Multitwinned\nIce Nanocrystals

Multitwinned\nnanocrystals are commonly found in substances that\npreferentially adopt tetrahedral local arrangements, but not yet in\nwater crystals. Ice nanocrystals are pivotal in cloud microphysics,\nand their surfaces become increasingly prominent in determining structure\nas crystal size decreases. Nevertheless, discussions on nanocrystal\nstructures have predominantly centered on ice polymorphs observed\nin bulk: hexagonal (Ih), cubic (Ic), and stacking-disordered (Isd)\nices. Here, we demonstrate, through molecular dynamics (MD) simulations,\nthat decahedral and icosahedral nanocrystals form from liquid water\ndroplets of a few nanometers in size without violating the ice rule.\nThe brute force spontaneous crystallization is conducted using the\nmW model, and the thermodynamic stability is examined using the TIP4P/Ice\nmodel. During the crystallization process, the formation of twin boundaries\nprecedes the emergence of centers exhibiting 5-fold and icosahedral\nsymmetry. The free energy calculation suggests the icosahedron has\ncomparable stability with ice Ih nanocrystal. The frequent occurrence\nof these unreported ice nanocrystals aligns with the fact that natural\npolycrystalline snow crystals predominantly display a 70.5-degree\nangle between the Ih <i>c</i>-axes of adjacent branches.\nMoreover, we show that the formation of multitwinned ice nanocrystals\nis enhanced within a fullerene, providing a potential avenue for experimental\nobservations.