The Origin of the Thermochromic Property Changes in Doped Vanadium Dioxide

Vanadium dioxide is a promising material for novel smart window applications due to its reversible metal-insulator transition which is accompanied by a change in its optical properties. The transition temperature (T_MIT) can be controlled via elemental doping, but the reduction of T_MIT is generally coupled with a decrease of the optical contrast between the two phases. To better understand how the contrast is fundamentally connected to T_MIT, the thermochromic properties of doped VO₂ were theoretically investigated across the metal-insulator transition from first principles. Different dopants and their interaction with the VO₂ host structure as well as different modes of doping were studied in detail. It was found that the transition temperature change is mainly related to the stabilization of the high-temperature metallic phase due to lattice deformations which are caused by the presence of the dopant ion. Inherent limitations to the thermochromic performance of VO₂ substitutionally doped by the replacement of vanadium cations with other species were found, and alternative approaches were proposed. Specifically, a charge-neutral substitution of oxygen or an oxygen substitution in combination with interstitial doping without net charge transfer between the dopant atoms and VO₂ were identified as promising avenues to ensure a low T_MIT and no loss of optical contrast in vanadia-based smart window materials.