Metal-insulator Transition in a Pyrochlore-type Ruthenium oxide, Hg2Ru2O7(2)

A new pyrochlore ruthenium oxide, Hg2Ru2O7 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_2g_3 electron configuration.

Transition metal oxides with the pyrochlore structure (of the formula A2B2O7) that possess magnetic 4d- and 5d-ions in the B-site often reveal an interesting interplay between strong electron correlation and magnetism1). The relatively wide bandwidth of 4d- and 5d-systems, compared with 3d-transition metal oxides, renders them metallic at room temperature more frequently2). Nevertheless, correlations among 4d- and 5d-electrons still play an important role and often bring about a metal-insulator transition (MIT) as a function of temperature (T)3,4) and chemical substitution5). The presence of geometrical frustration on the network of corner sharing tetrahedra in the B ion-sublattice makes the pyrochlore structure particularly interesting to study from the point of view of local moment magnetism. The pyrochlore lattice is known to give rise to one of the strongest degrees of frustration and prevents the long range ordering of spins, charge, and perhaps orbitals. Indeed, we may anticipate that frustration modifies the manner in which the MIT occurs and gives rise to a novel self-organization of a spin-charge-orbital complex.

Among a wide variety of 4d- and 5d-pyrochlore oxides, pyrochlores with tetravalent ruthenium Ru4+ occupying the B-site, A3+2Ru4+2O7, have been one of the most intensively studied4-6).Ru4+ is in the low spin state with four electrons in its nearly degenerate t2g orbitals. The wide variety of ground states in Ru4+ pyrochlores is likely due to their proximity to a MIT and the fact that their bandwidths depend strongly on the ionic radius of the trivalent A3+ ions through Ru-O-Ru bond bending7). RE3+2Ru4+2O7 (where RE is a rare earth ion, e.g., Y5)) is known to be a Mott insulator with a spin-glass ground state. Bi3+2Ru4+2O7 remains a metal down to the lowest measured temperatures6). Tl3+2Ru4+2O7 is a metal at room temperature but exhibits a metal to spin-singlet insulator transition with decreasing T 8,9). A novel interplay between orbital ordering and the S = 1 spins of Ru4+ has been suggested as the physics behind the spin-singlet formation.

In contrast to Ru4+ pyrochlores, only two Ru5+ pyrochlores,Cd2Ru2O710) and

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