Distinctive finite size effects on the phase diagram and metal-insulator transitions of tungsten-doped vanadium(iv) oxide

The influence of finite size in altering the phase stabilities of strongly correlated materials gives rise to the interesting prospect of achieving additional tunability of solid-solid phase transitions such as those involved in metal-insulator switching, ferroelectricity, and superconductivity. We note here some distinctive finite size effects on the relative phase stabilities of insulating (monoclinic) and metallic (tetragonal) phases of solid-solution W_xV_1-xO₂. Ensemble differential scanning calorimetry and individual nanobelt electrical transport measurements suggest a pronounced hysteresis between metal → insulator and insulator → metal phase transformations. Both transitions are depressed to lower critical temperatures upon the incorporation of substitutional tungsten dopants but the impact on the former transition seems far more prominent. In general, the depression in the critical temperatures upon tungsten doping far exceeds corresponding values for bulk W_xV_1-xO₂ of the same composition. Notably, the depression in phase transition temperature saturates at a relatively low dopant concentration in the nanobelts, thought to be associated with the specific sites occupied by the tungsten substitutional dopants in these structures. The marked deviations from bulk behavior are rationalized in terms of a percolative model of the phase transition taking into account the nucleation of locally tetragonal domains and enhanced carrier delocalization that accompany W⁶⁺ doping in the W_xV_1-xO₂ nanobelts.