (W/mK) 3.0 2.5 2.0 1.5 0 0.1 0.2 0.3 Pressure (GPa) Ice Ih Typical solid Ice Thermal Conductivity vs Pressure
Thermal conductivity of ice Ih compared to a typical solid under increasing pressure.

Everyday Relevance

Although this effect occurs at extreme pressures and is hardly noticeable in daily life, it matters for understanding ice sheets and glaciers. In the deep layers of ice caps — such as those in Antarctica or Greenland — considerable pressures exist. The reduced thermal conductivity affects how quickly geothermal heat is transported upward through the ice, influencing melt rates at the base of glaciers. For climate models that predict the behavior of large ice sheets, these material properties are therefore significant.