Water is one of the most familiar substances on Earth, yet its solid form, ice, is a subject of profound scientific complexity. Unlike most materials that have a single solid structure, water is polymorphous, meaning it can freeze into many different crystal arrangements. The unique behavior of the water molecule, \(\text{H}_2\text{O}\), allows its frozen state to adopt a surprising variety of distinct molecular architectures. These different “types” of ice are not mere variations in appearance but fundamentally different phases, each governed by specific conditions of temperature and pressure. Understanding this array of solid phases is important in environments from Earth’s surface to the depths of icy planets and interstellar space.
The Unique Structure of Standard Ice (\(\text{Ice I}_{\text{h}}\))
The ice encountered in nature, from snowflakes to ice cubes, is almost universally known as hexagonal ice, or \(\text{Ice I}_{\text{h}}\) (ice one-h). This familiar phase forms at standard atmospheric pressure and temperatures at or below \(0^\circ \text{C}\). The structure of \(\text{Ice I}_{\text{h}}\) is defined by a highly organized, open, hexagonal lattice created by hydrogen bonds. Every water molecule in this lattice is tetrahedrally coordinated, meaning it is bonded to four neighbors, forming a structure with large voids.
This open-cage arrangement is responsible for one of water’s most famous anomalies: solid ice is less dense than liquid water, allowing it to float. The density of \(\text{Ice I}_{\text{h}}\) is approximately \(0.92 \text{ g/cm}^3\), while liquid water reaches its maximum density of \(1.00 \text{ g/cm}^3\) at \(4^\circ \text{C}\). The lattice contains six-membered rings of water molecules, which stack in layers to form the characteristic six-fold symmetry seen in snowflakes. Within this structure, the oxygen atoms are fixed, but the hydrogen atoms are generally disordered, fluctuating around their bond positions, which classifies \(\text{Ice I}_{\text{h}}\) as a proton-disordered phase.
The Crystalline Polymorphs: Ice Under Pressure
The variety of ice phases expands dramatically under extreme conditions, particularly high pressure. These distinct forms are called crystalline polymorphs and are designated by Roman numerals in the chronological order of their discovery, starting with \(\text{Ice I}\). Scientists have confirmed the existence of at least 20 different crystalline ice phases, ranging from \(\text{Ice I}\) through \(\text{Ice XX}\) and beyond. For example, \(\text{Ice II}\) forms from \(\text{Ice I}_{\text{h}}\) when subjected to pressures over \(200 \text{ MPa}\) (about 2,000 times atmospheric pressure) at temperatures below \(-35^\circ \text{C}\).
As pressure increases, the open-cage structure of \(\text{Ice I}_{\text{h}}\) collapses, forcing the water molecules into denser arrangements. \(\text{Ice VI}\) and \(\text{Ice VII}\), for instance, are significantly denser than liquid water. \(\text{Ice VI}\), which exists at pressures above \(600 \text{ MPa}\) and temperatures between \(0^\circ \text{C}\) and \(-22^\circ \text{C}\), has a complex structure where two interpenetrating, non-hydrogen-bonded lattices coexist. \(\text{Ice VII}\), stable at pressures exceeding \(2.1 \text{ GPa}\) (over 20,000 times atmospheric pressure), is even denser, featuring a body-centered cubic structure.
Many of these crystalline phases are classified as proton-disordered, like \(\text{Ice I}_{\text{h}}\), but some are proton-ordered variants that form at extremely low temperatures. \(\text{Ice XI}\), for example, is the proton-ordered counterpart of \(\text{Ice I}_{\text{h}}\), where the hydrogen atoms settle into fixed, symmetrical positions. Similarly, \(\text{Ice XV}\) is the proton-ordered version of \(\text{Ice VI}\), requiring temperatures below \(-143^\circ \text{C}\) and high pressure to form. These denser forms are theorized to exist naturally within the interiors of icy moons and giant planets, where pressures reach millions of atmospheres.
Amorphous and Other Non-Standard Ice Phases
Beyond the ordered, crystalline forms, ice can also exist in a non-crystalline state known as amorphous ice, often described as glassy water. This ice lacks the long-range, repeating molecular pattern of a crystal, resembling a snapshot of liquid water frozen instantly. Amorphous ice is predominantly found in the cold vacuum of space, such as in interstellar clouds, making it arguably the most abundant form of ice in the universe.
Amorphous ice types are distinguished by density: Low-Density Amorphous (\(\text{LDA}\)), High-Density Amorphous (\(\text{HDA}\)), and Very-High Density Amorphous (\(\text{VHDA}\)). \(\text{LDA}\) (density \(\sim 0.94 \text{ g/cm}^3\)) is formed by condensing water vapor onto a surface at temperatures below \(-143^\circ \text{C}\) or by hyperquenching liquid water. \(\text{HDA}\) (density \(\sim 1.17 \text{ g/cm}^3\)) is created by compressing \(\text{Ice I}_{\text{h}}\) or \(\text{LDA}\) at cryogenic temperatures, forcing the water molecules into a much tighter, more disordered packing.
The existence of a significant density gap between \(\text{LDA}\) and \(\text{HDA}\) led to the recent discovery of Medium-Density Amorphous (\(\text{MDA}\)) ice, formed by intensely grinding crystalline ice at cryogenic temperatures. This polyamorphism, the ability to exist in multiple amorphous states, suggests that liquid water itself may have two distinct liquid forms at very low temperatures. Furthermore, at the theoretical extreme, scientists predict the existence of other non-standard phases, such as superionic ice at pressures above \(50 \text{ GPa}\), where protons can move freely through a solid oxygen lattice, or even metallic ice at pressures exceeding \(1.55 \text{ TPa}\).