Structural instability and the Mott-Peierls transition in a half-metallic hollandite: K2Cr8 O16

In order to explore the driving mechanism of the concomitant metal-insulator and structural transitions in quasi-one-dimensional hollandite K2Cr8O16, electronic structures and phonon properties are investigated by employing the ab initio density functional theory (DFT) calculations. We have found that the imaginary phonon frequency reflecting the structural instability appears only in the DFT + U (U: Coulomb correlation) calculation, which indicates that the Coulomb correlation plays an essential role in the structural transition. The lattice displacements of the softened phonon at X explain the observed lattice distortions in K2Cr8O16 perfectly well, suggesting the Peierls distortion vector Q of X (0, 0, 1/2). The combined study of electronic and phonon properties reveals that half-metallic K2Cr8O16, upon cooling, undergoes the correlation-assisted Peierls transition to become a Mott-Peierls ferromagnetic insulator at low temperature.