  Semimetallic osmium pyrochlore oxide Cd2Os2O7 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 Hg2Os2O7 through thermodynamic measurements, muSR
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 Hg2Os2O7 is most likely an AIAO semimetal, which
is analogous to the intermediate-temperature state of Cd2Os2O7. Hg2Os2O7 exists
on the verge of the metal-insulator boundary on the metal side and provides an
excellent platform for studying the electronic instability of 5d electrons with
moderate electron correlations and strong spin-orbit interactions.
