Discovery of Ferromagnetic-Half-Metal–to–Insulator Transition in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi mathvariant="bold">K</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi>Cr</mml:mi><mml:mn>8</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="bold">O</mml:mi><mml:mn>16</mml:mn></mml:msub></mml:math>

The hollandite chromium oxide K2Cr8O16 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 K2Cr8O16 is a ferromagnetic metal (or half-metal) with T(C)=180 K and shows a transition to an insulator at 95 K without any apparent structural change but retaining ferromagnetism. K2Cr8O16 is quite unique in three aspects: It has a rare mixed valence of Cr3+ and Cr4+; 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.