Metal-insulator Transition in a Pyrochlore-type Ruthenium oxide, Hg2Ru2O7(5)

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.

resistivity should be substantially lower. Undoubtedly, Hg2Ru2O7 is a metal above ~107 K. The Seebeck coefficient S shown in Fig. 2, which is known to be insensitive to grain boundaries, is positive; more importantly, it shows a nearly T-linear, metallic T-dependence and is as small as 6 µV/K at 300 K typical of conventional metals.

The χ(T) data shown in Fig. 2 may indicate that the metallic state above ~107 K has a large electronic density of states at the Fermi level (N(E F)). Above ~107 K, we observe a weakly T dependent paramagnetic χ. A Curie-Weiss (CW) fit in the range ~150?300 K yielded a large ΘCW ~ –2700 K with an effective moment µeff ~3.7 µB, which is close to that expected for localized Ru5+S = 3/2 spins (3.873 µB). This value of ΘCW appears to be uncharacteristically large in comparison with insulating pyrochlores, where ΘCW often ranges from ~?500 K to ?1100 K (Y2Ru2O75)). We note that a large, weakly T-dependent Pauli contribution to χ(T) also easily explains the data. Given the rather unphysical value of ΘCW, we believe the paramagnetic χ(T) above 107 K may originate from conduction electron Pauli paramagnetism. A room temperature value of χ ~ 0.95×10–3 emu/mole Oe implies an N(E F) ~ 14.7 /eV/Ru atom, which corresponds to a T-linear specific heat (C) γ coefficient of ~ 22 mJ/mol K2. This estimation of γ is substantially larger than those of conventional metals, strongly suggesting the presence of a relatively narrow 4d band and thus also electron correlations in this system.Interestingly, the XPS data also indicate the presence of strong correlation through the appearance of screened and unscreened peaks in the Ru3d core level spectra15).

The magnitude of ρ(Τ) jumps by about one order of magnitude at ~107 K, followed by an insulating T-dependence (dρ/d T < 0). We observe a very clear anomaly in χ, S, and C p at the same T shown in Fig. 2. Since ρ(T) and χ(T) were accompanied by a small hysteresis, the MIT is definitely of first order.

The low T insulating phase shows a nearly T-independent, significantly reduced χ, much lower value than that in the metallic state, as shown in Fig. 2. Given the presence of strong

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