Size-asymmetric binary charged colloidal solutions can
assemble
into ionic colloidal crystals. These are often stabilized by ionic-type
bonding, where the components with smaller size and charge sit at
fixed points within the lattice of large particles. Here, we study
the transition termed ionic to metallic bonding transition, by which
the lattice of the smaller component melts while the crystal of the
large particles is preserved, as in metallic bonding. We simulate
a charged colloidal crystal in equilibrium with a solution containing
small colloidal particles and counterions using the Coulomb interaction
between the finite-size components. We find ionic to metallic first-order
transitions by increasing either the temperature or the concentration
of the small particles in the solution. The transition is accompanied
by a lattice expansion and increased absorption of small particles
into the crystal. We compute the free energies of the ionic and metallic
states using the Madelung constant and Wigner–Seitz cell approaches,
respectively, combined with the quasi-harmonic lattice model. The
calculation reproduces the simulated transition and reveals that the
enthalpic gain is more pronounced than the entropic gain in the transition
from ionic to metallic bonding when material is exchanged with the
solution.