Toward Room-Temperature Thermochromism of VO₂ by Nb Doping: Magnetic Investigations

Abstract

Using a synthesis route based on carbon as the reducing agent mixed with Nb-doped V 2 O 5 nanopowders, highly crystallized Nb-doped VO 2 (M) particles, with tunable content of niobium (Nb), are successfully prepared. The morphology and the crystalline structure are first studied in regard to synthesis parameters and chemical composition for various thermal treatment temperatures/durations and niobium doping concentrations. The second part of this paper is devoted to the control, in Nb-doped VO 2 , of the phase transition temperature from monoclinic to rutile (M → R transition) versus the niobium doping concentration associated with thermochromic properties and different property changes characterized by differential scanning calorimetry and magnetic and electrical measurements. The niobium introduction allows the decrease of the thermochromic transition from 68 °C down to around room temperature for an effective Nb concentration of approximatively 6 mol % for the V 0.94 Nb 0.06 O 2±δ composition. A significant result is the decrease of both the hysteresis width as well as the phase transition amplitude supported by latent heat, magnetic behavior, and electrical transport versus niobium concentration. These phenomena are correlated to the breaking of the V-V pairing occurring in the monoclinic form when niobium ions are introduced. Finally, a deep interpretation of the Nb-doped VO 2 magnetic behavior is performed considering the impact of niobium concentration and crystallite sizes on the intensity of the Curie magnetic residue plus Van Vleck susceptibility in the low temperature form (M) and on the intensity of the Pauli magnetism of the high temperature form (R).

Doubling of the Phase Transition Temperature of VO2 by Fe Doping

“…Vanadium dioxide (VO 2 ) is a well-known thermochromic material, which undergoes a fully reversible first-order metal–insulator transition (MIT) from a low-temperature monoclinic M1 phase ( P 2 1 / c ) to a high-temperature tetragonal phase ( P 4 2 / mnm ) at around 68 °C. At the structural transition, significant changes in atomic organization are intertwined with changes in the electrical, optical, and magnetic properties, , making VO 2 a promising candidate for switching applications, , but with controversies remaining about the Peierls or Mott origin for the phase transition. − In our own previous studies, considering that the phase transition is well linked to structural origin, geometrical calculations in agreement with experimental studies suggested that the VO 2 phase transition is a consequence of very short atomic shifts mainly associated with a decrease in the level of second-sphere Coulombic interactions . For practical use, various transition temperatures of VO 2 are desired to fulfill the different demands depending on the applications.…”

From multilayers to V1-xWxO2±δ films elaborated by magnetron sputtering for decreasing thermochromic transition temperature

“…As mentioned above, the metal-insulator transition of VO2 occurred at 68 °C which is too high for room temperature applications. One way to reduce the transition temperature is to substitute V 4+ ions by metal ions with higher valence such as W 6+ , Nb 5+ , Mo 6+ [12,13,14,15]. Among them, W has been considered as one of the most effective ion, lowering the transition temperature (Tc) by 24 °C per 1 at.% [12] while 7.5 °C per 1 at.% and approximatively 8 °C per 1 at.% have been reported for Nb [13] and Mo [16], respectively.…”

Multi‐stimuli‐responsive and Multi‐functional Smart Windows

“…Doping has been reported to be effective for this purpose [3] . Various elements such as H and W have been theoretically proposed, as well as experimentally verified, to lower the transition temperature of VO 2 [34,35,70–79] . It is also known that continuous VO 2 films have a low T lum and Δ T sol ; [3,18] therefore, various synthetic and fabrication methods, under optimal thickness, have been developed to improve the thermochromic performance of these films [80–90] .…”