Metal-insulator transition in a spin-orbital-lattice coupled Mott system: K2 V8 O16

We have explored the underlying mechanism of the metal-insulator transition (MIT) in hollandite-type vanadate, K2V8O16, which has a quasi-one-dimensional chain structure and undergoes the MIT and Peierls-like structural transition upon cooling. For this purpose, we have investigated its electronic and magnetic properties in comparison to those of Rb2V8O16 that also undergoes the MIT but without the Peierls-like structural distortion. We have found that K2V8O16 is a spin-orbital-lattice coupled Mott system and manifests the orbital-selective Mott transition. The interplay of on-site Coulomb interaction, the magnetic-exchange interaction, and the Jahn-Teller-type tetragonal distortion plays an essential role in driving the MIT of K2V8O16, inducing the the charge ordering (CO) and orbital ordering of V t2g bands. The CO of V3+ and V4+ occurs in separate chains, preserving the inversion symmetry of the crystal. The dxy orbitals form the spin-singlet state along the chain direction. The Peierls-like distortion does not play an essential role in the MIT.