We show that the valence electrons of Ba 3 NaRu 2 O 9 , which has a quasimolecular 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 Ru 2 O 9 dimers lead to competition with a quasidegenerate charge-melted phase under photoexcitation 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.
Charge Order at the Frontier between the Molecular and Solid States in Ba3NaRu2O9
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