= Solution
\b[<Symmetry breaking in nanocrystal growth> makes growth rates along initially equivalent crystallographic directions unequal], so the final shape has lower symmetry than an unperturbed seed. It need not change the underlying <face-centered cubic lattice>. A cubic lattice does not require a cubic external shape.
Anisotropy can originate in a twin or <stacking fault>, unequal ligand coverage of nominally equivalent sites, selective adsorption or etching, a substrate or external field, or a local fluctuation amplified because newly deposited material changes subsequent growth. Facet-selective binding suppresses some normal growth rates; concentration and reduction kinetics determine whether atoms can redistribute towards a near-equilibrium shape or remain in a kinetically selected one. A facet with slow normal growth tends to remain exposed, while a fast-growing facet can disappear.
For a one-dimensional silver example, begin with a small seed with multiple regions related by <crystal twinning> containing five sectors around a common axis. Ligand-protected lateral faces and more active ends favour axial addition, producing an elongated pentagonal rod and then a wire. The defect geometry and surface kinetics jointly distinguish the axial direction. Fivefold twinning is a composite structure with strain accommodation, not a perfect single fcc crystal with forbidden fivefold lattice symmetry.
For a two-dimensional example, begin with a seed containing a planar twin or stacking fault. Basal-face protection and enhanced incorporation at suitable edges favour lateral extension over thickening, producing a thin triangular or hexagonal plate. The accompanying schematic emphasizes seed, symmetry-breaking event and anisotropic growth stages. These are representative routes, not universal requirements for every rod or plate. Experimentally, silver-seed growth can also be redirected into lower-symmetry forms by controlling reduction kinetics.<Symmetry Breaking during Seeded Growth of Nanocrystals by Xiaohu Xia and Younan Xia>[2012, seeded-silver growth and kinetic symmetry breaking]{ref}
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