The formation of RDs in our system may appear surprising, given that the NPs interact predominantly by electrostatic forces. RD-shaped colloidal crystals with the CsCl structure were previously observed in binary mixtures of NPs interacting via local, highly specific intermolecular interactions (between complementary DNA chains and a synthetic hydrogen-bond donor-acceptor pair ). Although electrostatic interactions are typically long-range, we note that self-assembly in our system occurs in a concentrated salt solution. In fact, we experimentally determined that oppositely charged 5.28 nm NPs initially immersed in a saturated (~10.5 M) (NH4)2CO3 solution begin to interact when the concentration of the evaporating salt is still very high, at ~4.28 M (i.e., at this salt concentration, the NH4 + and CO3 2-ions can no longer screen out the Coulombic attraction between oppositely charged NPs). Next, we performed atomistic molecular dynamics (MD) simulations of pairs of oppositely charged (Fig. ) and like-charged NPs (Supplementary Fig. ) in a 4.28 M (NH4)2CO3 solution. In such an environment, the Debye screening length is very short (λ ≈ 0.1 nm) and the NPs interact only locally by screened Coulomb coupling between the charged tips of their ligands (i.e., a surface-centered Yukawa-type interaction). Indeed, the free energies of interparticle interactions calculated by an umbrella sampling technique (for details, see Supplementary Section 3) show that the NPs do not experience attractive or repulsive interactions unless they are in close contact (Fig. ). For a pair of oppositely charged NPs, the interaction energy increases rapidly from ~0 to -50 kcal/mol over a distance of only ~0.7 nm (Fig. , green trace). Analogously, likecharged NPs couple via sharp repulsive potentials, but not before their ligands are practically in direct contact (within ~0.3 nm of each other) (red and blue traces in Fig. ; note that TMA is slightly longer than MUA). Overall, these simulations indicate that electrostatic interactions between superionic NPs at high ionic strengths resemble (in terms of range) specific intermolecular interactions.