Ice XXII: New Metastable Ice Phase

• Ice XXII is a metastable ice phase characterized by a large unit cell (Z = 304) and a face-centered orthorhombic structure with regular tetrahedral hydrogen bonding.
• It is generated by isothermal compression of emulsified water at pressures from 1.39 to 1.79 GPa and temperatures between 220 and 250 K, demonstrating controlled supercooling.
• Advanced crystallographic techniques, including synchrotron X-ray diffraction and neutron scattering, were pivotal in revealing Ice XXII's unique structure and its implications for water polymorphism.

Ice XXII is a recently discovered metastable crystalline phase of water formed under high-pressure and deeply supercooled conditions using emulsified water. Distinguished by an exceptionally large unit cell and a face-centered orthorhombic structure, Ice XXII expands the known polymorphism of ice and reveals new insights into the complex structural possibilities of water. First reported in 2025 through combined experimental and computational analyses, this phase demonstrates the critical role of controlled supercooling and nanoscale confinement for elucidating metastable regions of the water phase diagram.

1. Generation and Stabilization of Ice XXII

Ice XXII is synthesized by isothermal compression of emulsified water, a process in which water is dispersed in an organic matrix at high pressure and low temperature. Typical water:matrix ratios are approximately 7:10. This emulsified state allows supercooling of the water phase while preventing immediate crystallization into conventional ice phases. Critical experimental formation conditions involve pressures ranging from 1.39 to 1.79 GPa and temperatures between 220 and 250 K. For example, key diffraction data (Exp306-007) correspond to 230 K and 1.79 GPa, with other successful syntheses at 220 K/1.39 GPa and 250 K/1.58–1.65 GPa.

Emulsification enables access to the deeply supercooled region near the homogeneous nucleation temperature by suppressing heterogeneous nucleation and stabilizing the supercooled state long enough for a distinct metastable phase—Ice XXII—to form. This technique is essential for probing parts of the water phase diagram that are normally bypassed due to rapid crystallization into other ice polymorphs. The result is an unusual ordering of water molecule orientations that underpins the unique structure of Ice XXII (Kobayashi et al., 18 Jul 2025).

2. Crystallographic and Structural Features

The crystal structure of Ice XXII was determined through a multi-step process involving high-resolution synchrotron x-ray diffraction and neutron scattering analyses. Autoindexing of diffraction patterns established a face-centered orthorhombic lattice. Comprehensive structure solution efforts (Pawley fitting, maximum entropy method (MEP) optimization, and charge-flipping calculations) consistently converged on the space group Fdd2 (International Tables number 43, setting 1). The resulting unit cell contains Z = 304 water molecules—an exceptionally large repeating unit compared to other known ice phases.

Within the Fdd2 space group, the high-symmetry Wyckoff position 8a constrains the placement of oxygen atoms. The best fit to experimental data was achieved using these symmetry constraints, resulting in a calculated density (at 250 K, 1.65 GPa) that is intermediate between those of ices VI and VII. After identification of the oxygen sublattice by charge-flipping, only minimal positional adjustments were required during Rietveld refinement. Hydrogen atom positions were added subsequently, informed both by symmetry and by molecular dynamics (MD) simulations.

Molecular dynamics simulations initiated from the refined oxygen lattice demonstrated that hydrogen bonding adopts a regular tetrahedral arrangement. The simulated hydrogen positions were found to agree well with the Fdd2 symmetry when merged with the experimental oxygen framework. Unlike some other new ices where local hydrogen disorder is substantial, all water molecules in Ice XXII form a nearly ideal network.

A representative structural analysis employs the Patterson function

$$P(u,v,w) = \sum_{hkl} |F_{hkl}|^2 \exp[-2\pi i (h\cdot u + k\cdot v + l\cdot w)]$$

as part of the MEP calculation foundational to electron density mapping and charge-flipping.

3. Relation to Other Ice Phases

Ice XXII is topologically distinct from both previously known and contemporary metastable ice phases. For example, Ice XXI, which forms under similar conditions, adopts a body-centered tetragonal lattice (space group I̅42d) with local hydrogen-bond network complexity and disorder, unlike the regular tetrahedral motif of Ice XXII. Upon cooling, Ice XXI can transform into Ice XXIII, an orientationally ordered phase derived via symmetry reduction. However, Ice XXII does not participate in this sequence, instead maintaining its unique Fdd2 symmetry and tetrahedral hydrogen network.

The structural distinctions can be summarized as follows:

Phase	Space Group	Z (unit cell)	Hydrogen Network
Ice XXI	I̅42d	152	Complex/disordered
Ice XXII	Fdd2	304	Regular tetrahedral
Ice XXIII	P2₁2₁2₁	—	Ordered (derived)

The discovery of Ice XXII and its clear separation from both structurally and topologically related phases emphasizes the structural diversity accessible in the deeply supercooled region, particularly under conditions of suppressed nucleation.

4. Experimental and Simulation Methodologies

The identification and structural characterization of Ice XXII relies on a combination of advanced experimental and computational approaches:

• Powder X-ray diffraction: High-pressure experiments, often involving diamond anvil cells, yield high-quality data. Autoindexing tools (CONOGRAPH) facilitate rapid lattice determination.
• Pattern fitting: Pawley fitting is used to extract precise unit cell parameters prior to structure solution.
• MEP optimization and charge-flipping algorithms: Superflip software implements charge-flipping to generate three-dimensional electron-density maps, leading to reliable assignment of oxygen positions.
• Rietveld refinement: Applied to both x-ray and neutron diffraction data, this approach finalizes the model, with neutron work assuming complete hydrogen disorder consistent with Fdd2 and MD results.
• Molecular dynamics simulations: These start from the experimental oxygen sublattice and are used to both predict and validate the hydrogen-bonding arrangement. In Ice XXII, MD confirms a regular tetrahedral network unperturbed by higher-order disorder.

Concordance between experiment and simulation, especially concerning the large Z = 304 cell and hydrogen-bond network, underscores the robustness of the experimental assignment in Ice XXII.

5. Implications for Water Polymorphism and Future Research

The establishment of Ice XXII unveils new branches in the phase diagram of water, particularly within the previously inaccessible deeply supercooled, high-pressure domain. Its unique oxygen framework with an unusually large unit cell and regular hydrogen arrangement suggests that metastable high-pressure ice phases can exhibit remarkable structural diversity, contingent on fine-tuned pressure, temperature, and nucleation-control strategies.

Key implications and directions for future work include:

• Detailed investigation of the thermodynamic and kinetic frontiers of the phase transitions leading to Ice XXII, including its relationship to other metastable phases such as Ice XXI and Ice XXIII.
• Enhanced experimental efforts utilizing high-resolution neutron and single-crystal diffraction to further resolve subtle order/disorder phenomena and possible low-temperature variants of Ice XXII.
• Ongoing molecular dynamics and ab initio studies to probe local fluctuations, potential emerging hydrogen order at even lower temperatures, and the melting behavior of Ice XXII.
• Insight into the influence of nanoscale confinement (as achieved by emulsification) on polymorph nucleation and stabilization, which holds relevance for understanding ice behavior in natural and technological environments.

6. Broader Context within Water Science

Ice XXII demonstrates that the metastable landscape of water is more intricate than previously appreciated, with nanoscale control and supercooling providing routes to large, complex unit cell arrangements not realized in equilibrium crystallization. The structural simplicity of hydrogen bonding within Ice XXII—contrasted with the complexity seen in other metastable and high-pressure ices—highlights the interplay between confinement, symmetry, and hydrogen ordering that underpins water’s polymorphism. These findings will inform both the refinement of computational models of water and experimental strategies for the exploration of new ice phases within and beyond the known phase diagram.