ESR study of the equimolar PbO-V2O5 system

The semiconducting system 50 mol% PbO-V₂O₅ has been studied by electron spin resonance (ESR) in the temperature range 150 to 675 K. Depending on the cooling rate, this system can be obtained from the melt as an amorphous phase, as polycrystalline leadmetavanadate (PbV₂O₆), or as two different mixtures containing polycrystalline and amorphous phases. In all these materials the ratio V⁴⁺/V_tot has been measured by wet analysis showing that the V⁴⁺ concentration is nearly constant. It has been possible to show that the appearance of very different ESR line-shapes for the various materials is due to the difference in the hopping rate of the charge carriers. In particular the presence of a well-defined hyperfine structure is characteristic of the amorphous phase, in which the mobility of the charge carriers is lowered by the presence of a termW_D in the expression of the hopping rate. The lead-metavanadate ESR spectrum shows the presence of two lines of different widths; this fact has been explained assuming the presence of two distinct jumping rates. No temperature variation of the spectra has been observed at low temperatures. The high temperature spectra have permitted us to follow the thermal evolution of the system.