Electron Paramagnetic Resonance Study of Amorphous V$_{2}$O$_{5}$

Vitreous V$_{2}$O$_{5}$ is generally non-stoichiometric when cooled from above the melt, leading to the formation of paramagnetic V$^{4+}$ ions. At low temperature ($\sim $120 K), previous workers have observed well-resolved EPR spectra associated with these centers. By contrast, recent results from our laboratory show EPR spectra that remain broad ($\sim $20 mT, peak-to-peak) from ambient down to 120 K, with no apparent hyperfine interactions. EPR spin counting analyses also indicate high concentrations ($\sim $10$^{20}$ spins/cm$^{3})$ of paramagnetic centers, suggesting that linewidths are limited by spin-spin relaxation of the electron spin system. In addition, current results reveal that EPR signal intensities increase more rapidly than (Temperature)$^{-1}$, as indicated by monitoring signal intensities over the temperature range from 120-300 K. In particular, when temperature is decreased by a factor of two (from 240 to 120 K), signal intensity increases by a factor of approximately 10. Such observations indicate the possibility of superparamagnetic or spin glass behavior within the amorphous V$_{2}$O$_{5}$ system. In order to further investigate the magnetic behavior of amorphous V$_{2}$O$_{5}$, several experimental EPR parameters are evaluated over the temperature range from 120-370 K.