Cubic Ice Ic: How Researchers Made a Defect-Free Form

This article explains what cubic ice (ice Ic) is, why it has been so hard to study, and how researchers finally made a nearly defect-free sample. You will learn the difference between cubic and hexagonal ice, the methods used to create pure cubic ice, and why this matters for fields like atmospheric science and cryobiology.

What Is Cubic Ice (Ice Ic)?

Cubic ice, also called ice Ic (the “c” stands for cubic), is a crystalline form of ice where the water molecules arrange in a cubic lattice, unlike the hexagonal arrangement of ordinary ice (ice Ih) that forms in freezers and snowflakes. In ice Ic, each oxygen atom is tetrahedrally bonded to four others, but the overall symmetry is cubic (specifically, a diamond-like structure). This form is metastable at low temperatures and pressures, meaning it can exist but is not the most stable phase under those conditions. Ice Ic typically forms when water vapor deposits onto a cold surface below about -80°C, or when high-pressure ice phases are decompressed at low temperatures.

Why Is Pure Cubic Ice So Hard to Make?

The challenge is that ice Ic almost always forms with stacking faults—random errors in the sequence of layers that make the structure a mix of cubic and hexagonal stacking. Even when researchers think they have cubic ice, X-ray diffraction often shows broad peaks that indicate a high density of faults. This “stacking-disordered ice” (ice Isd) has properties that differ from ideal cubic ice, making it difficult to measure the true characteristics of pure ice Ic. The faults also affect thermodynamic properties like entropy and heat capacity, leading to inconsistent experimental results.

How Researchers Made a Defect-Free Form

In 2020, a team led by Alexander Rosu-Finsen and Christoph Salzmann at University College London reported the first creation of essentially defect-free cubic ice. Their method involved a clever sequence:

• First, they compressed ordinary hexagonal ice (Ih) at low temperature (around -200°C) to form ice XI, an ordered proton phase.
• Then they heated this ice XI to about -150°C while keeping it under pressure, transforming it into a high-density amorphous ice.
• Finally, they warmed the amorphous ice to about -150°C at ambient pressure, allowing it to crystallize into cubic ice with very few stacking faults.

This process, called “ice XI memory,” works because the ordered ice XI template guides the crystallization into a pure cubic structure, avoiding the random stacking that normally occurs. The resulting ice Ic showed sharp X-ray diffraction peaks consistent with a nearly perfect cubic lattice.

How the Defect-Free Form Was Verified

The researchers used X-ray diffraction to confirm the structure. In a perfect cubic ice, the diffraction pattern shows only certain peaks (e.g., 111, 220, 311) without the extra peaks that arise from hexagonal stacking. Their sample showed exactly those peaks, with no evidence of stacking faults. They also performed calorimetry to measure the heat release during transformation to hexagonal ice, which matched the expected energy difference for pure cubic ice. This combination of structural and thermodynamic evidence confirmed that the sample was indeed defect-free cubic ice.

Why It Matters: Properties and Applications

Having a pure sample allows scientists to measure the true properties of cubic ice, such as its density, heat capacity, and vapor pressure. For example, the density of cubic ice is about 0.92 g/cm³, slightly less than hexagonal ice (0.917 g/cm³) but the difference is small. More importantly, cubic ice is believed to exist in the upper atmosphere, where water vapor freezes at very low temperatures, and in comets. Understanding its formation and stability helps model atmospheric processes and the behavior of ice in space. In cryobiology, cubic ice can form during rapid cooling of biological samples, potentially damaging cells; knowing its properties helps improve preservation techniques.

Comparison with Other Ice Phases

Ice has at least 18 known crystalline phases. The most common is hexagonal ice (Ih), which is the stable form at normal pressures. Cubic ice (Ic) is metastable and forms under specific conditions. Other phases like ice II, III, V, VI, etc., require high pressures. The key difference between Ih and Ic is the stacking sequence of the layers: Ih has an ABAB pattern, while Ic has an ABCABC pattern. This difference affects the symmetry but not the local bonding. The defect-free cubic ice is important because it represents the ideal Ic structure, while most natural cubic ice is actually stacking-disordered.

Common Misconceptions and Challenges

One common misconception is that cubic ice is a separate substance with entirely different chemistry; in fact, it is just a different crystal arrangement of water molecules. Another is that you can make it by simply freezing water quickly—while rapid cooling can produce cubic ice, it often contains many faults. The main challenge in studying ice Ic is that it is metastable and transforms to hexagonal ice when heated above about -80°C, so experiments must be done at very low temperatures. Also, the term “cubic ice” is sometimes used loosely to include stacking-disordered ice, but the defect-free form is now the gold standard for research.

Frequently Asked Questions

What is the difference between cubic ice and regular ice?

Regular ice (ice Ih) has a hexagonal crystal structure, while cubic ice (ice Ic) has a cubic structure. Both are made of water molecules with tetrahedral bonding, but the arrangement of layers differs. Cubic ice is metastable and typically forms at low temperatures or under specific pressure conditions.

Can I make cubic ice in my freezer?

No, a standard freezer (around -18°C) produces hexagonal ice. Cubic ice requires temperatures below about -80°C, which is not achievable in a home freezer. Even then, the ice would likely be stacking-disordered unless you use the specialized method described above.

Why is defect-free cubic ice important?

Defect-free cubic ice allows accurate measurement of its physical properties, which is essential for understanding its role in atmospheric science, planetary science, and cryobiology. It also helps validate theoretical models of ice behavior.

Does cubic ice exist in nature?

Yes, cubic ice is thought to form in the upper atmosphere, in polar stratospheric clouds, and on comets where temperatures are very low. However, most natural cubic ice is stacking-disordered, not the pure cubic form.

How long does cubic ice last?

At temperatures below about -80°C, cubic ice is metastable and can persist indefinitely. Above that temperature, it slowly transforms to hexagonal ice. The transformation is exothermic and releases a small amount of heat.

What is the density of cubic ice?

Defect-free cubic ice has a density of about 0.92 g/cm³ at -150°C, which is slightly lower than hexagonal ice (0.917 g/cm³) but the difference is within measurement uncertainty. The exact value depends on temperature and pressure.

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in t | Temperature (°C) | °C | -150 | -273
in p | Pressure (GPa) | GPa | 0 | 0
out phase | Expected phase | - | if(t < -80 and p < 0.5, "Cubic ice (Ic)", if(t < 0 and p < 0.5, "Hexagonal ice (Ih)", "Liquid/other")) | 0