The hollandite chromium oxide ${\mathrm{K}}_2{\mathrm{Cr}}_8{\mathrm{O}}_{16}$ has been synthesized in both powder and single-crystal form under high pressure. Combining electrical resistivity, magnetic susceptibility, and x-ray diffraction, we found that ${\mathrm{K}}_2{\mathrm{Cr}}_8{\mathrm{O}}_{16}$ is a ferromagnetic metal (or half-metal) with $T_C=180\mathrm{K}$ and shows a transition to an insulator at 95 K without any apparent structural change but retaining ferromagnetism. ${\mathrm{K}}_2{\mathrm{Cr}}_8{\mathrm{O}}_{16}$ is quite unique in three aspects: It has a rare mixed valence of ${\mathrm{Cr}}^{3+}$ and ${\mathrm{Cr}}^{4+}$; it has a metal (or half-metal)-to-insulator transition in a ferromagnetic state; and the resulting low-temperature phase is a rare case of a ferromagnetic insulator. This discovery could open a new frontier on the relation of magnetism and conducting properties in strongly correlated electron systems.

• Received 17 June 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.146403