First, we used the above method to coassemble identically sized (i.e., obtained from the same batch of Au•OLA), 4.83 nm Au•MUA and Au•TMA. Inspection of the resulting black precipitate by scanning electron microscopy (SEM) revealed that the NPs coassembled, in a near-quantitative fashion, into colloidal crystals with a rhombic dodecahedron (RD) morphology (Fig. ), each bound by 12 identical (110) crystal facets. The average edge length of these crystals is 1.2 μm, meaning that each RD is composed of approximately 10 million individual NPs. The crystals are large enough to be imaged by a benchtop optical microscope (Fig. , inset). The procedure is readily scalable and can be used to prepare multimilligram quantities of RDs in a single experiment. Small-angle X-ray scattering (SAXS) experiments showed that the NPs are arranged in a body-centered cubic lattice (Fig. ); i.e., the packing of oppositely charged superions within RDs mimics that of Cs + and Cl -in the crystal lattice of cesium chloride.