Metal-insulator Transition in a Pyrochlore-type Ruthenium oxide, Hg2Ru2O7(8)

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.

novel Ru5+ valence in this compound was confirmed by XPS. The insulating state is likely to be a spin singlet; however, future studies are needed in order to confirm this. Hg2Ru2O7 provides us an intriguing topic for the study of metal-insulator transitions in the presence of strong geometrical frustration.

Note: Just before the submission of this paper, we became aware of a parallel study on Hg2Ru2O7 that has been reported by Klein et al17). Their crystal structure at room temperature and basic physical properties are consistent with our results. However, our sample showed a sharper M-I transition, and this enables us to observe thermal hysteresis and a structural change, which are evidences of a first-order transition.

Acknowledgements

We thank K. Watanabe from the advanced development and supporting center (RIKEN) for the measurements of EPMA. We also thank J. Matsuno, A. Chainani, and M. Uchida for useful discussion. This work was partly supported by a Grant-in-Aid for scientific research (No. 16GS50219) from the ministry of Education, Culture, Sports, Science and Technology as well as by the RIKEN DRI Research Grant.

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