  A compound with metallic electrical conductivity usually has a considerable
total thermal conductivity because both electrons and photons contribute to
thermal transport. Here, we show an exceptional example of iridium oxide,
Bi3Ir3O11, that concurrently displays metallic electrical conductivity and
ultralow thermal conductivity approaching 0.61 W m-1 K-1 at 300 K. The compound
crystallizes into a cubic structural framework with space group Pn-3. The edge-
and corner-sharing IrO6 octahedra with a mixed Ir4.33+ charge state favor
metallic electrical transport. Bi3Ir3O11 exhibits an extremely low lattice
thermal conductivity close to the minimum limit in theory owing to its
tunnel-like structure with filled heavy atoms Bi rattling inside. Theoretical
calculations reveal the underlying mechanisms for the extraordinary
compatibility between metallic electrical conductivity and ultralow thermal
conductivity. This study may establish a new avenue for designing and
developing unprecedented heat-insulation metals.
