Elastic constants and the effect of strain on monovacancy concentration in fcc hard-sphere crystals

We investigate the free energy and the concentration of monovacancies in strained face-centered-cubic (fcc) hard-sphere crystals for several densities at and above melting. We use the conventional molecular dynamics method for simulations and employ a bias insertion method to extract properties of a monovacancy. We study two distinct constant-volume strains, considering a simple shear and an orthogonal expansion and contraction. Strains are examined across the linear elastic region and include also some nonlinear elastic deformations. Second-order elastic constants are reported as a function of density. The concentration of monovacancies decreases as density increases for both strained and unstrained crystals. The effect of strain is to cause the monovacancy concentration to increase by up to 72% for the expansion-contraction strain at the largest deformation studied. The effect of the shear strain is considerably less, and produces an increase in monovacancy concentration of at most 9% for the conditions studied here.