K2Cr8O16 predicted as a half-metallic ferromagnet: Scenario for a metal-insulator transition

Based on the first-principles electronic structure calculations, we predict that a chromium oxide K₂Cr₈O₁₆ of hollandite type should be a half-metallic ferromagnet where the Fermi level crosses only the majority-spin band, whereas the minority-spin band has a semiconducting gap. We show that the double-exchange mechanism is responsible for the observed saturated ferromagnetism. We discuss possible scenarios of the metal-insulator transition observed at low temperature and we argue that the formation of the incommensurate long-wavelength density wave of spinless fermions caused by the Fermi-surface nesting may be the origin of the opening of the charge gap.