Random glass structure and electron localisation in amorphous V2O5

ESR experiments performed on crystalline and amorphous WO 3 -doped V 2 O 5 indicate different behaviours of both spectra with temperature and microwave frequency. A line broadening is observed for the crystalline oxide as the temperature increases while the ESR spectrum of amorphous V 2 O 5 remains well resolved up to 300K. This suggests that the electron hopping frequency between V 4+ and V 5+ ions is much smaller in the amorphous oxide. A strong frequency dependence of the ESR linewidth is observed for the amorphous sample, but not for the crystalline one. This has been attributed to a distribution of g-values among the different vanadium sites caused by a fluctuation of the local crystal field Delta . An analysis of the ESR linewidth as a function of the microwave frequency leads to a value delta Delta =0.24 eV of the crystal field fluctuations. Such a random distribution of Delta contributes to increase the activation energy of the hopping process and thus to the localisation of the charge carriers in the amorphous oxide.