Magnetic resonance study of vanadium pentoxide gels

Abstract

This work describes an Electron Paramagnetic Resonance (EPR) and Nuclear Magnetic Resonance (NMR) study of the vanadium pentoxide xerogel V2O5:nH(2)O with n approximate to 1.8 Experiments were performed in the temperature range 65 K - 310 K. The EPR spectrum at high temperatures exhibits the typical liquid-like eight lines-hyperfine structure. At low temperatures the EPR spectrum change to a V4+ anisotropic powder spectra. These two different regimes can be delimited by a transition temperature region centered at 280 K. Numerical simulations of the EPR spectra in the two temperature limits are in good agreement with the experimental data. Proton (H-1) NMR lineshapes, as functions of temperature were measured in the range 150-323 K and indicate that nuclear motional narrowing is effective at temperatures above 210 K, with an activation energy of 0.14 eV. The NMR spin-lattice relaxation recovery, associated to protons in the water molecules, was found to be non-exponential throughout the temperature range and described by two different relaxation processes. The slow relaxing component is temperature-independent and was attributed to water molecules located far from V4+ ions. The temperature dependence of the fast relaxing component shows maximum at around 260 K, suggesting a relaxation process sensitive to the temperature induced dynamic structural changes in the vanadium oxide matrix.