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

Iridates show fascinating properties due to the unpredictable ground states of their Ir cations. Generally, Ir 5+ (5 d 4 ) systems exhibit insulating nonmagnetic states owing to the strong spin-orbit coupling (SOC). Herein a new pyrochlore iridate Hg 2 Ir 2 O 7 with an Ir 5+ charge state synthesized by high-pressure technique is reported. Hg 2 Ir 2 O 7 crystallizes in the typical cubic pyrochlore crystal structure. The Ir 5+ valence state is evidenced by the XAS spectrum. Surprisingly, Hg 2 Ir 2 O 7 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 Ir 5+ in Hg 2 Ir 2 O 7 deviates from SOC-dominated insulating nonmagnetic states.