Microgravity experiments confirm densest stable hard sphere crystal is most symmetric

Packing of hard spheres fascinated mankind for millennia before it was also recognized as a versatile model for crystallization and other phenomena in materials. Theories in the 1930’s-60's predicted that they form a liquid phase for a particle volume fraction up to φ=0.494, a crystalline phase above φ =0.545 and coexistence in between1,2. In the mid 1980's, Pusey and van Megan confirmed this phase transition in colloids of closely matched particle/solvent densities3. However, their work and numerous experiments on Earth that followed observed small sedimented crystallites with random hexagonal close-packed (RHCP) structure and a high-density glass phase, in disagreement with theoretical predictions in the 1970’s-2020’s for high-symmetry faced-centered cubic (FCC) structure1,4,5. Here we report unique data with single-particle resolution on an FCC hard-sphere colloidal crystal, 27mm×1.2mm× 0.15 mm with φ ≈0.65, grown on the International Space Station. We confirm that FCC is the stable state for this most elementary system. The discovery that the absence of gravity makes it possible to equilibrate the FCC phase suggests routes for manufacturing colloidal crystals for mid-infrared photonics in a low-Earth orbit.