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 90MV(2)O(5)center dot 10P(2)O(5) (M = Li, Na, Mg) samples were prepared by melt-quenching technique at 1300 degrees 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 degrees 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 degrees 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 degrees C. All these phenomena around 67 degrees C are due to metal-insulator transition in VO2 impurities in the samples.