Scientists found ice XXI (152-molecule repeating unit) and ice XXII (304 molecules), the most structurally complex ice phases ever observed.
Key Takeaways
Over 20 ice phases have been observed since 1900; a 2018 simulation catalogued 75,000 mathematically possible forms, though most are non-viable.
Ice XXI was discovered accidentally at KRISS by squeezing water between diamonds; it was too complex to appear in existing prediction simulations.
A University of Tokyo follow-up found ice XXII (304-molecule unit cell) and also reliably produced the long-elusive metastable phase ice IV.
Both new phases support Ostwald’s step rule: systems transition to the nearest reachable state, not the most thermodynamically stable one.
Pharmaceutical relevance is direct: uncontrolled phase transitions in drug crystals can degrade entire production batches; Ostwald’s rule helps predict this.
