Visualization of the Insulator-Insulator and the Insulator-mixed-Metal-Insulator-transitions in self-heated Al_xV_1-xO₂ single crystals.

In addition to the well-known insulator-metal-transition (IMT) from a monoclinic -M1 to rutile structure exhibited above room temperature by pure vanadium dioxide, the title materials with 0.005<x<0.02 exhibit an insulator-insulator-transition (IIT) from triclinic-T to monoclinic -M2 structure at temperatures RT<T_IIT<T_IMT. The decrease in resistance of single crystals at the T_IMT is of several orders of magnitude; its increase at the T_IIT is by a factor of < 2. While IIT is easily induced by heating or self–heating in single crystals of Al_xV_1-xO₂, a mixed metal-insulator state sets in at T_IMT because the low power in the metal cannot cover the losses. Reported herein is the investigation of the IIT and the Insulator-mixed-metal-insulator-transition (ImMIT) in self-heated single crystals of Al_xV_1-xO₂ by optical microscopy synchronous with I-V tracing at RT. Prominent features on the current trace are easily identified with the onset of these self-heating induced transitions. In selected samples the video recordings showed that with increasing current, the embedded phase expands and with decreasing current it contracts, its boundaries receding in the first case or approaching until annihilation, in the second.