Metal-insulator Transition in a Pyrochlore-type Ruthenium oxide, Hg2Ru2O7(6)

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.

electron correlations inferred from the enhanced γ, it may be natural to suggest that the low T insulating phase is a sort of Mott insulator. Charge density wave formation (CDW) is rather difficult to imagine in this three-dimensional system. The formation of the Mott-insulating state is often accompaniedly unusual magnetism such that we anticipate some sort of S = 3/2 pyrochlore magnet due to the t2g3 configuration of Ru5+.

The strong geometrical frustration inherent in the pyrochlore structure may lead to an S = 3/2 spin liquid followed by a marginally achieved ordered state, as seen in the S = 3/2 spinel antiferromagnets (with pyrochlore magnetic sublattices) ZnCr2O412) and CdCr2O413). However, the χ(T) observed in Hg2Ru2O7 is more than one order of magnitude smaller than that of the S = 3/2 pyrochlore antiferromagnets. This paramagnetic χ is comparable to that of a spin-singlet insulator such as MgTi2O4(S = 1/2) 16) and Tl2Ru2O7(S = 1) 11). These comparisons lead us to suggest that the S = 3/2 moment is quenched into a spin-singlet insulator ground state in Hg2Ru2O7.

A structural distortion associated with the MIT is inferred from the XRD pattern and the T dependence of the thermal conductivity (κ).As shown in Fig. 3(a), κ(Τ) , a kink is observed at the MIT. Judging from the Weidemann-Franz law, κ is dominated by phonon heat transport such that the kink observed at T MI is consistent with a structural transition. The cubic a lattice parameter discontinuously increases at the MIT. Clearly, the lattice expands from the metallic phase to the insulating phase, which is consistent with the importance of electronic correlations. Importantly, the 444 and 880 peaks in the simple cubic unit cell split below T MI, indicating the lowering of the crystal symmetry. Our preliminary electron diffraction measurements at low T have indicated the appearance of superlattice spots in addition to those seen in the cubic phase. These observations suggest the presence of a complicated atomic displacement, which might stabilize the spin-singlet state suggested by χ(T).

The behavior of the t2g3 Hg2Ru2O7 system has a striking parallel with that of the t2g4

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