Size Asymmetry in Multidomain Single Crystals and the Development of Curved Surfaces

Straight facets and sharp edges are among the most distinctive indicators of well-defined crystals, and often reflect the polyhedral geometry and symmetry of the underlying close-packed, molecular structure. Curved morphologies are sometimes observed in biogenic crystals where templating or non-classical crystallization processes (e.g., crystallization via amorphous states) occur. Here we report the formation and growth of copper-based metal-organic frameworks (MOFs) that crystallize in the hexagonal space group, P622. The crystals begin as hexagon-like structures, developing over time into complex flower-like structures having two decks joined in the center. Each layer has 6 well-defined petals with curved lateral surfaces. The growth mechanism shows initial straight petals that become progressively more curved with increased faceting. Remarkably, despite its multi-domain appearance, the entire entity is a single-crystal. The curved morphology is correlated with the crystallographic structure and the arrangement of nanosized channels within this structure. Crystal habits are typically considered inconsistent with curved morphologies. This work suggests that crystallographic explanations can support the development of such surfaces for low-density structures.