Metal-insulator Transition in a Pyrochlore-type Ruthenium oxide, Hg2Ru2O7(7)

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.

system Tl2Ru2O7. Tl2Ru2O7 displays a MIT at ~125 K, accompanied by a structural phase transition from cubic to orthorhombic4). It has been argued that the Tl2Ru2O7 orbital ordering in the insulating state gives rise to an essentially one-dimensional chain of Ru S = 1 antiferromagnetically coupled moments9). A one-dimensional chain of integer spins would then be unstable towards the formation of a valence bond singlet state, associated with the appearance of the Haldane gap. Hg2Ru2O7, with its t2g3 configuration, should have S = 3/2 spins without any orbital degree of freedom. The spin gap formation of half-integer spins is not uncommon in materials without quasi-one-dimensional structures. The similarity between the two system may suggest that the spin singlet formation is quite a common phenomena of correlated pyrochlores and not necessarily the consequence of S = 1 quantum magnetism.

A comparison of the other Ru5+ pyrochlores, Cd2Ru2O710) and Ca2Ru2O711), is also rather informative, though the physical and structural properties of those two compounds have yet to be clarified. These two pyrochlores do not show any clear evidence for a MIT as in Hg2Ru2O7, but ρ(T) weakly increases with decreasing T; this suggests a close proximity to an insulator. The presence of frustrated magnetic moments is inferred from the spin-glass behavior observed in Ca2Ru2O710), indicative of the importance of electron correlations. These observations appear to imply that all these pyrochlores are commonly located in the vicinity of a correlation driven MIT and that a subtle difference in local structure, hybridization with A2+ ions, disorde,r and so on brings these systems to distinctly different ground states. The unit cell parameter, a, of Hg2Ru2O7 (a = 10.199(1) Å) is comparable to that of Ca2Ru2O7 (a = 10.197(2) Å11)) and distinctly larger than that of Cd2Ru2O7 (a = 10.1291) Å10)). It is rather difficult to find a definite correlation between the unit cell parameter, i.e. Ru–Ru distance, and the ground state.

In conclusion, we found a first-order metal-insulator transition at 107 K accompanied by a structural change in the new pyrochlore Hg2Ru2O7, prepared under high pressure. The

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