Metal-insulator Transition in a Pyrochlore-type Ruthenium oxide, Hg2Ru2O7(4)

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.

atomic positions assumed the regular position of the pyrochlore structure except for x(O1) in the 48f site. Occupancy in all atoms was fixed at 1.0, and B(O1) and B(O2) fixed at 0.6 Å2 and 1.0 Å2, respectively. The refined structure parameters are as follows: a = 10.199(1) Å, x(O1) = 0.3182(4), B(Hg) = 0.861(8) Å2, and B(Ru) = 0.554(11) Å2. The reliability factors are R wp = 10.95 %, R e = 8.08 %, S = 1.35, R I = 2.90 %, and R F = 2.49%. The T dependence of XRD was measured using conventional CuKαradiation (MacScience, MXP21TA). The lattice parameter below 300 K was determined by the least-squares method. The metal ratio determined by an electron microprobe analyzer (JEOL) was Hg/Ru = 1.01(2). The chemical analysis of the oxygen content could not be carried out in this study since it was very difficult to dissolve the sample in any acid. KCl remained in the sample as a byproduct of KClO3 and was washed with water as necessary.

X-ray photoemission spectra (XPS) were measured using a VG ESCALAB 250 spectrometer (Thermo-Fisher Electron Co.) employing monochromatic Al-K radiation at room temperature. The spectra of the peaks of Ru 3d were recorded with an energy step of 0.1 eV. The sample was mounted on Au disks as a sample holder that is also used for the standardization of Au 4f7/2 (83.9 eV). XPS data of Ru3d for Hg2Ru2O7 and RuO2 are shown in Fig. 1. The binding energy of the 3d5/2 peak of Hg2Ru2O7 (281.3 eV) is located 0.6 eV below that of RuO2 (280.7 eV) with Ru4+. This clearly demonstrates that a novel valence of Ru5+ (t2g3) is stabilized in this compound.

The electrical resistivity, thermoelectric power, heat capacity, and thermal conductivity were measured in a Physical Property Measurement System (Quantum Design). The dc-susceptibility was measured with a Magnetic Property Measurement System (Quantum Design).

A MIT is observed in the ρ(Τ) of Hg2Ru2O7 at T MI ~ 107 K, as shown in Fig. 2. The ρ(Τ) curve shows a metallic behavior (dρ/d T > 0) for T > T MI. The magnitude of ρ(Τ) at 300 K was as high as 10 m? cm. Considering that the sample is a porous ceramic, its intrinsic

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