Pyrochlore oxide Hg 2 Os 2 O 7 on verge of metal-insulator boundary.

Semimetallic osmium pyrochlore oxide Cd 2 Os 2 O 7 undergoes a magnetic transition to an all-in-all-out (AIAO)-type order at 227 K, followed by a crossover to an AIAO insulator at around 210 K. Here, we studied the isostructural and isoelectronic compound Hg 2 Os 2 O 7 through thermodynamic measurements, muon spin rotation (μSR) spectroscopy and neutron diffraction experiments. A similar magnetic transition, probably to an AIAO-type order, was observed at 88 K, while the resistivity showed a decrease at the transition and remained metallic down to 2 K. Thus, the ground state of Hg 2 Os 2 O 7 is most likely an AIAO semimetal, which is analogous to the intermediate-temperature state of Cd 2 Os 2 O 7 . Hg 2 Os 2 O 7 exists on the verge of the metal-insulator boundary on the metal side and provides an excellent platform for studying the electronic instability of 5 d electrons with moderate electron correlations and strong spin-orbit interactions.