Metal-to-insulator transition and its effective manipulation studied from investigations in V1-xNbxO2 bulks

Nb doping effects on crystalline structure, transport transition and magnetic properties are investigated for V1-xNbxO2 (0 ≤x≤ 5%) bulks. At room temperature, doping does not change structure of monoclinic phase but causes an increase in lattice parameters for the range of x≤ 3%, further doping leads to a structure transition to rutile tetragonal phase. Each sample simultaneously displays a transition to insulating behavior and a sharp decrease in susceptibility below an almost same critical temperature (Tc). The Tc is shown to shift from ~ 340 K at x = 0 to ~ 277 K at x = 5% at a rate of ~ −13 K/at%. It is argued that the observed sharp decrease in susceptibility originates from the formation of V-V dimers with spins in anti-parallel, while the insulating behavior below Tc is due to the electron movement from one dimerized state to another with the help of Variable-Range-Hopping mechanism. Furthermore, it is proposed that doping by larger cations, which would suppress the formation of dimers, can be used to manipulate the metal-insulator transition to lower temperatures.