We address the crystallization of monodisperse hard spheres in terms of theproperties of finite- size crystalline clusters. By means of large scaleevent-driven Molecular Dynamics simulations, we study systems at differentpacking fractions {\phi} ranging from weakly supersaturated state points toglassy ones, covering different nucleation regimes. We find that such regimesalso result in different properties of the crystalline clusters: compactclusters are formed in the classical-nucleation-theory regime ({\phi} \leq0.54), while a crossover to fractal, ramified clusters is encountered uponincreasing packing fraction ({\phi} \geq 0.56), where nucleation is morespinodal-like. We draw an analogy between macroscopic crystallization of ourclusters and percolation of attractive systems to provide ideas on how thepacking fraction influences the final structure of the macroscopic crystals. Inour previous work (Phys. Rev. Lett., 106, 215701, 2011), we have demonstratedhow crystallization from a glass (at {\phi} > 0.58) happens via a gradual(many-step) mechanism: in this paper we show how the mechanism of gradualgrowth seems to hold also in super-saturated systems just above freezingshowing that static properties of clusters are not much affected by dynamics.
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