Ice IhEdit
Ice Ih is the hexagonal crystalline form of water ice that dominates Earth's ice at ambient pressure. It forms when liquid water freezes or when water vapor condenses and deposits onto a surface at temperatures below freezing. The structure is defined by a three-dimensional network of oxygen atoms linked by hydrogen bonds, resulting in a tetrahedral coordination around each oxygen and a characteristic hexagonal stacking of layers. The arrangement in Ice Ih is often described as disordered with respect to the orientations of individual water molecules, subject to the constraints known as the ice rules. This proton disorder gives Ice Ih a notable configurational entropy even at temperatures well below the freezing point.
As the most common solid phase of water on Earth, Ice Ih underpins many natural and human activities. It is found in sea ice, glaciers, snow, and frost, and it plays a central role in climate processes, ocean dynamics, and cryosphere hydrology. The study of Ice Ih has driven fundamental questions about hydrogen bonding, crystal structure, and phase behavior in water, and it provides a reference point for understanding the broader family of crystalline ices that can form under varying pressure and temperature conditions. For comparisons with other forms of ice, see Ice Ic and the broader phase diagram of water.
Structure
Ice Ih crystallizes in the hexagonal system with the space group P6_3/mmc and a layered, close-packed oxygen lattice. Each unit cell contains 12 water molecules, and the oxygen sublattice forms a network that can be visualized as stacked hexagonal layers. The hydrogen atoms are arranged so that each water molecule participates in two short covalent O–H bonds and participates in two longer hydrogen bonds with neighboring oxygens. This arrangement satisfies the Bernal–Fowler ice rules, often summarized as: each oxygen is bonded to four hydrogens in a tetrahedral geometry, with two hydrogens acting as donors and two as acceptors across the lattice. The result is a robust hydrogen-bond network that endows Ice Ih with distinctive vibrational and thermodynamic properties.
A key aspect of Ice Ih is proton disorder: the orientations of the water molecules are not fixed in a long-range ordered fashion at ordinary temperatures, even though the overall network obeys the ice rules. This disorder contributes to a residual configurational entropy, a topic long discussed in the theory of ice. At sufficiently low temperatures, Ice Ih can undergo partial proton ordering if dopants are present; the proton-ordered phase is known as Ice XI and requires impurities to stabilize the ordered arrangement. Neutron and X-ray diffraction studies have been essential in mapping the positions of hydrogen atoms and in testing models of proton disorder and order.
In addition to the hexagonal Ice Ih, another crystalline form of ice, Ice Ic, exhibits cubic stacking and can arise under different conditions, such as rapid cooling or deposition. The stability and transformation between Ice Ih and Ice Ic (and the many high-pressure ices) are described in the broader phase diagram of water. Stacking faults and defects in Ice Ih contribute to real-world samples that may include small percentages of Ice Ic-like domains, particularly in rapidly grown or radiatively processed ice.
For structural details, see crystal structure and neutron diffraction studies that reveal hydrogen positions, together with complementary information from X-ray diffraction.
Proton disorder, order, and residual entropy
A defining feature of Ice Ih is the disordered arrangement of protons (hydrogen atoms) at typical temperatures. The ice rules ensure that each oxygen maintains a tetrahedral coordination with two hydrogens near and two hydrogens participating in hydrogen bonds with neighboring oxygens. This local constraint allows many possible global arrangements of water molecules, leading to a residual entropy even as the crystal remains solid.
The theoretical treatment of this residual entropy was famously advanced by Linus Pauling, who estimated S0 ≈ (R/2) ln(3/2) for Ice Ih. This “Pauling entropy” captures the combinatorial nature of proton disorder under the ice rules and has long informed discussions about disorder in ice. Later experimental and theoretical work has refined the understanding of residual entropy, taking into account correlations and the possibility of partial ordering under certain conditions. The ordered counterpart at low temperatures, achieved with dopants, is Ice XI, a proton-ordered phase that demonstrates the subtle balance between entropy and order in crystalline ice.
Proton ordering in Ice Ih is a topic of ongoing study. Neutron diffraction, calorimetry, and spectroscopic methods continue to test how much of the disorder remains at various temperatures and dopant levels. The presence of impurities (for example, dopants like potassium hydroxide or other acids) can facilitate the transition toward ordered arrangements, which has implications for the interpretation of low-temperature ice behavior and the potential for novel ice phases.
Physical properties and dynamics
Ice Ih is less dense than liquid water, which is a characteristic feature of most crystalline solids compared with their liquid counterparts. The hydrogen-bond network gives Ice Ih distinctive vibrational modes and thermal properties, including a relatively high heat capacity at low to moderate temperatures and a low core conductivity for ions, since protonic movement occurs primarily along hydrogen-bond pathways.
Under standard conditions, Ice Ih is the thermodynamically stable solid phase of water at atmospheric pressure and temperatures below 0°C. Its mechanical integrity derives from an extensive hydrogen-bond network, while its optical properties reflect the clarity and anisotropy associated with a crystalline ice lattice. When energy is added (heating), Ice Ih passes into liquid water at the melting point near 0°C at one atmosphere of pressure; increasing pressure can drive transitions to other ice phases (such as Ice II or higher-pressure forms) before melting, in accordance with the phase diagram of water.
The diffusion of protons within Ice Ih occurs via a mechanism often described as Grotthuss-like hopping along hydrogen bonds. This protonic conduction is much slower than ionic conduction in liquids but is a notable transport channel in the solid state. Isotopic substitution (e.g., using D2O) changes the dynamics and can stabilize different low-temperature phases, including variants related to proton ordering.
Formation and occurrence
Ice Ih forms naturally whenever liquid water freezes at atmospheric pressure or when water vapor deposits onto a cold surface. In the environment, it dominates the structure of sea ice, seasonal snow, and most glacier ice. The microscopic structure of natural ice can include defects, inclusions, and minute fractions of other ice forms, shaped by temperature history, salinity, radiation, and impurities. The phase behavior of water, including the existence of multiple crystalline ices at different pressures, is summarized in the phase diagram of water.
Laboratory studies of Ice Ih employ a range of techniques to probe structure and dynamics, including neutron diffraction to locate hydrogen positions, X-ray diffraction for the overall oxygen lattice, infrared and Raman spectroscopy for vibrational information, and calorimetry to characterize thermodynamic properties. The comparison between Ice Ih and its cubic counterpart Ice Ic, as well as the low-temperature Ice XI, illuminates fundamental aspects of hydrogen bonding and orientational order in solids.