In superionic compounds one component pre-melts providing high ionic conductivity to solid state electrolytes. Here, we find sublattice melting in colloidal crystals of oppositely charged particles that are highly asymmetric in size and charge in salt solutions. The small particles in ionic compounds melt when the temperature increases forming a superionic phase. These delocalized small particles in a crystal of large oppositely charged particles, in contrast to superionic phases in atomic systems, form crystals with non-electroneutral stoichiometric ratios. This generates structures with multiple domains of ionic crystals in percolated superionic phases with adjustable stoichiometries. [1][2][3], and have served as experimental models to study phase behaviors [4,5] and self-assembly processes [1,6]. Unlike chemical compounds in atomic systems, colloidal assemblies do not have constraints from the number, the symmetry, or the energy of orbitals. This significantly diversifies possible crystal structures. In the past few decades, several types of binary colloidal crystals with different component ratios have been studied [7][8][9]. Experiments and computer simulations have shown that the size ratio [10] or charge ratio [9,11] of the two components as well as the ionic strength in the solution [9] are important factors in the assembling process of binary superlattices. Most of the work on binary charged colloidal crystals retain the restricted structures of classical atomic ionic compounds in which all particle positions are fixed. Colloids of various components have been assembled into diverse crystalline structuresIn contrast, atomic "superionics" such as superionic conductors possess different kinds of structures and properties. In superionic conductors, due to the low energy barrier along cation migration paths [12], one of the components, termed the fast ions, are mobile and have a delocalized density distribution within the crystal lattice [12,13]. As a result, superionic conductors have high ionic conductivity at room temperature, making them the core component of high-performance solid state batteries. Superionic phases also have been found in other microscopic condensed systems such as ammonia [14], ice [14,15], and polymers [16].Here, we explore the possibility of assembling superionic conductors from oppositely charged colloids in salt solutions. The charge neutrality restriction in traditional ionic and superionic atomic crystals can be removed in charged colloidal crystals when the screening from small ions is present [9,17]. Moreover, the range of the interaction potential can be readily tuned * m-olvera@northwestern.edu by controlling the salt concentration in regimes where the Debye-Huckel approximation is valid [11,17]. Therefore, these colloidal systems may substantially expand the scope of colloidal science and superionic materials. Recently, a related superionic-like phase was found in binary colloidal mixtures of large DNA-functionalized gold nanoparticles (DNA-AuNPs) and complementary small DN...

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.

Nanoparticles with highly asymmetric sizes and charges that self-assemble into crystals via electrostatics may exhibit behaviors reminiscent of those of metals or superionic materials. Here, we use coarse-grained molecular simulations with underdamped Langevin dynamics to explore how a binary charged colloidal crystal reacts to an external electric field. As the field strength increases, we find transitions from insulator (ionic state), to superionic (conductive state), to laning, to complete melting (liquid state). In the superionic state, the resistivity decreases with increasing temperature, which is contrary to metals, yet the increment decreases as the electric field becomes stronger. Additionally, we verify that the dissipation of the system and the fluctuation of charge currents obey recently developed thermodynamic uncertainty relation. Our results describe charge transport mechanisms in colloidal superionic conductors.