  Colloidal crystals formed by size-asymmetric binary particles co-assemble
into a wide variety of colloidal compounds with lattices akin to ionic
crystals. Recently, a transition from a compound phase with a sublattice of
small particles to a metal-like phase in which the small particles are
delocalized has been predicted computationally and observed experimentally. In
this colloidal metallic phase, the small particles roam the crystal maintaining
the integrity of the lattice of large particles, as electrons do in metals. A
similar transition also occurs in superionic crystals, termed sublattice
melting. Here, we use energetic principles and a generalized molecular dynamics
model of a binary system of functionalized nanoparticles to analyze the
transition to sublattice delocalization in different co-assembled crystal
phases as a function of T, number of grafted chains on the small particles, and
number ratio between the small and large particles $n_s$:$n_l$. We find that
$n_s$:$n_l$ is the primary determinant of crystal type due to energetic
interactions and interstitial site filling, while the number of grafted chains
per small particle determines the stability of these crystals. We observe
first-order sublattice delocalization transitions as T increases, in which the
host lattice transforms from low- to high-symmetry crystal structures,
including A20 to BCT to BCC, Ad to BCT to BCC, and BCC to BCC/FCC to FCC
transitions and lattices. Analogous sublattice transitions driven primarily by
lattice vibrations have been seen in some atomic materials exhibiting an
insulator-metal transition also referred to as metallization. We also find
minima in the lattice vibrations and diffusion coefficient of small particles
as a function of $n_s$:$n_l$, indicating enhanced stability of certain crystal
structures for $n_s$:$n_l$ values that form compounds.
