An NMR Analysis of Transitions in FexNbxV2-2xO4

The first order semiconductor - metall phase transition with the lattice symmetry change from monoclinic to tetragonal and the conductivity jump of several orders of magnitude is observed in pure vanadium dioxide and the dioxide doped by varios impurities I]. Transition temperature T_t for pure VO₂ is 68°C and depends on the impurity. For example, the impurities which enter the insulating phase as pentavalent ions Nb⁵⁺ 2] give rise to a decrease in metal-semiconductor transition temperature at low concentrations and the trivalent ions Cr³⁺, Fe³⁺ 3,4] lead to an increase in T_t. In the latter case three different monoclinic phases are stabilized at a temperature below T_t. It has been already shown 5] that for double doping of vanadium dioxide by pentavalent Nb⁵⁺ and trivalent Fe³⁺ or Cr³⁺ ions three semiconductor phases exist (M_I, M₂, M₃) but the transition temperature decreases as the impurity concentration increases.