Size Asymmetry in\nMultidomain Single Crystals and\nthe Development of Curved Surfaces

Straight facets and sharp edges are among the most distinctive\nindicators of well-defined crystals and often reflect the polyhedral\ngeometry and symmetry of the underlying close-packed, molecular structure.\nCurved morphologies are sometimes observed in biogenic crystals where\ntemplating or nonclassical crystallization processes (e.g., crystallization\nvia amorphous states) occur. Here we report the formation and growth\nof copper-based metal–organic frameworks (MOFs) that crystallize\nin the hexagonal space group <i>P</i>622. The individual\nMOFs are homochiral and are formed from achiral compounds. The crystals\nbegin as hexagonal-like structures, developing over time into complex\nflower-like structures having two decks joined in the center. Each\ndeck layer has six well-defined petals with curved lateral surfaces.\nThe growth mechanism shows initial straight petals that become progressively\nmore curved with increased faceting. Remarkably, despite its multidomain\nappearance, the entire entity is a single crystal. The curved morphology\nis correlated to the crystallographic structure and the arrangement\nof nanosized channels within this structure. Crystal habits are typically\nconsidered to be inconsistent with curved morphologies. This work\nsuggests that crystallographic explanations can support the development\nof such surfaces for low-density structures.