Overview
Most of us think of ice as the solid cubes that clink in a glass of water or the frozen layer on a winter pond. But that everyday ice—known as Ice Ih—is just one of many possible crystalline forms of water. Since the early 1900s, physicists have identified more than 20 distinct phases of ice, each with unique properties and structures. These include hot ice that remains solid at high temperatures and even conductive ice that can carry electricity. In this guide, you’ll learn what defines a phase of ice, how these phases are discovered, and what makes some of them so extraordinary.
Prerequisites
Basic Knowledge
- Understand the three common states of matter (solid, liquid, gas).
- Familiarity with the concept of crystal lattices and unit cells.
- Basic grasp of temperature and pressure as thermodynamic variables.
Tools & Resources
- A scientific calculator or graphing software (optional, for phase diagram plots).
- Access to online phase diagram databases (e.g., the IAPWS water phase data).
- Pen and paper to sketch structures described.
Step-by-Step Instructions
Step 1: Start with the Familiar – Ice Ih
The most common phase, Ice Ih, forms under normal atmospheric pressure below 0 °C. Its structure is a hexagonal lattice—think of overlapping hexagons of water molecules. Each water molecule forms hydrogen bonds with four neighbors, giving it a relatively open structure that explains why ice floats. This phase serves as the baseline for all other forms.
Step 2: Increase Pressure – The High-Density Phases (Ice II, III, V, VI, etc.)
When you apply high pressure (hundreds of megapascals) at cold temperatures, the hydrogen bonds reorganize into denser crystal lattices. For example:
- Ice II forms at about 200 MPa and –50 °C. It has a rhombohedral structure, about 20% denser than Ice Ih.
- Ice III appears at similar pressures but higher temperatures (around –20 °C) and has a tetragonal lattice.
- Ice VI emerges near 1 GPa (10,000 atmospheres) at room temperature, featuring two interpenetrating hydrogen-bonded networks.
These phases are not stable at normal room pressure—they revert to Ice Ih or liquid upon decompression.
Step 3: Go to Extremes – Hot Ice and Conductive Ice
Hot ice refers to phases like Ice VII and Ice X. Ice VII forms at pressures above 2 GPa and remains solid well above 100 °C. Its structure is cubic and very dense. Ice X forms at even higher pressures (around 60 GPa) and is symmetric: hydrogen bonds become more covalent-like, and the material behaves like a proton conductor—hence conductive ice. This electro-conductive ice challenges our typical notion of electrical insulation in water.
In recent years, physicists have discovered even more exotic phases, such as Ice XIX (a hydrogen-ordered variant of Ice VI), pushing the count beyond 20.
Step 4: Use a Phase Diagram to Visualize Regions
A phase diagram maps temperature, pressure, and the stable ice phase. For a quick mental exercise, sketch a graph with Pressure on the y-axis (log scale from 0.1 MPa to 100 GPa) and Temperature on the x-axis (from –100 °C to +200 °C). Mark the regions:
- Ice Ih: low pressure, low temperature (below 0 °C).
- Liquid water: around 0 °C to 100 °C at 0.1 MPa.
- Ice III, V, VI: moderate pressure (0.2–2 GPa) at subzero temperatures.
- Ice VII, X: high pressure (2–100 GPa) and high temperature (up to 200 °C).
You can find accurate diagrams on the Wikipedia page for phases of ice.
Step 5: Analyze Crystal Structures with a Simple Code (Python Example)
To grasp the unit cell dimensions, you can use a Python script with the ase library (Atomic Simulation Environment) to visualize a basic hexagonal lattice for Ice Ih. Below is a minimal example (requires pip install ase):
from ase import Atoms
from ase.spacegroup import crystal
# Ice Ih: hexagonal lattice (space group P6_3/mmc)
# Lattice parameters a=4.52 Å, c=7.36 Å (typical)
water = crystal('H2O', basis=[(0.333, 0.666, 0.062), (0.333, 0.666, 0.438), (0.333, 0.666, 0.188), (0.333, 0.666, 0.312)],
spacegroup=194, cellpar=[4.52, 4.52, 7.36, 90, 90, 120])
print(water.cell)
print(water.positions)
# Save as .xyz file for visualization
water.write('ice_ih.xyz')This gives you the coordinates of oxygen and hydrogen atoms in an idealized unit cell. For high-pressure phases, the cell parameters shrink.
Common Mistakes
Mistake 1: Confusing Ice Phases with Amorphous or Clathrate Ice
Not every solid water is a “phase of ice” in the crystallographic sense. Amorphous ice (like that found in interstellar space) lacks long-range order. Clathrate hydrates trap gas molecules. Stick to the phase diagram for true crystalline phases.
Mistake 2: Assuming All Ice Is Cold
Some phases (Ice VII, Ice X) exist at temperatures above 100 °C. “Hot ice” is a real phenomenon—the high pressure forces molecules into a solid despite high thermal energy.
Mistake 3: Forgetting That Phase Boundaries Are Sharp
The transition from Ice Ih to Ice II happens at a specific pressure and temperature. Small changes can cause the ice to transform or melt incorrectly if you’re not precise.
Mistake 4: Neglecting Hydrogen Disorder–Order Transitions
Many ice phases have disordered hydrogen positions at high temperature. Cooling them can produce ordered variants (e.g., Ice XI from Ice Ih). Always check the proton ordering state when identifying a phase.
Summary
You now have a roadmap to navigate the more than 20 known ice phases—from the everyday hexagonal Ice Ih to exotic, conductive Ice X. By understanding the roles of pressure, temperature, and crystal structure, you can appreciate why physicists continue to discover new forms of this seemingly simple substance. The key takeaway: ice is far from ordinary; it’s a material that morphs under extreme conditions, revealing water’s hidden complexity.