A charge-stabilized Jahn-Teller Distortion of the Mixed Valence System NaBa2Cu3O6

The crystal structure of NaBa₂M₃O₆ contains an unusual hexagonal prism composed of linked MO₄ squares. This unit is distorted in the M = Cu compound but undistorted in the M = Ni material. Electronic structure calculations show an orbital degeneracy and electronic stabilization on distortion of the hexagonal prism at the electron count appropriate for the Cu compound. The preferred distortion is determined by considering how the nature of both the occupied and unoccupied metal d orbitals are consistent with the metal oxidation states set by the stoichiometry. The results provide added insights into the nature of mixed valence compounds. The synthesis of Na₂BaM₃O₆ is encouraged.