Depressed transition temperature of WxV1−xO2: mechanistic insights from the X-ray absorption fine structure (XAFS) spectroscopy

Abstract

The mechanism for the decreasing critical temperature (T_C) of the metal–insulator transition (MIT) in vanadium dioxide (VO₂) by tungsten (W) doping is a matter of debate. Here, to clarify the correlation between W doping and T_C, the electronic and geometrical structures around W and V atoms in W_xV_1−xO₂ samples are systematically investigated by X-ray absorption fine structure (XAFS) spectroscopy. The evidence of electron doping of W⁶⁺ ions in VO₂ is obtained from the reduction of V⁴⁺ to V³⁺ ions. This kind of electron doping has been considered to favor the MIT process. Moreover, from the XAFS results, the local rutile structure around W dopants is identified even at low doping, and acts as the structure-guided domain to facilitate the MIT in VO₂. Considering the electronic band structures of W_xV_1−xO₂ samples, the internal stresses induced by W⁶⁺ doping yield the detwisting of the nearby monoclinic VO₂ lattice. This lattice detwisting will drive the downward shift of the π* electron band and a smaller separation between antibonding and bonding d_∥ orbitals in the band structure of VO₂, which induces the decreased band gaps of W_xV_1−xO₂ samples. As a consequence, the potential energy barrier for phase transition is lowered and the reduced T_C is observed.