Observation of the metal-insulator transition of VO2 in glasses and nanomaterials of MV2O5-P2O5 system (M Li, Na, Mg)

In this research, special attention has been paid to an observation of a metal-insulator transition (MIT) in lithium/sodium/magnesium-vanadate-phosphate glasses and nanomaterials. Two series (A & B) of glassy 90MV2O5 ⋅10P2O5 (M Li, Na, Mg) samples were prepared by melt-quenching technique at 1300 °C using double-crucible technique. Series B was synthesized in more reducing conditions. DSC traces of the synthesized samples were typical for glassy materials. Glass transition and crystallization occurred in series A at significantly higher temperatures than in series B. In addition, an endothermic peak was observed at low temperature (67 °C). XRD patterns of the samples contained amorphous halo, but also some diffraction reflexes corresponding to monoclinic VO2. In series A, the reflexes had low intensity, whereas in series B the intensity of the peaks at the same positions was significantly higher. Dependences of electrical conductivity were typical for nanocrystallized glasses in case of series A, whereas in series B a reversible change of the conductivity was observed at 67 °C. Samples of series A exhibited positive and constant Seebeck coefficient S within wide temperature range, whereas in series B a change in thermoelectric power was observed above 67 °C. All these phenomena around 67 °C are due to metal-insulator transition in VO2 impurities in the samples.