A new pyrochlore ruthenium oxide, Hg2Ru2O 7, was synthesized under a high pressure of 6 GPa. In contrast to the extensively studied Ru4+ oxides, this compound possesses a novel Ru5+ valence state, corresponding to a half-filled t 2g3 electron configuration. Hg2Ru 2O7 exhibits a first order metal-insulator transition at 107 K, accompanied by a structural transition from a cubic to lower symmetry. The behavior of the magnetic susceptibility suggests the possible formation of a spin singlet in the insulating low temperature state.
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