Semiconductor-Metal Transitions in NbO₂ and Nb_1−xV_xO₂

NbO₂, Nb_0.98V_0.02O₂, and Nb_0.95V_0.05O₂ transform from semiconducting to metallic state at temperatures higher than the DTA phase-transition temperatures. Vibrational mode softening and c-axis Nb–Nb pairing appear to be important factors in the mechanism of these transitions. In the solid solutions, c/a ratio is maximum at x = 0.5 since the metal-metal interaction along the c axis becomes minimum; conductivity is minimum at this composition since there are no mobile electrons.