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The stability of a crystal with diamond structure for patchy particles with tetrahedral symmetry

Eva G. Noya, Carlos Vega, Jonathan P. K. Doye and Ard A. Louis

J. Chem. Phys. 132, 234511 (2010)


The phase diagram of model anisotropic particles with four attractive patches in a tetrahedral arrangement has been computed at two diļ¬€erent values for the range of the potential, with the aim of investigating the conditions under which a diamond crystal can be formed. We find that the diamond phase is never stable for our longer-ranged potential. At low temperatures and lower pressures, the fluid freezes into a body-centred cubic solid that can be viewed as two interpenetrating diamond lattices with a weak interaction between the two sublattices. Upon compression, an orientationally ordered face-centred-cubic crystal becomes more stable than the body-centred cubic crystal, and at higher temperatures a plastic face-centered-cubic phase is stabilized by the increased entropy due to orientational disorder. A similar phase-diagram is found for the shorter-ranged potential, but at low temperatures and pressures, we also find a region over which the diamond phase is thermodynamically favored over the body-centered cubic phase. The higher vibrational entropy of the diamond structure with respect to the body-centered cubic solid explains why it is stable even though the enthalpy of the latter phase is lower. Some preliminary studies on the growth of the diamond structure starting from a crystal seed were performed. Even though the diamond phase is never thermodynamically stable for the longer-ranged model, direct coexistence simulations of the interface between the fluid and the body-centred-cubic crystal and between the fluid and the diamond crystal show that, at sufficiently low pressures, it is quite probable that in both cases the solid grows into a diamond crystal, albeit involving some defects. These results highlight the importance of kinetic effects in the formation of diamond crystals in systems of patchy particles.

The full paper is available from JCP Online and arXiv.org.