Metal-insulator transition in Cr-doped hollandite vanadate K₂V₈O₁₆

Abstract

The hollandite K2V8O16 is a metal with ferromagnetic spin fluctuations in the high temperature (tetragonal I4/m) phase and encounters metal-insulator transition (MIT) at 170 K. The mechanism behind the MIT is still controversial due to inadequate information on the electronic structure. Furthermore, substitution of Rb for K or Ti for V increases the transition temperature. We have investigated whether break in the mirror symmetry is responsible for the insulating ground state of K2V8O16 and since the Cr counterpart is a robust ferromagnet, whether Cr doping which leads to break in mirror symmetry can help in driving a gap. We find that both 25% and 75% Cr doping break the mirror symmetry but only the former leads to an insulating ground state. Ferromagnetism (FM) remains intact in the insulating phase of K2V6Cr2O16. The structural, electronic and magnetic properties of pure and doped K2V8O16 were investigated within first-principles calculations using density functional theory (DFT). Electron correlation suppresses orbital fluctuations between the partially occupied Cr and V-3dt2g states. Consequently, transfer of charge (electron) from V-3d to Cr-3dt2g states is observed which facilitates Cr3+-V4+ charge ordering. Furthermore, Peierls like structural distortion is associated to the breaking of mirror symmetry in the Cr-V rectangular four chain columns within the crystal. Therefore, the simultaneous effect of Peierls instability, charge ordering and Coulomb correlation is responsible for the MIT in K2V6Cr2O16. Besides, Cr-3d O-2p hybridization and Cr-O coupling increases with Cr doping. These two effects are cooperatively responsible for the observed FM in insulating K2V6Cr2O16. More interestingly, the strength of FM is augmented with Cr concentration in K2V8O16.