We show that the valence electrons of Ba3NaRu2O9, which has a quasi-molecular
structure, completely crystallize below 210 K. Using an extended Hubbard model,
we show that the charge ordering instability results from long-range Coulomb
interactions. However, orbital ordering, metal-metal bonding and formation of a
partial spin gap enforce the magnitude of the charge separation. The striped
charge order and frustrated hcp lattice of Ru2O9 dimers lead to competition
with a quasi-degenerate charge-melted phase under photo-excitation at low
temperature. Our results establish a broad class of simple metal oxides as
models for emergent phenomena at the border between the molecular and solid
states.Comment: Minor changes, with supporting information. To appear in Phys. Rev.
Let
