Point defects in crystals of charged colloids

Abstract

Charged colloidal particles—on both the nano and micron scales—have been instrumental in enhancing our understanding of both atomic and colloidal crystals. These systems can be straightforwardly realized in the lab and tuned to self-assemble into body-centered-cubic (BCC) and face-centered-cubic (FCC) crystals. While these crystals will always exhibit a finite number of point defects, including vacancies and interstitials—which can dramatically impact their material properties—their existence is usually ignored in scientific studies. Here, we use computer simulations and free-energy calculations to characterize vacancies and interstitials in FCC and BCC crystals of point-Yukawa particles. We show that, in the BCC phase, defects are surprisingly more common than in the FCC phase, and the interstitials manifest as so-called crowdions: an exotic one-dimensional defect proposed to exist in atomic BCC crystals. Our results open the door to directly observe these elusive defects in the lab.

Defects in crystals of soft colloidal particles

“…This was also found for the FCC crystal of hard spheres 32 and repulsive point Yukawa particles. 10 More interesting are the deformations of the other crystals. In the BCC crystal, the vacancy causes a symmetry-broken, three-dimensional deformation in which only 4 of the 8 nearest neighbors are displaced, each leaving a trail of inwards moving particles behind.…”

“…This one-dimensional deformation was also found in the BCC crystal of repulsive point Yukawa particles. 10 …”

“…Following ref. 10 , we determine f vac(int) ( ρ M , T ) using thermodynamic integration in combination with MC simulations.…”

“…Similarly, for f int , we measured the free-energy difference between a system where one of the lattice sites contains two particles, and a system where that lattice site contains one normal particle and one ideal particle. 10 This was then combined with the free energy of inserting the ideal gas particle into the relevant Wigner–Seitz cell f insert = − f remove .…”

“… 7–9 Moreover, crowdions have recently been predicted to exist in relatively high quantities in the colloidal BCC crystal of repulsive point Yukawa particles. 10 This all leads to the question of the importance of the crystal structure in determining the defect behavior and whether other non-FCC crystal structures show interesting defect behaviors as well.…”