Anomalous metallic conductivity and short-range ferromagnetic correlation in high-pressure synthesized pyrochlore Hg2Ir2O7

Iridates show fascinating properties due to the unpredictable ground states of their Ir cations. Generally, Ir5+(5d4) systems exhibit insulating nonmagnetic states owing to the strong spin-orbit coupling (SOC). Herein a new pyrochlore iridates Hg2Ir2O7 with an Ir5+ charge state synthesized by high-pressure technique is reported. Hg2Ir2O7 crystallizes in the typical cubic pyrochlore crystal structure. The Ir5+ valence state is evidenced by the XAS spectrum. Surprisingly, Hg2Ir2O7 displays short-range ferromagnetic correlations at low temperatures as evidenced by S-shape field-dependent magnetization curves, positive magnetoresistance, and magnetic excitations in specific heat. Furthermore, it also shows metallic conduction and large electron component of specific heat. These results all indicate that Ir5+ in Hg2Ir2O7 deviates from SOC-dominated insulating nonmagnetic states.